The Sun is the star located at the centre of the Solar System. It is a massive sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light and infrared radiation with 10% at ultraviolet energies. It is the main source of energy for life on Earth. The Sun has been an object of veneration in many cultures and a central subject of astronomical research since antiquity.
The Sun orbits the Galactic Center at a distance of 24,000 to 28,000 light-years. Its mean distance from Earth is about 1.496×108 kilometres or about 8 light-minutes. The distance between the Sun and the Earth was used to define a unit of length called the astronomical unit (au), now defined to be 149.5978707×106 kilometres. It is the largest and most massive object in the Solar System; its diameter is about 1,391,400 km (864,600 mi), around 109 times that of Earth. The Sun's mass is around 330,000 times that of Earth, making up about 99.86% of the total mass of the Solar System. The mass of the Sun's surface layer, its photosphere, consists mostly of hydrogen (~73%) and helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.
The Sun formed approximately 4.6 billion[a] years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the centre; the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is now classified as a G-type main-sequence star (G2V). Every second, the Sun's core fuses about 600 billion kilograms (kg) of hydrogen into helium and converts 4 billion kilograms of matter into energy.
About 4 to 7 billion years from now, when hydrogen fusion in the Sun's core diminishes to the point where the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature which will cause its outer layers to expand, eventually transforming the Sun into a red giant. After the red giant phase, models suggest the Sun will shed its outer layers and become a dense type of cooling star (a white dwarf), and no longer produce energy by fusion, but will still glow and give off heat from its previous fusion for perhaps trillions of years. After that, it is theorised to become an extremely dense black dwarf, giving off negligible energy.
Mercury is the first planet from the Sun and the smallest in the Solar System. It is a rocky planet with a trace atmosphere and a surface gravity slightly higher than that of Mars. The surface of Mercury is similar to Earth's Moon, being cratered, with an expansive rupes system generated from thrust faults, and bright ray systems, formed by ejecta. Its largest crater, Caloris Planitia, has a diameter of 1,550 km (960 mi), which is about one-third the diameter of the planet (4,880 km or 3,030 mi). Being the most inferior orbiting planet, it always appears close to the Sun in Earth's sky, as either a "morning star" or an "evening star". It is the planet with the highest delta-v required for travel from Earth, as well as to and from the other planets in the Solar System.
Mercury's sidereal year (88.0 Earth days) and sidereal day (58.65 Earth days) are in a 3:2 ratio, in a spin–orbit resonance. Consequently, one solar day (sunrise to sunrise) on Mercury lasts for around 176 Earth days, or twice Mercury's sidereal year. This means that one side of Mercury will remain in sunlight for one Mercurian year of 88 Earth days; while during the next orbit, that side will be in darkness all the time until the next sunrise after another 88 Earth days.
Above the planet's surface is an extremely tenuous exosphere and a faint magnetic field just strong enough to deflect solar winds. Combined with its high orbital eccentricity, the planet's surface has widely varying sunlight intensity and temperature, with the equatorial regions ranging from −170 °C (−270 °F) at night to 420 °C (790 °F) during sunlight. Despite being the closest planet to the sun, it is the second hottest after Venus, lacking an atmosphere to trap heat. Due to its very small axial tilt, the planet's poles are permanently shadowed. This strongly suggests that water ice could be present in the craters.
Like the other planets in the Solar System, Mercury formed approximately 4.5 billion years ago. There are competing hypotheses about Mercury's origins and development, some of which incorporate collision with planetesimals and rock vaporization; as of the early 2020s, many broad details of Mercury's geological history are still under investigation or pending data from space probes. Its mantle is highly homogeneous, which suggests that Mercury had a magma ocean early in its history, like the Moon. According to current models, Mercury may have a solid silicate crust and mantle overlaying a solid outer core, a deeper liquid core layer, and a solid inner core. Mercury is expected to be destroyed, along with Venus, and possibly the Earth and the Moon, when the Sun becomes a red giant in approximately seven or eight billion years.
Mercury is a classical planet that has been observed and recognized throughout history as a planet (or wandering star). In English, it is named after the ancient Roman god Mercurius (Mercury), god of commerce and communication, and the messenger of the gods. The first successful flyby of Mercury was conducted by Mariner 10 in 1974, and it has since been visited and explored by the MESSENGER and BepiColombo orbiters.
Venus is the second planet from the Sun. Similar in size and mass to Earth, Venus has no liquid water, and its atmosphere is far thicker and denser than that of any other rocky body in the Solar System. The atmosphere is composed mostly of carbon dioxide and has a thick cloud layer of sulfuric acid that spans the whole planet. At the mean surface level, the atmosphere reaches a temperature of 737 K (464 °C; 867 °F), making it the hottest planet in the solar system and also a pressure 92 times greater than Earth's at sea level, turning the lowest layer of the atmosphere into a supercritical fluid. From Earth, Venus is visible as a star-like point of light, appearing brighter than any other natural point of light in the sky as either the brightest "morning star" or "evening star".
Venus orbits closer to the Sun than the Earth does. The orbits of Venus and Earth make the two planets approach each other in synodic periods of 1.6 years. In the course of this, Venus comes closer to Earth than any other planet. In interplanetary spaceflight from Earth, Venus is frequently used as a waypoint for gravity assists, offering a faster and more economical route. Venus has no moons and a very slow retrograde rotation about its axis, a result of competing forces of solar tidal locking and differential heating of Venus's massive atmosphere. As a result, a Venusian day is 116.75 Earth days long, about half a Venusian solar year, which is 224.7 Earth days long.
Venus has a weak magnetosphere; lacking an internal dynamo, it is induced by the solar wind interacting with the atmosphere. Internally, Venus has a core, a mantle, and a crust. Internal heat escapes through active volcanism, resulting in resurfacing, instead of plate tectonics. Venus may have had liquid surface water early in its history with a habitable environment, before a runaway greenhouse effect evaporated any water and turned Venus into its present state. There are atmospheric conditions at cloud layer altitudes that are the most similar ones to Earth in the Solar System and have been identified as possibly favourable for life on Venus, with potential biomarkers found in 2020, spurring new research and missions to Venus.
Throughout history humans across the globe have observed Venus and it has acquired particular importance in many cultures. With telescopes, the phases of Venus became discernible and, by 1613, were presented as decisive evidence disproving the then-dominant geocentric model and supporting the heliocentric model. Venus was visited for the first time in 1961 by Venera 1, which flew past the planet, achieving the first interplanetary spaceflight. The first data from Venus were returned during the second interplanetary mission, Mariner 2, in 1962. In 1967, the first interplanetary impactor, Venera 4, reached Venus, followed by the lander Venera 7 in 1970, with the first images captured by Mariner 10 during its 1974 gravity assist en route to Mercury. As of 2025, Solar Orbiter is on its way to fly by Venus in 2026, and the next mission planned to launch to Venus is the Venus Life Finder, scheduled for 2026 as well.
Earth is the third planet from the Sun and the only astronomical object known to harbor life. This is made possible by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all of Earth's water is contained in its ocean, which covers 70.8% of Earth's crust. The remaining 29.2% of Earth's crust is land, which is predominantly located within Earth's land hemisphere in the form of continental landmasses.
Most of Earth's land is at least somewhat humid and covered by vegetation, while large ice sheets at Earth's polar deserts retain more water than Earth's groundwater, lakes, rivers, and atmospheric water combined. Earth's crust consists of slowly moving tectonic plates, which interact to produce mountain ranges, volcanoes, and earthquakes. Earth has a liquid outer core that generates a magnetosphere capable of deflecting most of the destructive solar winds and cosmic radiation
Earth has a dynamic atmosphere, which sustains Earth's surface conditions and protects it from most meteoroids and ultraviolet light at entry. It is composed primarily of nitrogen and oxygen. Water vapor is widely present in the atmosphere, forming clouds that cover most of the planet. The water vapor acts as a greenhouse gas and, together with other greenhouse gases in the atmosphere, particularly carbon dioxide (CO2), creates the conditions for both liquid surface water and water vapor to persist via the capturing of energy from the Sun's light.
This process maintains the current average surface temperature of 14.76 °C (58.57 °F), at which water is liquid under normal atmospheric pressure. Differences in the amount of captured energy between geographic regions (as with the equatorial region receiving more sunlight than the polar regions) drive atmospheric and ocean currents, producing a global climate system with different climate regions, and a range of weather phenomena such as precipitation, allowing components such as carbon and nitrogen to cycle.
Earth is rounded into an ellipsoid with a circumference of about 40,000 kilometres (24,900 miles). It is the densest planet in the Solar System. Of the four rocky planets, it is the largest and most massive. Overall, Earth is the sixth largest object in the Solar System after the Sun and the outer planets. Earth is about eight light-minutes (1 AU) away from the Sun and orbits it, taking a year (about 365.25 days) to complete one revolution. Earth rotates around its own axis in slightly less than a day (in about 23 hours and 56 minutes).
Earth's axis of rotation is tilted with respect to the perpendicular to its orbital plane around the Sun, producing seasons. Earth is orbited by one permanent natural satellite, the Moon, which orbits Earth at 384,400 km (238,855 mi)—1.28 light seconds—and is roughly a quarter as wide as Earth. The Moon's gravity helps stabilize Earth's axis, causes tides and gradually slows Earth's rotation. Likewise, Earth's gravitational pull has already made the Moon's rotation tidally locked, keeping the same near side facing Earth.
Earth, like most other bodies in the Solar System, formed about 4.5 billion years ago from gas and dust in the early Solar System. The formation of the ocean and the subsequent development of life occurred during the first billion years of Earth's history. Life spread globally and has been altering Earth's atmosphere and surface, leading to the Great Oxidation Event two billion years ago. Humans emerged 300,000 years ago in Africa and have spread across every continent on Earth. Humans depend on Earth's biosphere and natural resources for their survival, but have increasingly impacted the planet's environment. Humanity's current impact on Earth's climate and biosphere is unsustainable, threatening the livelihood of humans and many other forms of life, and causing widespread extinctions.
The Moon is the only natural satellite of Earth. It orbits around Earth at an average distance of 384,399 kilometers (238,854 mi),[f] a distance roughly 30 times the width of Earth. It completes an orbit (lunar month) in relation to Earth and the Sun (synodically) every 29.5 days. The Moon and Earth are bound by gravitational attraction, which is stronger on the sides facing each other. The resulting tidal forces are the main driver of Earth's tides, and have pulled the Moon to always face Earth with the same near side. This tidal locking effectively synchronizes the Moon's rotation period (lunar day) to its orbital period (lunar month).
In geophysical terms, the Moon is a planetary-mass object or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is 3,474 km (2,159 mi), roughly one-quarter of Earth's (about as wide as the contiguous United States). Within the Solar System, it is larger and more massive than any known dwarf planet, and the fifth-largest and fifth–most massive moon, as well as the largest and most massive in relation to its parent planet.[18] Its surface gravity is about one-sixth of Earth's, about half that of Mars, and the second-highest among all moons in the Solar System after Jupiter's moon Io. The body of the Moon is differentiated and terrestrial, with only a minuscule hydrosphere, atmosphere, and magnetic field.
The lunar surface is covered in regolith dust, which mainly consists of the fine material ejected from the lunar crust by impact events. The lunar crust is marked by impact craters; some younger ones feature bright ray-like streaks. The Moon was volcanically active until 1.2 billion years ago, surfacing lava mostly on the thinner near side of the Moon, filling ancient craters, which through cooling formed the today prominently visible dark plains of basalt called maria ('seas'). The origin of the Moon is not clear, although it is thought to have been formed out of material from Earth that was ejected by a giant impact of a Mars-sized body 4.51 billion years ago, not long after Earth's formation.
From a distance, the day and night phases of the lunar day are visible as the lunar phases, and when the Moon passes through Earth's shadow a lunar eclipse is observable. The Moon's apparent size in Earth's sky is about the same as that of the Sun, which causes it to cover the Sun completely during a total solar eclipse. The Moon is the brightest celestial object in Earth's night sky because of its large apparent size, while the reflectance (albedo) of its surface is comparable to that of asphalt. About 59% of the surface of the Moon is visible from Earth owing to the different angles at which the Moon can appear in Earth's sky (libration), making parts of the far side of the Moon visible.
The Moon has been an important source of inspiration and knowledge in human history, having been crucial to cosmography, mythology, religion, art, time keeping, natural science and spaceflight. The first spaceflights to an extraterrestrial body were to the Moon, starting in 1959 with the flyby of Luna 1 (sent by the Soviet Union), and the intentional impact of Luna 2, followed in 1966 by the first soft landing (by Luna 9) and orbital insertion (by Luna 10). Humans first arrived in orbit with Apollo 8 (sent by the United States) on December 24, 1968, and then on the surface with Apollo 11 on July 20, 1969, making the Moon the only celestial body beyond Earth that humans have visited. By 1972, six Apollo missions had landed twelve people on the Moon and stayed up to three days. Renewed robotic exploration of the Moon, in particular to confirm the presence of water on the Moon, has fueled plans to return humans to the Moon, starting with the Artemis program scheduled for the late 2020s.
Mars is the fourth planet from the Sun. It is also known as the "Red Planet" for its orange-red appearance. Mars is a desert-like rocky planet with a tenuous atmosphere that is primarily carbon dioxide (CO2). At the average surface level the atmospheric pressure is a few thousandths of Earth's, atmospheric temperature ranges from −153 to 20 °C (−243 to 68 °F), and cosmic radiation is high. Mars retains some water, in the ground as well as thinly in the atmosphere, forming cirrus clouds, fog, frost, larger polar regions of permafrost and ice caps (with seasonal CO2 snow), but no bodies of liquid surface water. Its surface gravity is roughly a third of Earth's or double that of the Moon. Its mean diameter, 6,779 km (4,212 mi),[26] is about half the Earth's, or twice the Moon's, and its surface area is the size of all the dry land of Earth.
Fine dust is prevalent across the surface and the atmosphere, being picked up and spread at the low Martian gravity even by the weak wind of the tenuous atmosphere. The terrain of Mars roughly follows a north–south divide, the Martian dichotomy, with the northern hemisphere mainly consisting of relatively flat, low lying plains, and the southern hemisphere of cratered highlands. Geologically, the planet is fairly active with marsquakes trembling underneath the ground, but also hosts many enormous volcanoes that are extinct (the tallest is Olympus Mons, 21.9 km or 13.6 mi tall), as well as one of the largest canyons in the Solar System (Valles Marineris, 4,000 km or 2,500 mi long). Mars has two natural satellites that are small and irregular in shape: Phobos and Deimos. With a significant axial tilt of 25 degrees, Mars experiences seasons, like Earth (which has an axial tilt of 23.5 degrees). A Martian solar year is equal to 1.88 Earth years (687 Earth days), a Martian solar day (sol) is equal to 24.6 hours.
Mars formed along with the other planets approximately 4.5 billion years ago. During the martian Noachian period (4.5 to 3.5 billion years ago), its surface was marked by meteor impacts, valley formation, erosion, the possible presence of water oceans and the loss of its magnetosphere. The Hesperian period (beginning 3.5 billion years ago and ending 3.3–2.9 billion years ago) was dominated by widespread volcanic activity and flooding that carved immense outflow channels. The Amazonian period, which continues to the present, is the currently dominating and remaining influence on geological processes. Because of Mars's geological history, the possibility of past or present life on Mars remains an area of active scientific investigation, with some possible traces needing further examination.
Being visible with the naked eye in Earth's sky as a red wandering star, Mars has been observed throughout history, acquiring diverse associations in different cultures. In 1963 the first flight to Mars took place with Mars 1, but communication was lost en route. The first successful flyby exploration of Mars was conducted in 1965 with Mariner 4. In 1971 Mariner 9 entered orbit around Mars, being the first spacecraft to orbit any body other than the Moon, Sun or Earth; following in the same year were the first uncontrolled impact (Mars 2) and first successful landing (Mars 3) on Mars. Probes have been active on Mars continuously since 1997. At times, more than ten probes have simultaneously operated in orbit or on the surface, more than at any other planet beyond Earth. Mars is an often proposed target for future crewed exploration missions, though no such mission is currently planned.
Phobos (/ˈfoʊboʊs/) is the innermost and larger of the two natural satellites of Mars, the other being Deimos. The two moons were discovered in 1877 by American astronomer Asaph Hall. Phobos is named after the Greek god of fear and panic, who is the twin brother of Deimos and son of Ares (Mars).
Phobos is a small, irregularly shaped object with a mean radius of 11 km (7 mi). It orbits 6,000 km (3,700 mi) from the Martian surface, closer to its primary body than any other known natural satellite to a planet. It orbits Mars much faster than Mars rotates and completes an orbit in just 7 hours and 39 minutes. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 hours and 15 minutes or less, and set in the east, twice each Martian day.
Phobos is one of the least reflective bodies in the Solar System, with an albedo of 0.071. Surface temperatures range from about −4 °C (25 °F) on the sunlit side to −112 °C (−170 °F) on the shadowed side. The notable surface feature is the large impact crater Stickney, which takes up a substantial proportion of the moon's surface. The surface is also marked by many grooves, and there are numerous theories as to how these grooves were formed.
Images and models indicate that Phobos may be a rubble pile held together by a thin crust that is being torn apart by tidal interactions. Phobos gets closer to Mars by about 2 centimetres (0.79 in) per year.
Deimos is the smaller, outer moon of Mars, discovered in 1877 by Asaph Hall, and named after the Greek personification of fear. It's an irregularly shaped, cratered body, about 12 km across, that orbits Mars every 30.3 hours, appearing as a slow-moving star from the planet's surface. Like its sibling Phobos, it's thought to be a captured asteroid, with a surface covered in fine dust that makes it appear smoother than Phobos. Size: Approximately 12 x 15 x 11 km (7.45 x 9.3 x 6.8 miles). Orbit: Orbits Mars every 30.3 hours at a distance of about 23,460 km.
Appearance: A small, irregular, rocky body with a heavily cratered surface that looks smoother than Phobos due to a thick layer of dust (regolith). Discovery: Discovered by Asaph Hall on August 12, 1877, just days before Phobos. Origin: Believed to be a captured asteroid, similar to Phobos. Name: Named after the Greek god of fear, Deimos, who was the twin brother of Phobos (terror).
Jupiter is the fifth planet from the Sun, and the largest in the Solar System. It is a gas giant with a mass nearly 2.5 times that of all the other planets in the Solar System combined and slightly less than one-thousandth the mass of the Sun. The diameter of Jupiter is 11 times that of Earth and a tenth that of the Sun. It orbits the Sun at a distance of 5.20 AU (778.5 Gm), with an orbital period of 11.86 years. Jupiter is the third-brightest natural object in the Earth's night sky, after the Moon and Venus, and has been observed since prehistoric times. Its name derives from that of Jupiter, the chief deity of ancient Roman religion.
Jupiter was the first of the Sun's planets to form, and its inward migration during the primordial phase of the Solar System affected much of the formation history of the other planets. Jupiter's atmosphere consists of 76% hydrogen and 24% helium by mass, with a denser interior. It contains traces of the elements carbon, oxygen, sulfur, neon, and compounds such as ammonia, water vapour, phosphine, hydrogen sulfide, and hydrocarbons. Jupiter's helium abundance is 80% of the Sun's, similar to Saturn's composition.
The outer atmosphere is divided into a series of latitudinal bands, with turbulence and storms along their interacting boundaries; the most obvious result of this is the Great Red Spot, a giant storm that has been recorded since 1831.
Because of Jupiter's rapid rotation rate, one turn in ten hours, the shape of the planet is an oblate spheroid; the radius to the equator is about 7% larger than the radius to its poles. Its internal structure is believed to consist of an outer mantle of fluid metallic hydrogen and a diffuse inner core of denser material. The ongoing contraction of Jupiter's interior generates more heat than the planet receives from the Sun. Jupiter's magnetic field is the strongest and second-largest contiguous structure in the Solar System, generated by eddy currents within the fluid, metallic hydrogen core. The solar wind interacts with the magnetosphere, extending it outward and affecting Jupiter's orbit.
At least 115 moons orbit the planet; the four largest moons—Io, Europa, Ganymede, and Callisto—orbit within the magnetosphere and are visible with common binoculars. Ganymede, the largest of the four, is larger than the planet Mercury. Jupiter is surrounded by a faint system of planetary rings. The rings of Jupiter consist mainly of dust and have three main segments: an inner torus of particles known as the halo, a relatively bright main ring, and an outer gossamer ring. The rings have a reddish colour in visible and near-infrared light. The age of the ring system is unknown, possibly dating back to Jupiter's formation. Since 1973, Jupiter has been visited by nine robotic probes: seven flybys and two dedicated orbiters (with two more en route). Jupiter-like exoplanets have also been found in other planetary systems.
Ganymede is a natural satellite of Jupiter and is the largest and most massive moon in the Solar System. Like Saturn's largest moon, Titan, it is larger than the planet Mercury, but has somewhat less surface gravity than Mercury, Io, or Earth's Moon due to its lower density compared to the three. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.
Ganymede is composed of silicate rock and water in approximately equal proportions. It is a fully differentiated body with an iron-rich, liquid metallic core, giving it the lowest moment of inertia factor of any solid body in the Solar System. Its internal ocean potentially contains more water than all of Earth's oceans combined.
Ganymede is the only natural satellite in the solar system to possess an internally generated magnetic field. It is probably created by convection within its core, and influenced by tidal forces from Jupiter's far greater magnetic field. Ganymede has a thin oxygen atmosphere that includes O, O 2, and possibly O3. Atomic hydrogen is a minor atmospheric constituent. Whether Ganymede has an ionosphere associated with its atmosphere is unresolved.
Ganymede's surface is composed of two main types of terrain, the first of which are lighter regions, generally crosscut by extensive grooves and ridges, dating from slightly less than 4 billion years ago, covering two-thirds of Ganymede. The cause of the light terrain's disrupted geology is not fully known, but may be the result of tectonic activity due to tidal heating. The second terrain type are darker regions saturated with impact craters, which are dated to four billion years ago.
Ganymede's discovery is credited to Simon Marius and Galileo Galilei, who both observed it in 1610, as the third of the Galilean moons, the first group of objects discovered orbiting another planet.[26] Marius soon named it after Ganymede, a Trojan prince desired by Zeus, who carried him off to serve as cupbearer to the gods.
Beginning with Pioneer 10, several spacecraft have explored Ganymede. The Voyager probes, Voyager 1 and Voyager 2, refined measurements of its size, while Galileo discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moons Explorer (Juice), which was launched in 2023. After flybys of all three icy Galilean moons, it is planned to enter orbit around Ganymede.
Callisto (/kəˈlɪstoʊ/ kə-LIST-oh) is the second-largest moon of Jupiter, after Ganymede. It is also the third-largest moon in the Solar System, following Ganymede and Saturn's moon Titan, and nearly as large as the planet Mercury. With a diameter of 4,821 km, Callisto is roughly a third larger than Earth's Moon and orbits Jupiter on average at a distance of 1.883 million km, which is about five times further out than the Moon orbiting Earth. It is the outermost of the four large Galilean moons of Jupiter, which were discovered in 1610 with one of the first telescopes, and is today visible from Earth with common binoculars.
The surface of Callisto is the oldest and most heavily cratered in the Solar System, with the surface almost completely covered with impact craters. It does not show any signatures of subsurface processes such as plate tectonics or volcanism, and is thought to have evolved predominantly under the influence of impacts. Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters called catenae and associated scarps, ridges and deposits.
At a small scale, the surface is varied and made up of small, sparkly frost deposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. This is thought to result from the sublimation-driven degradation of small landforms, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants. The absolute ages of the landforms are not known.
Callisto is composed of approximately equal amounts of rock and ice, with a density of about 1.83 g/cm3, the lowest density and surface gravity of Jupiter's major moons. Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates and organic compounds. Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than 100 km.
It is not in an orbital resonance like the three other Galilean satellites—Io, Europa and Ganymede—and is thus not appreciably tidally heated. Callisto's rotation is tidally locked to its orbit around Jupiter, so that it always faces the same direction, making Jupiter appear to hang directly overhead over its near side. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit located just outside Jupiter's main radiation belt Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere.
Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation] Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation. The slow convection in the interior of Callisto, which commenced soon after formation, led to partial differentiation and possibly to the formation of a subsurface ocean at a depth of 100–150 km and a small, rocky core.
The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than those on nearby Europa. Various space probes, including Pioneers 10 and 11, Galileo, and Cassini, have studied Callisto. Because of its low radiation levels, Callisto has long been considered the most suitable to base possible future crewed missions on to study the Jovian system.
Io (/ˈaɪ.oʊ/) is the innermost and second smallest of the four Galilean moons of Jupiter. Slightly larger than Earth's Moon, Io is the fourth-largest natural satellite in the Solar System, has the highest density and strongest surface gravity of any natural satellite, and the lowest amount of water by atomic ratio of any known astronomical object in the Solar System.
With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity results from tidal heating from friction generated within Io's interior as it is pulled between Jupiter and two other Galilean moons—Europa and Ganymede. Several volcanoes produce plumes of sulfur and sulfur dioxide as high as 500 km (300 mi) above the surface. Io's surface is also dotted with more than 100 mountains uplifted by extensive compression at the base of Io's silicate crust. Some of these peaks are taller than Mount Everest, the highest point on Earth's surface. Unlike most moons in the outer Solar System, which are mostly composed of water ice, Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. Most of Io's surface is composed of extensive plains with a frosty coating of sulfur and sulfur dioxide.
Io's volcanism is responsible for many of its unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various subtle shades of yellow, red, white, black, and green, largely due to allotropes and compounds of sulfur. Numerous extensive lava flows, several more than 500 km (300 mi) in length, also mark the surface. The materials produced by this volcanism make up Io's thin, patchy atmosphere, and they also greatly affect the nature and radiation levels of Jupiter's extensive magnetosphere. Io's volcanic ejecta also produces a large, intense plasma torus around Jupiter, creating a hostile radiation environment on and around the moon
It was discovered along with the other Galilean moons in 1610 by Galileo Galilei and named after the mythological character Io, a priestess of Hera who became one of Zeus's lovers. The discovery of the Galilean moons played a significant role in the development of astronomy, furthering the adoption of the Copernican model of the Solar System and the development of Kepler's laws of planetary motion. Io in particular was used for the first measurement of the speed of light. In 1979, the two Voyager spacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, and a young surface with no obvious impact craters.
The Galileo spacecraft performed several close flybys in the 1990s and early 2000s, obtaining data about Io's interior structure and surface composition. These spacecraft also revealed the relationship between Io and Jupiter's magnetosphere and the existence of a belt of high-energy radiation centered on Io's orbit. Further observations have been made by Cassini–Huygens in 2000, New Horizons in 2007, and Juno since 2017, as well as from Earth-based telescopes and the Hubble Space Telescope.
Europa (/jʊˈroʊpə/ ⓘ) is the smallest and least massive of Jupiter's four Galilean moons. It is observable from Earth with common binoculars and is a planetary-mass moon, slightly smaller and less massive than Earth's Moon. Europa is an icy moon, and, of the three icy Galilean moons, the closest orbiting Jupiter. As a result, it exhibits a relatively young surface shaped by tidal heating.
Europa consists mainly of silicate rock, and potentially has an iron-nickel core It has a very thin atmosphere composed primarily of oxygen. Europa has a pale, geologically young surface striated by light tan cracks and streaks; the surface lacks large-scale features such as mountains or craters, making the moon the smoothest known solid object in the Solar System. The apparent youth and smoothness of the surface is due to a water ocean beneath the surface, which could conceivably harbor extraterrestrial life. The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to plate tectonics, absorbing chemicals from the surface into the ocean below.
Sea salt from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the sea floor. This may be important in determining whether Europa could be habitable. In addition, the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturn's moon Enceladus, which are thought to be caused by erupting cryogeysers.
In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated analysis of data obtained from the Galileo space probe, which orbited Jupiter from 1995 to 2003. Such plume activity could help researchers in a search for life from the subsurface Europan ocean without having to land on the moon. In March 2024, astronomers reported that the surface of Europa may have much less oxygen than previously inferred.
Europa was discovered independently by Simon Marius and Galileo Galilei. Marius later named it after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus (the Greek equivalent of the Roman Jupiter). In addition to Earth-bound telescope observations, Europa has been examined by a succession of space-probe flybys, the first occurring in the early 1970s. The Galileo mission, launched in 1989, provides the bulk of current data on Europa. No spacecraft has yet landed on Europa, although there have been several proposed exploration missions. In September 2022, the Juno spacecraft flew within around 320km (200 miles) of Europa for a more recent close-up view. The European Space Agency's Jupiter Icy Moons Explorer (Juice) is a mission to Ganymede launched on 14 April 2023, that will include two flybys of Europa. NASA's Europa Clipper was launched on 14 October 2024.
Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth of the average density of Earth, but is over 95 times more massive. Even though Saturn is almost as big as Jupiter, Saturn has less than a third of its mass. Saturn orbits the Sun at a distance of 9.59 AU (1,434 million km), with an orbital period of 29.45 years.
Saturn's interior is thought to be composed of a rocky core, surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium, and an outer layer of gas. Saturn has a pale yellow hue, due to ammonia crystals in its upper atmosphere. An electrical current in the metallic hydrogen layer is thought to give rise to Saturn's planetary magnetic field, which is weaker than Earth's, but has a magnetic moment 580 times that of Earth because of Saturn's greater size. Saturn's magnetic field strength is about a twentieth that of Jupiter.[27] The outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 kilometres per hour (1,100 miles per hour).
The planet has a bright and extensive system of rings, composed mainly of ice particles, with a smaller amount of rocky debris and dust. At least 293 moons orbit the planet, of which 63 are officially named; these do not include the hundreds of moonlets in the rings. Titan, Saturn's largest moon and the second largest in the Solar System, is larger (but less massive) than the planet Mercury and is the only moon in the Solar System that has a substantial atmosphere.
Titan is the largest moon of Saturn and the second largest in the Solar System. It is the only moon known to have a dense atmosphere (denser than Earth's), and is the only known object in the Solar System besides Earth with clear evidence of stable bodies of surface liquid. Titan is one of seven gravitationally rounded moons of Saturn and the second-most distant among them. Frequently described as a planet-like moon, Titan is 48.16% larger in diameter than Earth's Moon and 80% more massive. It is the second-largest moon in the Solar System after Jupiter's Ganymede and is larger than Mercury; yet Titan is only 40% as massive as Mercury, because Mercury is mainly iron and rock while Titan is mostly ice, which is less dense.
Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn and the sixth known planetary satellite (after Earth's moon and the four Galilean moons of Jupiter). Titan orbits Saturn at 20 Saturn radii or 1.2 million km above Saturn's apparent surface. From Titan's surface, Saturn, disregarding its rings, subtends an arc of 5.09 degrees, rendering it more than ten times as large in Titan's sky as the Moon is in Earth's sky.
Titan is primarily composed of ice and rocky material, with a rocky core surrounded by various layers of ice, including a crust of ice Ih and a subsurface layer of ammonia-rich liquid water. Much as with Venus before the Space Age, the dense opaque atmosphere prevented understanding of Titan's surface until the Cassini–Huygens mission in 2004 provided new information, including the discovery of liquid hydrocarbon lakes in Titan's polar regions and the discovery of its atmospheric super-rotation. The geologically young surface is generally smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found.
The atmosphere of Titan is mainly nitrogen and methane; minor components lead to the formation of hydrocarbon clouds and heavy organonitrogen haze. Its climate, including wind and rain, creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas (probably of liquid methane and ethane), and deltas, and is dominated by seasonal weather patterns as on Earth. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan's methane cycle nearly resembles Earth's water cycle, albeit at a much lower temperature of about 94 K (−179 °C; −290 °F). Due to these factors, Titan is sometimes called the most Earth-like celestial object in the Solar System.
Rhea (/ˈreɪ.ə/) is the second-largest natural satellite of Saturn and the ninth-largest moon in the Solar System, with a diameter of 1,528 kilometres (949 mi). Rhea is the smallest body in the Solar System that is confirmed to be in hydrostatic equilibrium. It has a nearly circular orbit around Saturn, but it is also tidally locked, like Saturn's other major moons. It rotates with the same period it revolves or orbits. Thus, one hemisphere always faces towards the planet.
The moon has a fairly low density, composed of roughly three-quarters ice and only one-quarter rock. The surface of Rhea is heavily cratered, with distinct leading and trailing hemispheres. Like the moon Dione, it has high-albedo ice cliffs that appear as bright wispy streaks visible from space. The surface temperature varies between −174 °C (−281.2 °F) and −220 °C (−364.0 °F).
Rhea was discovered in 1672 by Giovanni Domenico Cassini. Since then, it has been visited by both Voyager probes and was the subject of close targeted flybys by the Cassini orbiter in 2005, 2007, 2010, 2011, and once more in 2013.
Iapetus (/aɪˈæpətəs/) is the outermost of Saturn's large moons. With an estimated diameter of 1,469 km (913 mi), it is the third-largest moon of Saturn and the eleventh-largest in the Solar System. Named after the Titan Iapetus from Greek mythology, the moon was discovered in 1671 by Giovanni Domenico Cassini.
A relatively low-density body composed mostly of ice, Iapetus is home to several distinctive and unusual features, such as a striking difference in coloration between its dark leading hemisphere and its bright trailing hemisphere, as well as a massive equatorial ridge that runs three-quarters of the way around the moon.
Dione (/daɪˈoʊni/) is the fourth-largest moon of Saturn. With a mean diameter of 1,123 km and a density of about 1.48 g/cm3, Dione is composed of an icy mantle and crust overlying a silicate rocky core, with rock and water ice roughly equal in mass. Its trailing hemisphere is marked by large cliffs and scarps called chasmata; the trailing hemisphere is also significantly darker compared to the leading hemisphere.
The moon was discovered by Italian astronomer Giovanni Domenico Cassini in 1684 and is named after the Titaness Dione in Greek mythology. Dione was first imaged up-close by the Voyager 1 space probe in 1980. Later, the Cassini spacecraft made multiple flybys of Dione throughout the 2000s and 2010s as part of its campaign to explore the Saturn system.
Tethys (/ˈtiːθɪs, ˈtɛθɪs/) is the fifth-largest moon of Saturn, measuring about 1,060 km (660 mi) across. It was discovered by Giovanni Domenico Cassini in 1684, and is named after the titan Tethys of Greek mythology.
Tethys has a low density of 0.98 g/cm3, the lowest of all the major moons in the Solar System, indicating that it is made of water ice with just a small fraction of rock. This was confirmed by the spectroscopy of its surface, which identified water ice as the dominant surface material. A further, smaller amount of an unidentified dark material is present as well. The surface of Tethys is very bright, the second-brightest of the moons of Saturn after Enceladus, and neutral in color.
Tethys is heavily cratered and cut by a number of large faults and trench-like graben. The largest impact crater, Odysseus, is about 400 km in diameter, whereas the largest graben, Ithaca Chasma, is about 100 km wide and more than 2,000 km long; the two surface features may be related. A small part of the surface is covered by smooth plains that may be cryovolcanic in origin. Like the other regular moons of Saturn, Tethys formed from the Saturnian sub-nebula—a disk of gas and dust that surrounded Saturn soon after its formation.
Tethys has been approached and observed by several space probes, including Pioneer 11 (1979), Voyager 1 (1980) and Voyager 2 (1981), with Cassini–Huygens observing the moon the most, and in greatest detail, during its extensive mission to the Saturnian system (2004–2017)
Enceladus is the sixth-largest moon of Saturn and the 18th largest in the Solar System. It is about 500 kilometres (310 miles) in diameter, about a tenth of that of Saturn's largest moon, Titan. It is covered by clean, freshly deposited snow hundreds of meters thick, making it one of the most reflective bodies of the Solar System. Consequently, its surface temperature at noon reaches only −198 °C (75.1 K; −324.4 °F), far colder than a light-absorbing body would be. Despite its small size, Enceladus has a wide variety of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrain.
Enceladus was discovered on August 28, 1789, by William Herschel, but little was known about it until the two Voyager spacecraft, Voyager 1 and Voyager 2, flew by Saturn in 1980 and 1981. In 2005, the spacecraft Cassini started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered water-rich plumes venting from the south polar region. Cryovolcanoes near the south pole shoot geyser-like jets of water vapour, molecular hydrogen, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, totalling about 200 kilograms (440 pounds) per second.
More than 100 geysers have been identified.[ Some of the water vapour falls back as snow, now several hundred metres thick; the rest escapes and supplies most of the material making up Saturn's E ring. According to NASA scientists, the plumes are similar in composition to comets. In 2014, NASA reported that Cassini had found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km (6 mi). The existence of Enceladus's subsurface ocean has since been mathematically modelled and replicated.
These observations of active cryoeruptions, along with the finding of escaping internal heat and very few (if any) impact craters in the south polar region, show that Enceladus is currently geologically active. Like many other satellites in the extensive systems of the giant planets, Enceladus participates in an orbital resonance. Its resonance with Dione excites its orbital eccentricity, which is damped by tidal forces, tidally heating its interior and driving the geological activity.
Cassini performed chemical analysis of Enceladus's plumes, finding evidence for hydrothermal activity, possibly driving complex chemistry. Ongoing research on Cassini data suggests that Enceladus's hydrothermal environment could be habitable to some of Earth's hydrothermal vent's microorganisms, and that plume-found methane could be produced by such organisms.
Mimas is the seventh-largest natural satellite of Saturn. With a mean diameter of 396.4 kilometres or 246.3 miles, Mimas is the smallest astronomical body known to be roughly rounded in shape due to its own gravity. Mimas's low density, 1.15 g/cm3, indicates that it is composed mostly of water ice with only a small amount of rock, and study of Mimas's motion suggests that it may have a liquid ocean beneath its surface ice. The surface of Mimas is heavily cratered and shows little sign of recent geological activity.
A notable feature of Mimas's surface is Herschel, one of the largest craters relative to the size of the parent body in the Solar System. Herschel measures 139 kilometres (86 miles) across, about one-third of Mimas's mean diameter,[10] and formed from an extremely energetic impact event. The crater is named after William Herschel, who discovered Mimas in 1789. The moon's presence has created one of the largest 'gaps' in Saturn's ring, named the Cassini Division, due to orbital resonance destabilising the particles' orbit there.
Uranus has the third-largest diameter and fourth-largest mass among the Solar System's planets. Based on current models, inside its volatile mantle layer is a rocky core, and a thick hydrogen and helium atmosphere surrounds it. Trace amounts of hydrocarbons (thought to be produced via hydrolysis) and carbon monoxide along with carbon dioxide (thought to have originated from comets) have been detected in the upper atmosphere. There are many unexplained climate phenomena in Uranus's atmosphere, such as its peak wind speed of 900 km/h (560 mph), variations in its polar cap, and its erratic cloud formation. The planet also has very low internal heat compared to other giant planets, the cause of which remains unclear.
Like the other giant planets, Uranus has a ring system, a magnetosphere, and many natural satellites. The extremely dark ring system reflects only about 2% of the incoming light. Uranus's 29 natural satellites include 19 known regular moons, of which 14 are small inner moons. Further out are the larger five major moons of the planet: Miranda, Ariel, Umbriel, Titania, and Oberon. Orbiting at a much greater distance from Uranus are the ten known irregular moons. The planet's magnetosphere is highly asymmetric and has many charged particles, which may be the cause of the darkening of its rings and moons.
Uranus is visible to the naked eye, but it is very dim and moves very slowly relative to the background stars and was not classified as a planet until 1781, when it was first observed by William Herschel. About seven decades after its discovery, consensus was reached that the planet be named after the Greek god Uranus (Ouranos), one of the Greek primordial deities. As of 2026, it has been visited only once when in 1986 the Voyager 2 probe flew by the planet. Though nowadays it can be resolved and observed by telescopes, there is much desire to revisit the planet, as shown by Planetary Science Decadal Survey's decision to make the proposed Uranus Orbiter and Probe mission a top priority in the 2023–2032 survey, and the CNSA's proposal to fly by the planet with a subprobe of Tianwen-4.
Titania (/tɪˈtɑːniə, -ˈteɪ-/) is the largest moon of Uranus and the eighth-largest moon in the Solar System, with a diameter of 1,578 km (981 mi). Discovered by William Herschel in 1787, it is named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream. Its orbit lies inside Uranus's magnetosphere.
Titania consists of approximately equal amounts of ice and rock, and is probably differentiated into a rocky core and an icy mantle. A layer of liquid water may be present at the core–mantle boundary. Its surface, which is relatively dark and slightly red in color, appears to have been shaped by both impacts and endogenic processes. Although Titania is covered with numerous impact craters reaching up to 326 kilometres (203 mi) in diameter, it is less heavily cratered than Oberon, the outermost of Uranus's five large moons.
It may have undergone an early endogenic resurfacing event which obliterated its older, heavily cratered surface. The surface is cut by a system of enormous canyons and scarps, the result of the expansion of its interior during the later stages of its evolution. Like all major moons of Uranus, Titania probably formed from an accretion disk which surrounded the planet just after its formation. Uranus and Neptune (and thus their moons) may both appear apparent retrograde orbits when viewed from Earth.
Infrared spectroscopy conducted from 2001 to 2005 revealed the presence of water ice as well as frozen carbon dioxide on Titania's surface, suggesting it may have a tenuous carbon dioxide atmosphere with a surface pressure of about 10 nanopascals (10−13 bar). Measurements during Titania's occultation of a star put an upper limit on the surface pressure of any possible atmosphere at 1–2 mPa (10–20 nbar). The Uranian system has been studied up close only once, by the spacecraft Voyager 2 in January 1986. It took several images of Titania, which allowed mapping of about 40% of its surface.
Oberon likely formed from the accretion disk that surrounded Uranus just after the planet's formation. The moon consists of approximately equal amounts of ice and rock, and is probably differentiated into a rocky core and an icy mantle. A layer of liquid water may be present at the boundary between the mantle and the core. The surface of Oberon, which is dark and slightly red in color, appears to have been primarily shaped by asteroid and comet impacts. It is covered by numerous impact craters reaching 210 km in diameter. Oberon possesses a system of chasmata (graben or scarps) formed during crustal extension as a result of the expansion of its interior during its early evolution.
The Uranian system has been studied up close only once; the spacecraft Voyager 2 took several images of Oberon in January 1986, which allowed 40% of the moon's surface to be mapped.
Umbriel (/ˈʌmbriəl/) is the third-largest moon of Uranus. It was discovered on October 24, 1851, by William Lassell at the same time as neighboring moon Ariel. It was named after a character in Alexander Pope's 1712 poem The Rape of the Lock. Umbriel consists mainly of ice with a substantial fraction of rock, and may be differentiated into a rocky core and an icy mantle. The surface is the darkest among Uranian moons and appears to have been shaped primarily by impacts, however, the presence of canyons suggests early internal processes. The moon may have undergone an early endogenically driven resurfacing event that obliterated its older surface.
Covered by numerous impact craters reaching 210 km (130 mi) in diameter, Umbriel is the second-most heavily cratered satellite of Uranus after Oberon. The most prominent surface feature is a ring of bright material on the floor of Wunda crater. This moon, like all regular moons of Uranus, probably formed from an accretion disk that surrounded the planet just after its formation. Umbriel has been studied up close only once, by the spacecraft Voyager 2 in January 1986. It took several images of Umbriel, which allowed mapping of about 40% of the moon's surface.
Ariel is the fourth-largest moon of Uranus. Ariel orbits and rotates in Uranus's equatorial plane, which is almost perpendicular to the planet's orbit, giving the moon an extreme seasonal cycle.
It was discovered on 24 October 1851 by William Lassell and named for a character in two different pieces of literature. As of 2019, much of the detailed knowledge of Ariel derives from a single flyby of Uranus performed by the space probe Voyager 2 in 1986, which managed to image around 35% of the moon's surface. There are no active plans at present to return to study the moon in more detail, although various concepts such as a Uranus Orbiter and Probe have been proposed.
After Miranda, Ariel is the second-closest of Uranus's five major rounded satellites. Among the smallest of the Solar System's 19 known spherical moons (it ranks 14th among them in diameter), it is believed to be composed of roughly equal parts ice and rocky material. Its mass is approximately equal in magnitude to Earth's hydrosphere.
Like all of Uranus's moons, Ariel probably formed from an accretion disc that surrounded the planet shortly after its formation, and, like other large moons, it is likely differentiated, with an inner core of rock surrounded by a mantle of ice. Ariel has a complex surface consisting of extensive cratered terrain cross-cut by a system of scarps, canyons, grabens and ridges. The surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating.
Miranda is the smallest and innermost of Uranus's five round satellites. It was discovered by Gerard Kuiper on 16 February 1948 at McDonald Observatory in Texas, and named after Miranda from William Shakespeare's play The Tempest. Like the other large moons of Uranus, Miranda orbits close to its planet's equatorial plane. Because Uranus orbits the Sun on its side, Miranda's orbit is nearly perpendicular to the ecliptic and shares Uranus's extreme seasonal cycle.
At just 470 km (290 mi) in diameter, Miranda is one of the smallest closely observed objects in the Solar System that might be in hydrostatic equilibrium (spherical under its own gravity), and its total surface area is roughly equal to that of the U.S. state of Texas. The only close-up images of Miranda are from the Voyager 2 probe, which made observations of Miranda during its Uranus flyby in January 1986. During the flyby, Miranda's southern hemisphere pointed towards the Sun, so only that part was studied.
Miranda probably formed from an accretion disc that surrounded the planet shortly after its formation and, like other large moons, it is likely differentiated, with an inner core of rock surrounded by a mantle of ice. Miranda has one of the most extreme and varied topographies of any object in the Solar System, including Verona Rupes, a roughly 20-kilometre-high (12 mi) scarp that may be the highest cliff in the Solar System] and chevron-shaped tectonic features called coronae. The origin and evolution of this varied geology, the most of any Uranian satellite, are still not fully understood, and multiple hypotheses exist regarding Miranda's evolution.
Neptune is the eighth and farthest known planet orbiting the Sun. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth. Compared to Uranus, its neighbouring ice giant, Neptune is slightly smaller, but more massive and dense. Being composed primarily of gases and liquids,[23] it has no well-defined solid surface. Neptune orbits the Sun once every 164.8 years at an orbital distance of 30.1 astronomical units (4.5 billion kilometres; 2.8 billion miles). It is named after the Roman god of the sea and has the astronomical symbol ♆, representing Neptune's trident.
Neptune is not visible to the unaided eye and is the only planet in the Solar System that was not initially observed by direct empirical observation. Rather, unexpected changes in the orbit of Uranus led Alexis Bouvard to hypothesise that its orbit was subject to gravitational perturbation by an unknown planet. After Bouvard's death, the position of Neptune was mathematically predicted from his observations, independently, by John Couch Adams and Urbain Le Verrier. Neptune was subsequently directly observed with a telescope on 23 September 1846 by Johann Gottfried Galle within a degree of the position predicted by Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining moons were located telescopically until the 20th century.
The planet's distance from Earth gives it a small apparent size, and its distance from the Sun renders it very dim, making it challenging to study with Earth-based telescopes. Only the advent of the Hubble Space Telescope and of large ground-based telescopes with adaptive optics allowed for detailed observations. Voyager 2, which flew by Neptune on 25 August 1989, remains the only spacecraft to visit the planet. Like the gas giants (Jupiter and Saturn), Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, but contains a higher proportion of ices such as water, ammonia and methane. Similar to Uranus, its interior is primarily composed of ices and rock;[26] both planets are normally considered "ice giants" to distinguish them. Along with Rayleigh scattering, traces of methane in the outermost regions make Neptune appear faintly blue.
In contrast to the strongly seasonal atmosphere of Uranus, which can be featureless for long periods of time, Neptune's atmosphere has active and consistently visible weather patterns. At the time of the Voyager 2 flyby in 1989, the planet's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. In 2018, a newer main dark spot and smaller dark spot were identified and studied. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, as high as 2,100 km/h (580 m/s; 1,300 mph). Because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K (−218 °C; −361 °F). Temperatures at the planet's centre are approximately 5,400 K (5,100 °C; 9,300 °F). Neptune has a faint and fragmented ring system (labelled "arcs"), discovered in 1984 and confirmed by Voyager 2.
Triton is the largest natural satellite of the planet Neptune. It is the only moon of Neptune massive enough to be rounded under its own gravity and hosts a thin, hazy atmosphere. Triton orbits Neptune in a retrograde orbit—revolving in the opposite direction to the parent planet's rotation—the only large moon in the Solar System to do so. Triton is thought to have once been a dwarf planet from the Kuiper belt, captured into Neptune's orbit by the latter's gravity.
At 2,710 kilometers (1,680 mi) in diameter, Triton is the seventh-largest moon in the Solar System, the second-largest planetary moon in relation to its primary (after Earth's Moon), and larger than all of the known dwarf planets. The mean density is 2.061 g/cm3,[6] reflecting a composition of approximately 30–45% water ice by mass,: 866 with the rest being mostly rock and metal. Triton is differentiated, with a crust of primarily ice atop a probable subsurface ocean of liquid water and a solid rocky-metallic core at its center. Although Triton's orbit is nearly circular with a very low orbital eccentricity of 0.000016,[2] its interior may still experience tidal heating through obliquity tides.
Triton is one of the most geologically active worlds in the Solar System, with an estimated average surface age of less than 100 million years old. Its surface is covered by frozen nitrogen and is geologically young, with very few impact craters. Young, intricate cryovolcanic and tectonic terrains suggest a complex geological history. The atmosphere of Triton is composed primarily of nitrogen, with minor components of methane and carbon monoxide. Triton's atmosphere is relatively thin and strongly variable, with its atmospheric surface pressure varying by up to a factor of three within the past 30 years. Triton's atmosphere supports clouds of nitrogen ice crystals and a layer of organic atmospheric haze.
Triton was the first Neptunian moon to be discovered, on October 10, 1846, by English astronomer William Lassell. The 1989 flyby of Triton by the Voyager 2 spacecraft remains the only up-close visit to the moon as of 2026. As the probe was able to study only about 40% of the moon's surface, multiple concept missions have been developed to revisit Triton. These include a Discovery-class Trident and New Frontiers-class Triton Ocean Worlds Surveyor and Nautilus.
Proteus (/ˈproʊtiəs/ PROH-tee-əs), also known as Neptune VIII, is the second-largest moon of Neptune and the planet's largest inner satellite. Discovered by Voyager 2 in 1989, it is named after Proteus, the shape-shifting sea god in Greek mythology. Proteus is about 400 km (250 mi) in diameter and orbits Neptune in a nearly equatorial orbit at a distance of about 4.75 Neptune radii from the planet's center.
Proteus has a highly irregular shape that differs significantly from an ellipsoid. It resembles an irregular polyhedron with several slightly concave facets and surface relief of up to 20 km (12 mi). Its surface is dark, nearly neutral in color, and heavily cratered. The largest crater, Pharos, is more than 230 km (140 mi) across. The surface also contains numerous cliffs, grooves, and valleys associated with large impact craters.
Proteus is unlikely to have formed alongside Neptune. Instead, it probably accreted from debris produced after the capture of Neptune's largest moon, Triton.
Nereid, or Neptune II, is the third-largest moon of Neptune. It was the second moon of Neptune to be discovered, by Gerard Kuiper in 1949. It is notable for its unusually eccentric orbit and relatively large size for an irregular satellite. Nereid also exhibits large brightness variations over long periods of time, the origin of which has yet to be conclusively explained.
Size: Measures roughly 340 kilometers (210 miles) in diameter.
Orbit: Swings wildly from about 1.4 million kilometers (less than 1 million miles) to over 9.6 million kilometers (6 million miles) away from Neptune in an egg-shaped path that takes about 360 Earth days to complete.
Origin: Recent data from the James Webb Space Telescope suggests Nereid is not a captured Kuiper Belt object like many outer moons, but rather an original moon of Neptune that survived and was pushed outward when Neptune captured Triton.
Larissa (Neptune VII) is an inner natural satellite of Neptune located about 48,800 kilometers (30,320 miles) above the planet's clouds.
Shape: Irregular and heavily cratered with a rigid, dark surface. Size: Measures roughly 194 to 216 kilometers (120 to 134 miles) across.
Orbit: Circles Neptune in the same direction as the planet spins, taking about 13 hours and 18 minutes to complete one revolution. It is slowly spiraling inward and may eventually break apart or impact Neptune.
Initial Detection: Found on May 24, 1981, by astronomers Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky, and David J. Tholen using ground-based stellar occultation (blocking a distant star's light).
Confirmation: Officially recovered and photographed by the Voyager Mission flyby of Neptune in 1989.
Pluto (minor-planet number 134340) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest known trans-Neptunian object by volume by a small margin, but is less massive than Eris. Like other Kuiper belt objects, Pluto is made primarily of ice and rock and is much smaller than the inner planets. Pluto has roughly one-sixth the mass of the Moon and one-third of its volume. Originally considered a planet, its status was changed when astronomers adopted a new definition of the word with new criteria.
Pluto has a moderately eccentric and inclined orbit, ranging from 30 to 49 astronomical units (4.5 to 7.3 billion kilometres; 2.8 to 4.6 billion miles) from the Sun. Light from the Sun takes 5.5 hours to reach Pluto at its orbital distance of 39.5 AU (5.91 billion km; 3.67 billion mi). Pluto's eccentric orbit periodically brings it closer to the Sun than Neptune, but a stable orbital resonance prevents them from colliding.
Pluto has five known moons: Charon (the largest and the diameter of which is just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits (that is, their center of mass) does not lie within either body, and they are tidally locked. New Horizons was the first spacecraft to visit Pluto and its moons, making a flyby on July 14, 2015, and taking detailed measurements and observations.
Pluto was discovered in 1930 by Clyde W. Tombaugh, making it the first known object in the Kuiper belt. It was immediately hailed as the ninth planet, but its planetary status was questioned when it was found to be much smaller than expected. These doubts increased following the discovery of additional objects in the Kuiper belt starting in the 1990s, particularly the more massive scattered disk object Eris in 2005. In 2006, the International Astronomical Union (IAU) formally redefined the term planet to exclude dwarf planets such as Pluto. Many planetary astronomers, however, continue to consider Pluto and other dwarf planets to be planets.
Charon (/ˈkɛərɒn, -ən/ KAIR-on, -ən or /ˈʃærən/ SHARR-ən),[note 1] formal designation (134340) Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km (377 mi). Charon is a planetary-mass moon and the sixth-largest known trans-Neptunian object after Pluto, Eris, Haumea, Makemake, and Gonggong. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS).
With half the diameter and one-eighth the mass of Pluto, Charon is a very large moon in comparison to its parent body. Its gravitational influence is such that the barycenter of the Plutonian system lies outside Pluto, and the two bodies are tidally locked to each other. The dwarf planet systems Pluto–Charon and Eris–Dysnomia and the dwarf planet candidate system Salacia–Actaea are the only known examples of mutual tidal locking in the Solar System, though it is likely that Orcus–Vanth is another.
The reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be essential ingredients of life. These tholins were produced from methane, nitrogen, and related gases which may have been released by cryovolcanic eruptions on the moon, or may have been transferred over 19,000 km (12,000 mi) from the atmosphere of Pluto to the orbiting moon.
The New Horizons spacecraft, the only probe to visit the Pluto system, approached Charon to within 27,000 km (17,000 mi) in 2015.
Eris (minor-planet number 136199) is the most massive and second-largest-known dwarf planet in the Solar System. It is a trans-Neptunian object (TNO) in the scattered disk and has a high-eccentricity orbit. Eris was discovered in January 2005 by a Palomar Observatory–based team led by Mike Brown and verified later that year. It was named in September 2006 after the Greco-Roman goddess of strife and discord. Eris is the ninth-most massive known object orbiting the Sun and the sixteenth-most massive in the Solar System (counting moons). It is also the largest-known object in the Solar System that has not been visited by a spacecraft. Eris has been measured at 2,326 ± 12 kilometres (1,445 ± 7 mi) in diameter; its mass is 0.28% that of the Earth and 27% greater than that of Pluto, although Pluto is slightly larger by volume. Both Eris and Pluto have a surface area that is comparable to that of Russia or South America.
Eris has one large confirmed moon, Dysnomia, and it might have another much larger moon. In February 2016, Eris's distance from the Sun was 96.3 AU (14.41 billion km; 8.95 billion mi), more than three times that of Neptune or Pluto. With the exception of long-period comets, Eris and Dysnomia were the most distant known natural objects in the Solar System until the discovery of 2018 AG37 and 2018 VG18 in 2018.
Because Eris appeared to be larger than Pluto, NASA initially described it as the Solar System's tenth planet. This, along with the prospect of other objects of similar size being discovered in the future, motivated the International Astronomical Union (IAU) to define the term planet for the first time. Under the IAU definition approved on August 24, 2006, Eris, Pluto and Ceres are "dwarf planets", reducing the number of known planets in the Solar System to eight, the same as before Pluto's discovery in 1930. Observations of a stellar occultation by Eris in 2010 showed that it was slightly smaller than Pluto,[30][31] which was measured by New Horizons as having a mean diameter of 2,377 ± 4 kilometres (1,477 ± 2 mi) in July 2015.
Makemake (minor-planet number 136472) is a dwarf planet orbiting the Sun in the Kuiper belt, a disc of icy bodies beyond the orbit of Neptune. It is the fourth largest dwarf planet and trans-Neptunian object in the Solar System, having a diameter 60% that of Pluto. It was discovered on March 31, 2005, by American astronomers Michael E. Brown, Chad Trujillo, and David Rabinowitz at Palomar Observatory. As one of the largest objects found by this team, the discovery of Makemake contributed to the reclassification of Pluto as a dwarf planet in 2006.
Makemake's surface is similar to that of Pluto: it is highly reflective, covered largely by frozen methane, and stained reddish-brown by tholins. Makemake has one known moon, which has not been named. The orbit of this moon suggests that Makemake's rotation has a high axial tilt, which implies that it experiences extreme seasons. Makemake shows evidence of geochemical activity and cryovolcanism, which has led scientists to suspect that it might harbor a subsurface ocean of liquid water. Gaseous methane has been detected around Makemake, although it is unclear whether it is contained in an atmosphere or comes from temporary outgassing.
No high-resolution images of Makemake's surface exist because it has not been visited up close by a space probe. Makemake is so far from Earth that it appears as a star-like point of light even when viewed through a telescope. Scientists have expressed desire to send a space probe to explore Makemake because of its geological activity and potential subsurface ocean.
Haumea (minor-planet number 136108) is a dwarf planet located beyond Neptune's orbit.[26] It was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory, and formally announced in 2005 by a team headed by José Luis Ortiz Moreno at the Sierra Nevada Observatory in Spain, who had discovered it that year in precovery images taken by the team in 2003. From that announcement, it received the provisional designation 2003 EL
On 17 September 2008, it was named after Haumea, the Hawaiian goddess of childbirth and fertility, under the expectation by the International Astronomical Union (IAU) that it would prove to be a dwarf planet. Nominal estimates make it the third-largest known trans-Neptunian object, after Eris and Pluto, and approximately the size of Uranus's moon Titania. Precovery images of Haumea have been identified back to 22 March 1955.
Haumea's mass is about one-third that of Pluto and 1/1400 that of Earth. Although its shape has not been directly observed, calculations from its light curve are consistent with it being a Jacobi ellipsoid (the shape it would be if it were a dwarf planet), with its major axis twice as long as its minor. In October 2017, astronomers announced the discovery of a ring system around Haumea, representing the first ring system discovered for a trans-Neptunian object and a dwarf planet.
Haumea's gravity was until recently thought to be sufficient for it to have relaxed into hydrostatic equilibrium, though that is now unclear. Haumea's elongated shape, together with its rapid rotation, rings, and high albedo (from a surface of crystalline water ice), is thought to be the consequences of a giant collision, which left Haumea the largest member of a collisional family (the Haumea family) that includes several large trans-Neptunian objects and Haumea's two known moons, Hiʻiaka and Namaka.
Ceres is a dwarf planet in the main asteroid belt between the orbits of Mars and Jupiter. It was the first object identified in the asteroid belt, discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory in Sicily, and announced as a new planet. Ceres was later classified as an asteroid, and then more recently as the only confirmed dwarf planet within the asteroid belt, and the largest without a moon. It is also the only recognized dwarf planet in the solar system whose orbit lies within that of Neptune. In the minor planet numbering system, its designation is 1 Ceres or (1) Ceres.
Ceres's diameter is about a quarter that of the Moon. Its small size means that even at its brightest it is too dim to be seen by the naked eye, except under extremely dark skies. Its apparent magnitude ranges from 6.7 to 9.3, peaking at opposition (when it is closest to Earth) once every 15- to 16-month synodic period. As a result, its surface features are barely visible even with the most powerful telescopes, and little was known about it until the robotic NASA spacecraft Dawn approached Ceres for its orbital mission in 2015.
Dawn found Ceres's surface to be a mixture of water, ice, and hydrated minerals such as carbonates and clay. Gravity data suggest Ceres to be partially differentiated into a muddy (ice–rock) mantle/core and a less dense, but stronger crust that is at most thirty percent ice by volume. Although Ceres likely has an internal ocean of liquid water, brines still flow through the outer mantle and reach the surface, allowing cryovolcanoes such as Ahuna Mons to form roughly every fifty million years. This makes Ceres the closest known cryovolcanically active body to the Sun. Ceres has an extremely tenuous and transient atmosphere of water vapour, vented from localised sources on its surface.
Sedna (minor-planet number 90377) is a dwarf planet in the outermost reaches of the Solar System, orbiting the Sun far beyond the orbit of Neptune. It was discovered in 2003, and is roughly 1,000 km in diameter. Spectroscopic analysis has revealed its surface to be a mixture of the solid ices of water, carbon dioxide, and ethane, along with sedimentary deposits of methane-derived, reddish-colored tholins, a chemical makeup similar to the surfaces of other trans-Neptunian objects. Sedna is not expected to have a substantial atmosphere. Within the range of uncertainty, it is tied with Ceres in the asteroid belt as the largest dwarf planet not known to have a moon. Owing to its lack of known moons, Sedna's mass and density remain unknown.
Sedna takes approximately 11,400 years to complete one orbit around the Sun. Its orbit is one of the widest known in the Solar System. Its aphelion is located 937 astronomical units (AU) away, about 19 times farther than that of Pluto. Sedna's orbit is also one of the most elliptical discovered, with an eccentricity of 0.85. As of 2026, Sedna is 83.0 AU (12.4 billion km) from the Sun, 2.5 times as far away as Neptune.
Upon its discovery, Sedna was initially classified as a member of the scattered disc, a group of objects sent into high-eccentricity orbits by the gravitational influence of Neptune. Some astronomers instead referred to it as the first known member of the inner Oort cloud, as its perihelion at 76.2 AU (11.4 billion km) is far too distant for it to have been scattered by any of the known planets. It has since become the prototype of a new class of objects characterized by highly eccentric orbits with very distant perihelia, the sednoids. The astronomer Michael E. Brown, co-discoverer of Sedna, has argued that its unusual orbit could provide information on the early evolution of the Solar System. Sedna might have been perturbed into its orbit through a close gravitational encounter with the hypothetical Planet Nine.
Gonggong (minor-planet number 225088) is a dwarf planet and a member of the scattered disc beyond Neptune. It has a highly eccentric and inclined orbit during which it ranges from 33–101 astronomical units (4.9–15.1 billion kilometers; 3.1–9.4 billion miles) from the Sun. As of 2026, its distance from the Sun is 89.7 AU (13.42×109 km; 8.34×109 mi), and it is the sixth-farthest known Solar System object. According to the Deep Ecliptic Survey, Gonggong is in a 3:10 orbital resonance with Neptune, in which it completes three orbits around the Sun for every ten orbits completed by Neptune. Gonggong was discovered in July 2007 by American astronomers Megan Schwamb, Michael Brown, and David Rabinowitz at the Palomar Observatory, and the discovery was announced in January 2009.
At approximately 1,230 km (760 mi) in diameter, Gonggong is similar in size to Pluto's moon Charon, making it the fifth-largest known trans-Neptunian object (apart possibly from Charon). It may be sufficiently massive to be in hydrostatic equilibrium and therefore a dwarf planet. Gonggong's large mass makes retention of a tenuous atmosphere of methane just possible, though such an atmosphere would slowly escape into space. The object is named after Gònggōng, a Chinese water god responsible for chaos, floods and the tilt of the Earth. The name was chosen by its discoverers in 2019, when they hosted an online poll for the general public to help choose a name for the object, and the name Gonggong won.
Gonggong is very red, likely due to the presence of organic compounds called tholins on its surface. Water ice is also present on its surface, which hints at a brief period of cryovolcanic activity in the distant past. With a rotation period of around 22 hours, Gonggong rotates slowly compared to other trans-Neptunian objects, which typically have periods of less than 12 hours. The slow rotation of Gonggong may have been caused by tidal forces from its natural satellite, named Xiangliu.
Orcus (minor-planet number 90482) is a dwarf planet located in the Kuiper belt, with one large moon, Vanth. It has an estimated diameter of 870 to 960 km (540 to 600 mi), comparable to the Inner Solar System dwarf planet Ceres. The surface of Orcus is relatively bright with albedo reaching 23 percent, neutral in color, and rich in water ice. The ice is predominantly in crystalline form, which may be related to past cryovolcanic activity. Other compounds like methane or ammonia may also be present on its surface. Orcus was discovered by American astronomers Michael Brown, Chad Trujillo, and David Rabinowitz on 17 February 2004.
Orcus is a plutino, a trans-Neptunian object that is locked in a 2:3 orbital resonance with the ice giant Neptune, making two revolutions around the Sun to every three of Neptune's. This is much like Pluto, except that the phase of Orcus's orbit is opposite to Pluto's: Orcus is at aphelion (most recently in 2019) around when Pluto is at perihelion (most recently in 1989) and vice versa. Orcus is the second-largest known plutino, after Pluto itself. The perihelion of Orcus's orbit is around 120° from that of Pluto, while the eccentricities and inclinations are similar. Because of these similarities and contrasts, along with its large moon Vanth that can be compared to Pluto's large moon Charon, Orcus has been dubbed the "anti-Pluto". This was a major consideration in selecting its name, as the deity Orcus was the Roman/Etruscan equivalent of the Roman/Greek Pluto.
Vesta is thought to be the second-largest asteroid, both by mass and by volume, after the dwarf planet Ceres. Measurements give it a nominal volume only slightly larger than that of Pallas (about 5% greater), but it is 25% to 30% more massive. It constitutes an estimated 9% of the mass of the asteroid belt. Vesta is the only known remaining rocky protoplanet of the kind that formed the terrestrial planets. Numerous fragments of Vesta were ejected by collisions one and two billion years ago that left two enormous craters occupying much of Vesta's southern hemisphere. Debris from these events has fallen to Earth as howardite–eucrite–diogenite (HED) meteorites, which have been a rich source of information about Vesta.
Vesta is the brightest asteroid visible from Earth. It is regularly as bright as magnitude 5.1, at which times it is faintly visible to the naked eye. Its maximum distance from the Sun is slightly greater than the minimum distance of Ceres from the Sun, although its orbit lies entirely within that of Ceres.
NASA's Dawn spacecraft entered orbit around Vesta on 16 July 2011 for a one-year exploration and left the orbit of Vesta on 5 September 2012 en route to its final destination, Ceres. Researchers continue to examine data collected by Dawn for additional insights into the formation and history of Vesta.
Pallas (minor-planet designation: 2 Pallas) is the third-largest asteroid in the Solar System by volume and mass. It is the second asteroid to have been discovered, after Ceres, and is likely a remnant protoplanet. Like Ceres, it is believed to have a mineral composition similar to carbonaceous chondrite meteorites, though significantly less hydrated than Ceres. It is 79% the mass of Vesta and 22% the mass of Ceres, constituting an estimated 7% of the total mass of the asteroid belt. Its estimated volume is equivalent to a sphere 507 to 515 kilometers (315 to 320 mi) in diameter, 90–95% the volume of Vesta.
During the planetary formation era of the Solar System, objects grew in size through an accretion process to approximately the size of Pallas. Most of these protoplanets were incorporated into the growth of larger bodies, which became the planets, whereas others were ejected by the planets or destroyed in collisions with each other. Pallas, Vesta and Ceres appear to be the only intact bodies from this early stage of planetary formation to survive within the orbit of Neptune.
When Pallas was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 28 March 1802, it was considered to be a planet, as were other asteroids in the early 19th century. The discovery of many more asteroids after 1845 eventually led to the separate listing of "minor" planets from "major" planets, and the realization in the 1950s that such small bodies did not form in the same way as (other) planets led to the gradual abandonment of the term "minor planet" in favor of "asteroid" (or, for larger bodies such as Pallas, "planetoid").
With an orbital inclination of 34.8°, Pallas's orbit is unusually highly inclined to the plane of the asteroid belt, making Pallas relatively inaccessible to spacecraft, and its orbital eccentricity is nearly as large as that of Pluto.
The high inclination of the orbit of Pallas results in the possibility of close conjunctions to stars that other solar objects always pass at great angular distance. This resulted in Pallas passing Sirius on 9 October 2022, only 8.5 arcminutes southwards, while no planet can get closer than 30 degrees to Sirius.
10 Hygiea is a large asteroid in the outer region of the main asteroid belt, between the orbits of Mars and Jupiter. It was the tenth known asteroid, discovered on 12 April 1849 by Italian astronomer Annibale de Gasparis at the Astronomical Observatory of Capodimonte in Naples, Italy. It was named after Hygieia, the Greek goddess of health. It is the fourth-largest main-belt asteroid by both volume and mass, with a mean diameter of 433 km (269 mi) and a mass constituting 3% of the main asteroid belt's total mass.
Hygiea has a nearly spherical shape, with two known craters about 100 and 180 km (62 and 112 mi) in diameter. Because of its shape and large size, some researchers consider Hygiea a possible dwarf planet. Hygiea has a dark, carbonaceous surface consisting of hydrated and ammoniated silicate minerals, with carbonates and water ice. Hygiea's subsurface likely contains a large fraction of water ice. These characteristics make Hygiea very similar to the main-belt dwarf planet Ceres, which suggests the two objects have similar origins and evolutionary histories.
Hygiea is the parent body of the Hygiea family, an asteroid family comprising over 7,000 known asteroids that share similar orbital and compositional characteristics with Hygiea.[13] The Hygiea family is believed to have formed by a giant impact on Hygiea about 2 to 3 billion years ago. This impact is thought to have shattered Hygiea, which led to its reaccumulation as a nearly spherical body.
Arrokoth is a contact binary—a "snowman"-shaped object composed of two lobes connected by a narrow neck of material. The lobes of Arrokoth, named Wenu and Weeyo, are believed to be former planetesimals that once formed a binary system, but gently merged together. Wenu and Weeyo have flattened shapes consisting of distinct mounds, which indicate that they formed from a collection of smaller planetesimals 4.5 billion years ago. The surface of Arrokoth is tinted red by tholins and shows little cratering, which suggests that it has undergone little change since its formation. The primitive nature of Arrokoth is supported by its nearly circular and low-inclination orbit around the Sun, which suggests that it has never been disturbed by the gravitational influence of the planets.
Quaoar has two thin rings orbiting outside its Roche limit, which defies theoretical expectations that rings outside the Roche limit should be unstable. Quaoar has one moon named Weywot and a potential second moon that has not yet been confirmed. It is believed that Quaoar's elongated shape, gravitational influence of its moon(s), and extremely cold temperature help keep its rings stable.
174567 Varda is a large trans-Neptunian object, discovered on 21 June 2003 by American astronomer Jeffrey A. Larsen using survey images taken with the 0.9-meter Spacewatch telescope at the Kitt Peak National Observatory. As a dynamically hot classical Kuiper belt object, Varda orbits within the Kuiper belt region. Stellar occultations in 2018 show that Varda is likely an oblate spheroid with a flattening of 0.07 and an area equivalent diameter of 740±14 km. Other triaxial ellipsoid shape models remain mathematically unconstrained.] The total mass of the binary system is approximately 2.675+0.048 −0.049×1020 kg with Varda contributing around 86 to 90% of it. Varda's bulk density estimates remain highly ambiguous. James Webb Space Telescope data shows a surface temperature of 44 K. Abundant frozen carbon dioxide dominates its surface.
Because of its large size, some astronomers including Michael E. Brown and Noemi Pinilla-Alonso have considered it a possible dwarf planet.[18][19] However, it is hypothesized that trans-Neptunian objects in the size range of 400–1000 km, such as Varda, have probably never compressed into fully solid bodies, let alone fall into hydrostatic equilibrium, and so are unlikely to be dwarf planets, though the density calculations for Varda are highly ambiguous. It is not clear if Varda's density is high enough to compressed into a fully solid body.
Varda has one known moon named Ilmarë. The moon's diameter measures approximately 403±40 km. Varda and Ilmarë are likely mutually tidally locked. The binary system exhibits a very large mass ratio, comparable to that of Pluto and Charon.
704 Interamnia is a large F-type asteroid. With a mean diameter of around 330 kilometres, it is the fifth-largest asteroid, after Ceres, Vesta, Pallas and Hygiea. Its mean distance from the Sun is 3.067 AU. It was discovered on 2 October 1910 by Vincenzo Cerulli, and named after the Latin name for Teramo, Italy, where Cerulli worked. Its mass is probably between fifth and tenth highest in the asteroid belt, with a mass estimated to be 1.2% of the mass of the entire asteroid belt.[6] Observations by the Very Large Telescope's SPHERE imager in 2017–2019, combined with occultation results, indicate that the shape of Interamnia may be consistent with hydrostatic equilibrium for a body of its density with a rotational period of 7.6 hours. (Its current period is 8.7 hours.) This suggests that Interamnia may have formed as an equilibrium body, and that impacts changed its rotational period after it fully solidified.
216 Kleopatra is a large M-type asteroid with a mean diameter of 120 kilometers (75 miles) and is noted for its elongate bone or dumbbell shape. It was discovered on 10 April 1880 by Austrian astronomer Johann Palisa at the Austrian Naval Pola Observatory, in what is now Pula, Croatia, and was named after Cleopatra VII, the famous Egyptian queen. It has two small minor-planet moons which were discovered in 2008 and later named Alexhelios and Cleoselene.
Dog-Bone Form: Radar and telescope images show it has two thick ends connected by a thinner middle part.Rub-Pile Build: It is not a solid rock. Scientists think it is a loose pile of metal and stone pieces held together with lots of empty space inside.Composition: It is classed as an M-type asteroid because it contains a large amount of metal, like nickel and iron.
Location: It orbits the Sun in the main asteroid belt between Mars and Jupiter.The Moons: It has a tiny system of two moons discovered in 2008.Namesake: The asteroid and its moons are named after the ancient Egyptian Queen Cleopatra and her children.
Date Found: September 1, 1804.Discoverer: Karl Ludwig Harding.Name Origin: Named after the high Roman goddess Juno (queen of the gods and wife of Jupiter).Classification: Initially categorized as a planet before being reclassified as an asteroid in the 1850s.
Size: Mean diameter of approximately 247 km (153 mi).Composition: Made of a stony mix of rock and metal (siliceous S-type).Orbit Shape: Highly eccentric (elliptical) orbit bringing it as close as 1.99 AU and as far as 3.35 AU from the Sun.Orbital Period: Takes 4.36 Earth years to complete one trip around the Sun.Rotation: Spins on its axis once every 7.2 hours
High Albedo: Reflects more sunlight than typical dark carbonaceous asteroids, making it historically easier to spot with early telescopes.Surface Feature: Displays a large impact crater roughly 100 kilometers wide, pointing to a major past collision.
Halley's Comet is the only known short-period comet that is consistently visible to the naked eye from Earth, appearing roughly every 74–79 years. It last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061. Officially designated 1P/Halley, it is also commonly called Comet Halley, or sometimes simply Halley.
Halley's periodic returns to the inner Solar System have been observed and recorded by astronomers around the world since at least 240 BC, but it was not until 1705 that the English astronomer Edmond Halley understood that these appearances were re-appearances of the same comet. As a result of this discovery, the comet is named after Halley.
In March 1986, during its latest visit to the inner Solar System, Halley's Comet became the first comet to be observed in detail by a spacecraft, the European Space Agency's Giotto mission, providing the first observational data on the structure of a comet nucleus and the mechanism of coma and tail formation. These observations supported several longstanding hypotheses about comet construction, particularly Fred Whipple's "dirty snowball" model, which correctly predicted that Halley would be composed of a mixture of volatile ices—such as water, carbon dioxide, ammonia—and dust.
The missions also provided data that substantially reformed and reconfigured these ideas; for instance, it is now understood that the surface of Halley is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy. It was also visited by the two spacecraft of the Vega program, Vega 1 & 2, on 6 and 9 March, respectively. They went as close as 8,890 km (5,520 mi), and 8,030 km (4,990 mi), providing data on Halley's dimensions, shape, temperature, and surface properties.
Comet Shoemaker–Levy 9 (formally designated D/1993 F2) was a comet that broke apart in July 1992 and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of Solar System objects. This generated a large amount of coverage in the popular media, and the comet was closely observed by astronomers worldwide. The collision provided new information about Jupiter and highlighted its possible role in reducing space debris in the inner Solar System.
The comet was discovered by astronomers Carolyn and Eugene M. Shoemaker, and David Levy in 1993.[6] Shoemaker–Levy 9 (SL9) had been captured by Jupiter and was orbiting the planet at the time. It was located on the night of March 24 in two photographs taken with the 46 cm (18 in) Schmidt telescope at the Palomar Observatory in California. It was the first active comet observed to be orbiting a planet, and had probably been captured by Jupiter around 20 to 30 years earlier.
Calculations showed that its unusual fragmented form was due to a previous closer approach to Jupiter in July 1992. At that time, the orbit of Shoemaker–Levy 9 passed within Jupiter's Roche limit, and Jupiter's tidal forces had acted to pull the comet apart. The comet was later observed as a series of fragments ranging up to 2 km (1.2 mi) in diameter. These fragments collided with Jupiter's southern hemisphere between July 16 and 22, 1994 at a speed of approximately 60 km/s (37 mi/s) (Jupiter's escape velocity) or 216,000 km/h (134,000 mph). The prominent scars from the impacts were more visible than the Great Red Spot and persisted for many months.
Comet Tsuchinshan–ATLAS, also known as the Great Comet of 2024 and formally designated as C/2023 A3, is a non-periodic comet from the Oort cloud discovered by the Purple Mountain Observatory in China on 9 January 2023 and independently found by ATLAS South Africa on 22 February 2023. The comet passed perihelion at a distance of 0.39 AU (58 million km; 36 million mi) on 27 September 2024, when it became visible to the naked eye. Tsuchinshan–ATLAS peaked at its brightest magnitude on 9 October, shortly after passing the Sun, with an apparent magnitude of −4.9 per reported observations at the Comet Observation Database (COBS), making it the brightest comet observed in the Northern Hemisphere since Comet Hale–Bopp in 1997.image Credit Jim Vajda
C/2025 A6 (Lemmon) is a non-periodic comet discovered by the Mount Lemmon Survey in images obtained on 3 January 2025. It made its closest approach to Earth on 21 October 2025 and was visible to the naked eye.
Discovery Date: January 3, 2025Origin of Name: Found via the telescope survey at Mount Lemmon, ArizonaClosest Approach to Earth: October 21, 2025Orbital Period: Roughly 1,300 to 1,350 years, meaning it will not return to the inner solar system until around the year 3175.
Naked Eye / Binoculars: During its peak in mid-to-late October 2025, it was visible with binoculars and under dark skies, sometimes faintly by the naked eye.
Path: It tracked past bright stars like Arcturus in the constellation Boötes, making it easier for amateur astronomers using finder charts and binoculars to locate it shortly after sunset. Image Credit Antonio Ferretti; Attilio Bruzzone
3200 Phaethon is a near-Earth Apollo asteroid famous for being the parent body of the annual Geminids meteor shower.
Discovery: Found on October 11, 1983, using data from NASA's Infrared Astronomical Satellite (IRAS).
Size: Approximately 5.1 to 5.8 kilometers (3.2 to 3.6 miles) in diameter.Orbit: Completes one orbit around the Sun every 1.43 years (524 days) in a highly elliptical path.
Extreme Heat: Its perihelion (closest point to the Sun) brings it within 0.14 astronomical units of the Sun—closer than any other named asteroid—surging surface temperatures up to around 1,500°F (800°C).
Rock Comet Behavior: While it looks and acts mostly like a rocky asteroid, extreme solar heating fractures its surface and causes it to brighten. NASA observations revealed that its comet-like tail is made of sodium gas rather than dust.
Blue Hue: Phaethon has an unusual bluish surface color, setting it apart from typical gray or red asteroids.
Comet Swift–Tuttle (formally designated 109P/Swift–Tuttle) is a large periodic comet with a 1995 (osculating) orbital period of 133 years that is in a 1:11 orbital resonance with Jupiter. It fits the classical definition of a Halley-type comet, which has an orbital period between 20 and 200 years. The comet was independently discovered by Lewis Swift on 16 July 1862 and by Horace Parnell Tuttle on 19 July 1862.
Its nucleus is 26 km (16 mi) in diameter. Swift–Tuttle is the parent body of the Perseid meteor shower, perhaps the best known shower and among the most reliable in performance.
The comet made a return appearance in 1992, when it was rediscovered by Japanese astronomer Tsuruhiko Kiuchi and became visible with binoculars. It was last observed in April 1995 when it was 8.6 AU (1.3 billion km) from the Sun. In 2126, it will likely be a bright naked-eye comet, potentially reaching an apparent magnitude of about 0.7.
Perseids: Leaves behind the dust and debris that create the annual Perseid meteor shower every August. Size: More than twice the size of the object linked to the extinction of the dinosaurs. Comet 109P/Swift-Tuttle on December 15, 1992. Courtesy Michael Jaeger in Austria.
The Nancy Grace Roman Space Telescope (shortened as the Roman Space Telescope, Roman, NGRST, or RST) is a NASA infrared space telescope that was launched on a trajectory toward a Sun–Earth L2 orbit on 30 August 2026 by a Falcon Heavy launch vehicle. It is named after former NASA chief of astronomy Nancy Grace Roman.
The Roman Space Telescope is based on an existing 2.4-meter (7.9-foot) primary mirror with a wide field of view that was donated by the National Reconnaissance Office and will carry two scientific instruments. The Wide-Field Instrument (WFI) is a 300.8-megapixel multi-band visible and near-infrared camera, providing a sharpness of images comparable to that achieved by the Hubble Space Telescope over a 0.28-square-degree field of view, 100 times larger than imaging cameras on the Hubble. The Coronagraph Instrument (CGI) is a high-contrast, small-field-of-view camera and spectrometer covering visible and near-infrared wavelengths using novel starlight-suppression technology.
Stated objectives include a search for extra-solar planets using gravitational microlensing, along with probing the chronology of the universe and growth of cosmic structure, with the end goal of measuring the effects of dark energy, the consistency of general relativity, and the curvature of spacetime. Roman was recommended in 2010 by the United States National Research Council Decadal Survey committee as the top priority for the next decade of astronomy. It was approved for development and launch on 17 February 2016.
The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. It is the largest telescope in space, and is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, distant, or faint for the Hubble Space Telescope.[9] This enables investigations across many fields of astronomy and cosmology, such as observation of the first stars and the formation of the first galaxies, and detailed atmospheric characterization of potentially habitable exoplanets.
Despite Webb's mirror diameter being 2.7 times larger than that of the Hubble Space Telescope, it produces images of comparable resolution because it observes in the infrared spectrum, which has longer wavelengths than the Hubble's visible spectrum. The longer the wavelength the telescope is designed to observe, the larger the information-gathering surface (mirrors in the infrared spectrum or antenna area in the millimeter and radio ranges) required to achieve the desired resolution.
The Webb was launched on 25 December 2021 on an Ariane 5 rocket from Kourou, French Guiana. In January 2022, it arrived at its destination, a solar orbit near the Sun–Earth L2 Lagrange point, about 1.5 million kilometers (930,000 mi) from Earth. The telescope's first image was released to the public on 11 July 2022.
The U.S. National Aeronautics and Space Administration (NASA) led Webb's design and development and partnered with two central agencies: the European Space Agency (ESA) and the Canadian Space Agency (CSA). The NASA Goddard Space Flight Center in Maryland managed telescope development, while the Space Telescope Science Institute in Baltimore on the Homewood Campus of Johns Hopkins University operates Webb. The primary contractor for the project was Northrop Grumman.
The telescope is named after James E. Webb, who was the administrator of NASA from 1961 to 1968 during the Mercury, Gemini, and Apollo programs.
Webb's primary mirror consists of 18 hexagonal mirror segments made of gold-plated beryllium, which together create a 6.5-meter-diameter (21 ft) mirror, compared with Hubble's 2.4 m (7 ft 10 in). This gives Webb a light-collecting area of about 25 m2 (270 sq ft), about six times that of Hubble. Unlike Hubble, which observes in the near ultraviolet, visible, and near infrared spectra (0.1–2.5 μm), Webb observes a lower frequency range, from long-wavelength visible light (red) through mid-infrared (0.6–28.5 μm). The telescope must be kept extremely cold, below 50 K (−223 °C; −370 °F), so that the infrared radiation emitted by the telescope itself does not interfere with the collected light. Its five-layer sunshield protects it from warming by the Sun, Earth, and Moon.
Initial designs for the telescope, then named the Next Generation Space Telescope, began in 1996. Two concept studies were commissioned in 1999, for a potential launch in 2007 and a US$1 billion budget. The program saw enormous cost overruns and delays. A significant redesign was carried out in 2005, with construction completed in 2016, followed by years of exhaustive testing, at a total cost of US$10 billion.
The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versatile, renowned as a vital research tool and as a public relations boon for astronomy. The Hubble Space Telescope is named after astronomer Edwin Hubble and is one of NASA's Great Observatories. The Space Telescope Science Institute (STScI) selects Hubble's targets and processes the resulting data, while the Goddard Space Flight Center (GSFC) controls the spacecraft.
Hubble features a 2.4 m (7 ft 10 in) mirror, and its five main instruments observe in the ultraviolet, visible, and near-infrared regions of the electromagnetic spectrum. Hubble's orbit outside the distortion of Earth's atmosphere allows it to capture extremely high-resolution images with substantially lower background light than ground-based telescopes. It has recorded some of the most detailed visible light images, allowing a deep view into space. Many Hubble observations have led to breakthroughs in astrophysics, such as determining the rate of expansion of the universe.
The Hubble Space Telescope was funded and built in the 1970s by NASA with contributions from the European Space Agency. Its intended launch was in 1983, but the project was beset by technical delays, budget problems, and the 1986 Challenger disaster. Hubble was launched on STS-31 in 1990, but its main mirror had been ground incorrectly, resulting in spherical aberration that compromised the telescope's capabilities. The optics were corrected to their intended quality by a servicing mission, STS-61, in 1993.
Hubble is the only telescope designed to be maintained in space by astronauts. Five Space Shuttle missions repaired, upgraded, and replaced systems on the telescope, including all five of the main instruments. The fifth mission was initially canceled on safety grounds following the Columbia disaster (2003), but after NASA administrator Michael D. Griffin approved it, the servicing mission was completed in 2009. Hubble completed 30 years of operation in April 2020 and is predicted to last until 2030 to 2040.
Hubble is the visible light telescope in NASA's Great Observatories program; other parts of the spectrum are covered by the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope (which covers the infrared bands). The mid-IR-to-visible band successor to the Hubble telescope is the James Webb Space Telescope (JWST), which was launched on December 25, 2021, with the Nancy Grace Roman Space Telescope due to follow in 2026.
The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the Space Shuttle Columbia during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources 100 times fainter than any previous X-ray telescope, enabled by the high angular resolution of its mirrors. Since the Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes; therefore space-based telescopes are required to make these observations. Chandra is an Earth satellite in a 64-hour orbit, and its mission is ongoing as of 2025.
Chandra is one of the Great Observatories, along with the Hubble Space Telescope, Compton Gamma Ray Observatory (1991–2000), and the Spitzer Space Telescope (2003–2020). The telescope is named after the Nobel Prize-winning Indian-American astrophysicist Subrahmanyan Chandrasekhar. Its mission is similar to that of ESA's XMM-Newton spacecraft, also launched in 1999 but the two telescopes have different design foci, as Chandra has a much higher angular resolution and XMM-Newton higher spectroscopy throughput.
In response to a decrease in NASA funding in 2024 by the US Congress, Chandra is threatened with an early cancellation despite having more than a decade of operation left. The cancellation has sardonically been referred to as a potential "extinction-level" event for X-ray astronomy in the US. A group of astronomers have put together a public outreach project to try to get enough American citizens to persuade the US Congress to provide enough funding to avoid early termination of the observatory.
New Horizons is an interplanetary space probe launched as a part of NASA's New Frontiers program. Launched in 2006, it became the first spacecraft to perform a flyby study and capture high-resolution photographs of Pluto and its moons during its encounter.
It was engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a team led by Alan Stern. New Horizons is the fifth space probe to achieve the escape velocity needed to leave the Solar System.
On January 19, 2006, New Horizons was launched from Cape Canaveral Space Force Station by an Atlas V rocket directly into an Earth-and-solar escape trajectory with a speed of about 16.26 km/s (10.10 mi/s; 58,500 km/h; 36,400 mph) relative to the sun. It was the fastest (average speed with respect to Earth) human-made object ever launched from Earth. It is not the fastest speed recorded for a spacecraft, which, as of 2026, is that of the Parker Solar Probe. After a brief encounter with asteroid 132524 APL, New Horizons proceeded to Jupiter, making its closest approach on February 28, 2007, at a distance of 2.3 million kilometers (1.4 million miles). The Jupiter flyby provided a gravity assist that increased New Horizons' speed; the flyby also enabled a general test of New Horizons' scientific capabilities, returning data about the planet's atmosphere, moons, and magnetosphere.
Most of the post-Jupiter voyage was spent in hibernation mode to preserve onboard systems, except for brief annual checkouts. On December 6, 2014, New Horizons was brought back online for the Pluto encounter, and instrument check-out began. On January 15, 2015, the spacecraft began its approach phase to Pluto.
On July 14, 2015, at 11:49 UTC, it flew 12,500 km (7,800 mi) above the surface of Pluto, which at the time was 34 AU from the Sun, making it the first spacecraft to explore the dwarf planet. In August 2016, New Horizons was reported to have traveled at speeds of more than 84,000 km/h (52,000 mph). On October 25, 2016, at 21:48 UTC, the last recorded data from the Pluto flyby was received from New Horizons. Having completed its flyby of Pluto, New Horizons then maneuvered for a flyby of Kuiper belt object 486958 Arrokoth (then nicknamed Ultima Thule), which occurred on January 1, 2019, when it was 43.4 AU (6.49 billion km; 4.03 billion mi) from the Sun. In August 2018, NASA cited results by Alice on New Horizons to confirm the existence of a "hydrogen wall" at the outer edges of the Solar System. This "wall" was first detected in 1992 by the two Voyager spacecraft.
New Horizons is traveling through the Kuiper belt; it is 64.21 AU (9.61 billion km; 5.97 billion mi) from Earth and 64.45 AU (9.64 billion km; 5.99 billion mi) from the Sun as of April 2026. NASA has announced its plan to extend operations for New Horizons until the spacecraft exits the Kuiper belt, which is expected to occur in either 2028 or 2029. The White House's proposed budget for FY2026 would have cut funding for New Horizons, but the continued funding of the mission was heavily debated in the United States Congress and a final budget of 24.44 billion USD guaranteed the mission continuation.
Voyager 1 is a space probe launched by NASA on September 5, 1977, as part of the Voyager program, to study the outer Solar System and the interstellar space beyond the Sun's heliosphere. It was launched 16 days after its twin, Voyager 2. It communicates through the NASA Deep Space Network (DSN) to receive routine commands and to transmit data to Earth. Real-time distance and velocity data are provided by NASA and JPL. At a distance of 171.19 AU (25.6 billion km; 15.9 billion mi) as of August 2026, it is the most distant human-made object from Earth. Voyager 1 is projected to reach a distance of one light day from Earth in November 2026.
The probe made flybys of Jupiter, Saturn, and Saturn's largest moon, Titan. NASA had a choice of either conducting a Pluto or Titan flyby. Exploration of Titan took priority because it was known to have a substantial atmosphere. Voyager 1 studied the weather, magnetic fields, and rings of the two gas giants and was the first probe to provide detailed images of their moons.
As part of the Voyager program and like its sister craft Voyager 2, the spacecraft's extended mission is to locate and study the regions and boundaries of the outer heliosphere and to begin exploring the interstellar medium. Voyager 1 crossed the heliopause and entered interstellar space on August 25, 2012, making it the first spacecraft to do so. Two years later, Voyager 1 began experiencing a third wave of coronal mass ejections from the Sun that continued to at least December 15, 2014, further confirming that the probe is in interstellar space.
In 2017, the Voyager team successfully fired the spacecraft's trajectory correction maneuver (TCM) thrusters for the first time since 1980, enabling the mission to be extended by two to three years. Voyager 1 experienced successful revivals of several thrusters in 2018, 2019, and 2025.
Voyager 1's extended mission is expected to continue to return scientific data for several more years. Its radioisotope thermoelectric generators (RTGs) may supply enough electric power to return engineering data until 2036. As of 2026, only two instruments are operational, the Plasma Wave Subsystem and magnetometer.
Voyager 2 is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and Neptune). The only spacecraft to have visited either of the ice giant planets, it was the third of five spacecraft to achieve Solar escape velocity, which allowed it to leave the Solar System. Launched 16 days before its twin Voyager 1, the primary mission of the spacecraft was to study the outer planets and its extended mission is to study interstellar space beyond the Sun's heliosphere.
Voyager 2 successfully fulfilled its primary mission of visiting the Jovian system in 1979, the Saturnian system in 1981, Uranian system in 1986, and the Neptunian system in 1989. The spacecraft is now in its extended mission of studying the interstellar medium. It is at a distance of 143.05 AU (21.4 billion km; 13.3 billion mi) from Earth as of February 2026.
The probe entered the interstellar medium on November 5, 2018, at a distance of 119.7 AU (11.1 billion mi; 17.9 billion km) from the Sun and moving at 15.341 km/s (34,320 mph) relative to the Sun. Voyager 2 has left the Sun's heliosphere and is traveling through the interstellar medium, though still inside the Solar System, joining Voyager 1, which reached the interstellar medium in 2012. Voyager 2 has begun to provide the first direct measurements of the density and temperature of the interstellar plasma.
Voyager 2 is in contact with Earth through the NASA Deep Space Network. Communications are the responsibility of Australia's DSS 43 communication antenna, part of the Canberra communication complex; the craft's southern trajectory puts it out of range of the network's Goldstone and Madrid complexes.
The Parker Solar Probe (PSP; previously Solar Probe, Solar Probe Plus or Solar Probe+) is a NASA space probe launched in 2018 to make observations of the Sun's outer corona.
It used repeated gravity assists from Venus to develop an eccentric orbit, approaching within 9.86 solar radii (6.9 million km or 4.3 million miles) from the center of the Sun. At its closest approach in 2024, its speed relative to the Sun was 690,000 km/h (430,000 mph) or 191 km/s (118.7 mi/s), which is 0.064% the speed of light. It is the fastest object ever built on Earth.
The project was announced in the fiscal 2009 budget year. Johns Hopkins University Applied Physics Laboratory designed and built the spacecraft, which was launched on August 12, 2018. It became the first NASA spacecraft named after a living person, honoring the physicist Eugene Newman Parker, professor emeritus at the University of Chicago.
On October 29, 2018, at about 18:04 UTC, the spacecraft became the closest ever artificial object to the Sun. The previous record, 42.73 million kilometers (26.55 million miles) from the Sun's surface, was set by the Helios 2 spacecraft in April 1976. At its perihelion on September 27, 2023, the PSP's closest approach was 7.26 million kilometers (4.51 million miles), reaching this distance again on March 29, 2024.
On December 24, 2024, at 11:53 UTC, PSP made its closest approach to the Sun, coming to a distance of 6.1 million km (3.8 million miles) from the surface. Its beacon signal was received on December 26, showing that it had survived the passage through the corona. Detailed telemetry was received January 1, 2025.
In 2025, the teams from NASA, Johns Hopkins, and partners were awarded the 2024 Collier Trophy for their achievements.
The Kepler space telescope was a space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler (who, ironically, was convinced that planetary systems did not exist around any of the stars) the spacecraft was launched into an Earth-trailing heliocentric orbit. The principal investigator was William J. Borucki. After nine and a half years of operation, the telescope's reaction control system fuel was depleted, and NASA announced its retirement on October 30, 2018.
Designed to survey a portion of Earth's region of the Milky Way to discover Earth-size exoplanets in or near habitable zones and to estimate how many of the billions of stars in the Milky Way have such planets, Kepler's sole scientific instrument is a photometer that continually monitored the brightness of approximately 150,000 main sequence stars in a fixed field of view. These data were transmitted to Earth, then analyzed to detect periodic dimming caused by exoplanets that cross in front of their host star. Only planets whose orbits are seen edge-on from Earth could be detected. Kepler observed 530,506 stars, and had detected 2,778 confirmed planets as of June 16, 2023.
The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope that was active between 2003 and 2020. Spitzer was the third space telescope dedicated to infrared astronomy, following IRAS (1983) and ISO (1995–1998). It was the first spacecraft to use an Earth-trailing orbit, later used by the Kepler planet-finder telescope.
The planned mission period was to be 2.5 years with a pre-launch expectation that the mission could extend to five or slightly more years until the onboard liquid helium supply was exhausted. This occurred on 15 May 2009. Without liquid helium to cool the telescope to the very low temperatures needed to operate, most of the instruments were no longer usable. However, the two shortest-wavelength modules of the IRAC camera continued to operate with the same sensitivity as before the helium was exhausted, and continued to be used into early 2020 in the Spitzer Warm Mission.
During the warm mission, the two short wavelength channels of IRAC operated at 28.7 K and were predicted to experience little to no degradation at this temperature compared to the nominal mission. The Spitzer data, from both the primary and warm phases, are archived at the Infrared Science Archive (IRSA).
In keeping with NASA tradition, the telescope was renamed after its successful demonstration of operation, on 18 December 2003. Unlike most telescopes that are named by a board of scientists, typically after famous deceased astronomers, the new name for SIRTF was obtained from a contest open to the general public. The contest led to the telescope being named in honor of astronomer Lyman Spitzer, who had promoted the concept of space telescopes in the 1940s. Spitzer wrote a 1946 report for the RAND Corporation describing the advantages of an extraterrestrial observatory and how it could be realized with available or upcoming technology. He has been cited for his pioneering contributions to rocketry and astronomy, as well as "his vision and leadership in articulating the advantages and benefits to be realized from the Space Telescope Program."
The US$776 million Spitzer was launched on 25 August 2003 at 05:35:39 UTC from Cape Canaveral SLC-17B aboard a Delta II 7920H rocket. It was placed into a heliocentric (as opposed to a geocentric) orbit trailing and drifting away from Earth's orbit at approximately 0.1 astronomical units per year (an "Earth-trailing" orbit.
The primary mirror is 85 centimeters (33 in) in diameter, f/12, made of beryllium and was cooled to 5.5 K (−268 °C; −450 °F). The satellite contains three instruments that allowed it to perform astronomical imaging and photometry from 3.6 to 160 micrometers, spectroscopy from 5.2 to 38 micrometers, and spectrophotometry from 55 to 95 micrometers.
The Fermi Gamma-ray Space Telescope (FGST, also FGRST), formerly called the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is the Large Area Telescope (LAT), with which astronomers mostly intend to perform an all-sky survey studying astrophysical and cosmological phenomena such as active galactic nuclei, pulsars, other high-energy sources and dark matter. Another instrument aboard Fermi, the Gamma-ray Burst Monitor (GBM; formerly GLAST Burst Monitor), is being used to study gamma-ray bursts and solar flares.
Fermi, named for high-energy physics pioneer Enrico Fermi, was launched on 11 June 2008 at 16:05 UTC aboard a Delta II 7920-H rocket. The mission is a joint venture of NASA, the United States Department of Energy, and government agencies in France, Germany, Italy, Japan, and Sweden, becoming the most sensitive gamma-ray telescope on orbit, succeeding INTEGRAL. The project is a recognized CERN experiment (RE7).
Large Area Telescope (LAT): The primary instrument that detects high-energy gamma rays (20 MeV to over 300 GeV) by converting them into electron-positron pairs.Gamma-ray Burst Monitor (GBM): Detects sudden, short flashes of gamma rays known as gamma-ray bursts and solar flares across a lower energy range (8 keV to 40 MeV).
The Transiting Exoplanet Survey Satellite (TESS) is a space telescope for NASA's Explorer program, designed to search for exoplanets using the transit method in an area 400 times larger than that covered by the Kepler mission. It was launched on 18 April 2018, atop a Falcon 9 launch vehicle and was placed into a highly elliptical 13.70-day orbit around the Earth. The first light image from TESS was taken on 7 August 2018, and released publicly on 17 September 2018.
In the two-year primary mission, TESS was expected to detect about 1,250 transiting exoplanets orbiting the targeted stars, and an additional 13,000 orbiting stars not targeted but observed. After the end of the primary mission around 4 July 2020, scientists continued to search its data for more planets, while the extended missions acquire additional data. As of 3 May 2026, TESS had identified 7,931 candidate exoplanets, of which 885 had been confirmed.
The primary mission objective for TESS was to survey the brightest stars near the Earth for transiting exoplanets over a two-year period. The TESS satellite uses an array of wide-field cameras to perform a survey of 85% of the sky. With TESS, it is possible to study the mass, size, density and orbit of a large cohort of small planets, including a sample of rocky planets in the habitable zones of their host stars. TESS provides prime targets for further characterization by the James Webb Space Telescope (JWST), as well as other large ground-based and space-based telescopes of the future.
While previous sky surveys with ground-based telescopes have mainly detected giant exoplanets and the Kepler space telescope has mostly found planets around distant stars that are too faint for characterization, TESS finds many small planets around the nearest stars in the sky. TESS records the nearest and brightest main sequence stars hosting transiting exoplanets, which are the most favorable targets for detailed investigations. Detailed information about such planetary systems with hot Jupiters makes it possible to better understand the architecture of such systems.
The program is led by the Massachusetts Institute of Technology (MIT) with seed funding from Google. On 5 April 2013, it was announced that TESS, along with the Neutron Star Interior Composition Explorer (NICER), had been selected by NASA for launch. On 18 July 2019, after the first year of operation, the southern portion of the survey was completed, and the northern survey was started. The primary mission ended with the completion of the northern survey on 4 July 2020, which was followed by the first extended mission. The first extended mission concluded in September 2022 and the spacecraft entered its second extended mission which should last for another three years.
Pioneer 10 (originally designated Pioneer F) is a NASA space probe launched in 1972 that completed the first mission to the planet Jupiter. It was the first spacecraft to traverse the asteroid belt and the first of five artificial objects to achieve the escape velocity needed to leave the Solar System. The mission was managed by NASA Ames Research Center in California, and the spacecraft was built by TRW Inc.
The spacecraft was built around a hexagonal satellite bus with a 2.74 m (9.0 ft) diameter parabolic high-gain antenna and was spin-stabilized about the antenna axis. Electrical power was supplied by four radioisotope thermoelectric generators (RTGs), which produced a combined 155 watts at launch.
Pioneer 10 was launched on March 3, 1972, at 01:49:00 UTC (March 2 local time), aboard an Atlas-Centaur rocket from Cape Canaveral Launch Complex 36A. Between July 15, 1972, and February 15, 1973, it became the first spacecraft to pass through the asteroid belt. Imaging of Jupiter began on November 6, 1973, from a distance of 25 million km (16 million mi), and the spacecraft returned more than 500 images. Its closest approach to Jupiter was on December 3, 1973, passing within 132,252 km (82,178 mi) of the planet. During the mission, its scientific instruments investigated the asteroid belt, the environment of Jupiter, the solar wind, cosmic rays, and the outer heliosphere.
The last signal from Pioneer 10 was received on January 23, 2003, after declining electrical power from its RTGs left the spacecraft unable to operate its radio transmitter. At that time, it was about 80 AU (12 billion km; 7.4 billion mi) from Earth.
Pioneer 11 (also known as Pioneer G) is a NASA robotic space probe launched on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, the solar wind, and cosmic rays. It was the first probe to encounter Saturn, the second to fly through the asteroid belt, and the second to fly by Jupiter. Later, Pioneer 11 became the second of five artificial objects to achieve an escape velocity allowing it to leave the Solar System. Due to power constraints and the vast distance to the probe, the last routine contact with the spacecraft was on September 30, 1995, and the last good engineering data was received on November 24, 1995.
Jupiter Flyby: Reached Jupiter on December 2, 1974, passing closer than Pioneer 10 and sending back the first detailed images of Jupiter's polar regions. Saturn Flyby: Became the first spacecraft to fly past Saturn on September 1, 1979, passing 13,000 miles above the cloud tops to test the safety of the route for future Voyager missions. Saturn Findings: Discovered Saturn's thin F-ring, new moons, and provided the first evidence of a planetary magnetic field.
Interstellar Travel: Crossed Neptune's orbit on February 23, 1990, heading out of the solar system. Last Contact: Routine operations ended on September 30, 1995, due to fading power, with the final faint signal received on November 24, 1995. Plaque: Carries a gold-anodized aluminum plaque featuring a message and pictorial representations of humanity intended for potential extraterrestrial discovery.
Cassini–Huygens (/kəˈsiːni ˈhɔɪɡənz/ kə-SEE-nee HOY-gənz), commonly called Cassini, was a joint space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, including its rings and natural satellites. The Flagship-class robotic spacecraft comprised both NASA's Cassini space probe and ESA's Huygens lander, which landed on Saturn's largest moon, Titan. Cassini was the fourth space probe to visit Saturn and the first to enter its orbit, where it stayed from 2004 to 2017. The two craft took their names from the astronomers Giovanni Cassini and Christiaan Huygens.
Launched aboard a Titan IV-B with a Centaur D-1T upper stage on October 15, 1997, Cassini was active in space for nearly 20 years, spending almost 7 years in transit and 13 years orbiting Saturn, studying the planet and its system after entering orbit on July 1, 200
The voyage to Saturn included flybys of Venus (April 1998 and July 1999), Earth (August 1999), the asteroid 2685 Masursky, and Jupiter (December 2000). The mission ended on September 15, 2017, when Cassini's trajectory took it into Saturn's upper atmosphere and it burned up in order to prevent any risk of contaminating Saturn's moons, which might have offered habitable environments to stowaway terrestrial microbes on the spacecraft. The mission was successful beyond expectations – NASA's Planetary Science Division Director, Jim Green, described Cassini–Huygens as a "mission of firsts" that revolutionized human understanding of the Saturn system, including its moons and rings, and our understanding of where life might be found in the Solar System.
Galileo was an American robotic space probe that studied the planet Jupiter and its moons, as well as the asteroids Gaspra and Ida. Named after the Italian astronomer Galileo Galilei, it consisted of an orbiter and an entry probe. It was delivered into Earth orbit on October 18, 1989, by Space Shuttle Atlantis, during STS-34. Galileo arrived at Jupiter on December 7, 1995, after gravitational assist flybys of Venus and Earth, and became the first spacecraft to orbit an outer planet.
The Jet Propulsion Laboratory built the Galileo spacecraft and managed the Galileo program for NASA. West Germany's Messerschmitt-Bölkow-Blohm supplied the propulsion module. NASA's Ames Research Center managed the atmospheric probe, which was built by Hughes Aircraft Company. At launch, the orbiter and probe together had a mass of 2,562 kg (5,648 lb) and stood 6.15 m (20.2 ft) tall.
Spacecraft are normally stabilized either by spinning around a fixed axis or by maintaining a fixed orientation with reference to the Sun and a star. Galileo did both. One section of the spacecraft rotated at three revolutions per minute, keeping Galileo stable and holding six instruments that gathered data from many different directions, including the fields and particles instruments.
Galileo was intentionally destroyed in Jupiter's atmosphere on September 21, 2003 to prevent it from contaminating any of Jupiter's moons. The next orbiter to be sent to Jupiter was Juno, which arrived on July 5, 2016.
Release: Separated from the main orbiter on July 13, 1995, for a five-month unpowered freefall.Entry: Hit Jupiter's upper atmosphere on December 7, 1995, at roughly 170,000 km/h (106,000 mph).Survival: Endured deceleration forces of 228 Gs and temperatures twice as hot as the sun's surface, protected by a thick carbon-phenolic heat shield.Operation: Descended by parachute and transmitted science data for about 58 to 61 minutes before radio contact was lost.End: The probe descended roughly 160 to 180 km below the clouds before it was crushed, melted, and vaporized by extreme pressure and heat.
Measured Jupiter's atmospheric composition, revealing fewer organic compounds and less water vapor than expected.Provided direct data on high-speed winds, storm dynamics, and lightning activity.The companion orbiter later found strong evidence of subsurface saltwater oceans on moons like Europa, Ganymede, and Callisto.
Juno is a NASA space probe orbiting the planet Jupiter. Built by Lockheed Martin and operated by NASA's Jet Propulsion Laboratory, the spacecraft was launched from Cape Canaveral Air Force Station on August 5, 2011 UTC, as part of the New Frontiers program. Juno entered a polar orbit of Jupiter on July 5, 2016, UTC, to begin a scientific investigation of the planet. After completing its mission, Juno was originally planned to be intentionally deorbited into Jupiter's atmosphere,[9] but has since been approved to continue orbiting until contact is lost with the spacecraft. It will continue to explore Jupiter to study Jovian rings and inner moons area which is not well explored; this phase will also include close flybys of the moons Thebe, Amalthea, Adrastea, and Metis.
Juno's mission is to measure Jupiter's composition, gravitational field, magnetic field, and polar magnetosphere. It also searches for clues about how the planet formed, including whether it has a rocky core, the amount of water present within the deep atmosphere, mass distribution, and its deep winds, which can reach speeds up to 620 km/h (390 mph).
Juno is the second spacecraft to orbit Jupiter, after the RTG-powered Galileo orbiter, which orbited from 1995 to 2003. Unlike all earlier spacecraft sent to the outer Solar System and beyond—which used radioisotope thermoelectric generators for power—Juno is powered by solar panels, more commonly used by satellites orbiting Earth and working in the inner Solar System. Accordingly, Juno required the three largest solar panel wings ever deployed on a planetary probe (at the time of launching). These play an integral role in stabilizing the spacecraft as well as generating power.
As of August 6, 2026, Juno remained operational and in contact with Earth through the NASA Deep Space Network.
MESSENGER was a NASA robotic space probe that orbited the planet Mercury between 2011 and 2015, studying Mercury's chemical composition, geology, and magnetic field. The name is a backronym for Mercury Surface, Space Environment, Geochemistry, and Ranging, and a reference to the messenger god Mercury from Roman mythology.
MESSENGER was launched aboard a Delta II rocket in August 2004. Its path involved a complex series of flybys – the spacecraft flew by Earth once, Venus twice, and Mercury itself three times, allowing it to decelerate relative to Mercury using minimal fuel. During its first flyby of Mercury in January 2008, MESSENGER became the second mission, after Mariner 10 in 1975, to reach Mercury.
MESSENGER entered orbit around Mercury on March 18, 2011, becoming the first spacecraft to do so. It successfully completed its primary mission in 2012. Following two mission extensions, the spacecraft used the last of its maneuvering propellant to deorbit, impacting the surface of Mercury on April 30, 2015.
Opportunity, also known as MER-B (Mars Exploration Rover – B) or MER-1, and nicknamed Oppy, is a robotic rover that was active on Mars from 2004 until 2018. Opportunity was operational on Mars for 5111 sols (14 years, 138 days on Earth). Launched on July 7, 2003, as part of NASA's Mars Exploration Rover program, it landed in Meridiani Planum on January 25, 2004, three weeks after its twin, Spirit (MER-A), touched down on the other side of the planet.[9] With a planned 90-sol duration of activity (slightly less than 92.5 Earth days), Spirit functioned until it got stuck in 2009 and ceased communications in 2010, while Opportunity was able to stay operational for 5111 sols after landing, maintaining its power and key systems through continual recharging of its batteries using solar power, and hibernating during events such as dust storms to save power. This careful operation allowed Opportunity to operate for 57 times its designed lifespan, exceeding the initial plan by 14 years, 47 days (in Earth time). By June 10, 2018, when it last contacted NASA, the rover had traveled a distance of 45.16 kilometers (28.06 miles).
Mission highlights included the initial 90-sol mission, finding meteorites such as Heat Shield Rock (Meridiani Planum meteorite), and over two years of exploring and studying Victoria crater. The rover survived moderate dust storms and in 2011 reached Endeavour crater, which has been considered as a "second landing site". The Opportunity mission is considered one of NASA's most successful ventures.
Due to the planetary 2018 dust storm on Mars, Opportunity ceased communications on June 10 and entered hibernation on June 12, 2018. It was hoped it would reboot once the weather cleared, but it did not, suggesting either a catastrophic failure or that a layer of dust had covered its solar panels. NASA hoped to re-establish contact with the rover, citing a recurring windy period which was forecast for November 2018 to January 2019, that could potentially clean off its solar panels. On February 13, 2019, NASA officials declared that the Opportunity mission was complete, after the spacecraft had failed to respond to over 1,000 signals sent since August 2018.
Spirit, also known as MER-A (Mars Exploration Rover – A) or MER-2, is a Mars robotic rover, active from 2004 to 2010. Spirit was operational on Mars for 2208 sols or 3.3 Martian years (2269 days; 6 years, 77 days). It was one of two rovers of NASA's Mars Exploration Rover Mission managed by the Jet Propulsion Laboratory (JPL). Spirit landed successfully within the impact crater Gusev on Mars at 04:35 Ground UTC on January 4, 2004, three weeks before its twin, Opportunity (MER-B), which landed on the other side of the planet. Its name was chosen through a NASA-sponsored student essay competition. The rover got stuck in a "sand trap" in late 2009 at an angle that hampered recharging of its batteries; its last communication with Earth was on March 22, 2010.
The rover completed its planned 90-sol mission (slightly less than 92.5 Earth days). Aided by cleaning events that resulted in more energy from its solar panels, Spirit went on to function effectively over twenty times longer than NASA planners expected. Spirit also logged 7.73 km (4.8 mi) of driving instead of the planned 600 m (0.4 mi), allowing more extensive geological analysis of Martian rocks and planetary surface features. Initial scientific results from the first phase of the mission (the 90-sol prime mission) were published in a special issue of the journal Science.
On May 1, 2009 (5 years, 3 months, 27 Earth days after landing; 21 times the planned mission duration), Spirit became stuck in soft sand. This was not the first of the mission's "embedding events" and for the following eight months NASA carefully analyzed the situation, running Earth-based theoretical and practical simulations, and finally programming the rover to make extrication drives in an attempt to free itself. These efforts continued until January 26, 2010, when NASA officials announced that the rover was likely irrecoverably obstructed by its location in soft sand, though it continued to perform scientific research from its current location.
The rover continued in a stationary science platform role until communication with Spirit stopped on March 22, 2010 (sol 2208). JPL continued to attempt to regain contact until May 24, 2011, when NASA announced that efforts to communicate with the unresponsive rover had ended, calling the mission complete. A formal farewell took place at NASA headquarters shortly thereafter.
Perseverance is a NASA rover that has been exploring Mars since February 18, 2021, as part of the Mars 2020 mission. Built and managed by the Jet Propulsion Laboratory, the car-sized rover was launched on July 30, 2020, from Cape Canaveral aboard an Atlas V rocket and landed in Jezero Crater, a site chosen for its ancient river delta that may preserve evidence of past microbial life.
The rover's main goals are to search for signs of ancient life, study the planet's geology and climate, and collect rock and regolith samples for possible return to Earth by a future mission. Perseverance also tests technologies intended to support later human exploration, including an experiment that successfully produced oxygen from the thin carbon-dioxide atmosphere.
Perseverance carries seven primary scientific instruments, 19 cameras, and two microphones. It also deployed the experimental helicopter Ingenuity, which in April 2021 performed the first powered and controlled flight on another planet. Originally intended for up to five flights, Ingenuity completed dozens of sorties before being retired in 2024.
Powered by a radioisotope thermoelectric generator, Perseverance has an expected mission duration of over a decade. It has provided high-resolution panoramas, drilled and cached samples for later retrieval, and identified rocks which may have been habitable for ancient microbial life in Jezero Crater. In July 2024, it discovered the Cheyava Falls rock containing "possible biosignature." As of 29 August 2026, Perseverance has been active on Mars for 1964 sols (2018 total days; 5 years, 192 days) since its landing.
Curiosity is a Mars rover that is exploring Gale crater and Mount Sharp on Mars as part of NASA's Mars Science Laboratory (MSL) mission. Launched in 2011 and landed the following year, the car-sized rover continues to operate more than a decade after its original two-year mission.
Curiosity was launched from Cape Canaveral, Florida, on November 26, 2011, at 15:02:00 UTC and landed on Aeolis Palus inside Gale crater on Mars on August 6, 2012, 05:17:57 UTC. The Bradbury Landing site was less than 2.4 km (1.5 mi) from the center of the rover's touchdown target after a 560 million km (350 million mi) journey.
Mission goals include an investigation of the Martian climate and geology, an assessment of whether the selected field site inside Gale has ever offered environmental conditions favorable for microbial life (including investigation of the role of water), and planetary habitability studies in preparation for human exploration.
In December 2012, Curiosity's two-year mission was extended indefinitely. On August 6, 2022, a detailed overview of accomplishments by the Curiosity rover for the last ten years was reported. The rover is still operational, and as of August 30, 2026, Curiosity has been active on Mars for 5000 sols (5137 total days; 14 years, 24 days) since its landing (see current status).
The NASA/JPL Mars Science Laboratory/Curiosity Project Team was awarded the 2012 Robert J. Collier Trophy by the National Aeronautic Association "In recognition of the extraordinary achievements of successfully landing Curiosity on Mars, advancing the nation's technological and engineering capabilities, and significantly improving humanity's understanding of ancient Martian habitable environments." Curiosity's rover design served as the basis for NASA's 2021 Perseverance mission, which carries different scientific instruments.
The Sojourner rover was a vehicle that reached Mars on July 4, 1997 as part of the Mars Pathfinder mission. Sojourner was operational on Mars for 92 sols (95 Earth days), and was the first wheeled vehicle to operate on an astronomical object other than the Earth or Moon. The landing site was in the Ares Vallis channel in the Chryse Planitia region of the Oxia Palus quadrangle.
The rover was equipped with front and rear cameras, and hardware that was used to conduct several scientific experiments. It was designed for a mission ending 8 sols, with a possible extension to 30 sols, and was active for 83 sols (85 Earth days). The rover communicated with Earth through the Pathfinder base station, which had its last successful communication session with Earth at 3:23 a.m. PDT on September 27, 1997. The last signal from the rover was received on the morning of October 7, 1997.
Sojourner traveled just over 100 meters (330 ft) by the time communication was lost. Its final confirmed command was to remain stationary until October 5, 1997, (sol 91) and then drive around the lander; there is no indication it was able to do so. The Sojourner mission formally ended on March 10, 1998, after all further options were exhausted.
Mariner 2 (Mariner-Venus 1962), an American space probe to Venus, was the first robotic space probe to report successfully from a planetary encounter. The first successful spacecraft in the NASA Mariner program, it was a simplified version of the Block I spacecraft of the Ranger program and an exact copy of Mariner 1. The missions of the Mariner 1 and 2 spacecraft are sometimes known as the Mariner R missions. Original plans called for the probes to be launched on the Atlas-Centaur, but serious developmental problems with that vehicle forced a switch to the much smaller Agena B second stage. As such, the design of the Mariner R vehicles was greatly simplified. Far less instrumentation was carried than on the Soviet Venera probes of this period—for example, forgoing a TV camera—as the Atlas-Agena B had only half as much lift capacity as the Soviet 8K78 booster. The Mariner 2 spacecraft was launched from Cape Canaveral on August 27, 1962, and passed as close as 34,773 km (21,607 mi) to Venus on December 14, 1962.
The Mariner probe consisted of a 100 cm (39 in) diameter hexagonal bus, to which solar panels, instrument booms, and antennas were attached. The scientific instruments on board the Mariner spacecraft were: two radiometers (one each for the microwave and infrared portions of the spectrum), a micrometeorite sensor, a solar plasma sensor, a charged particle sensor, and a magnetometer. These instruments were designed to measure the temperature distribution on the surface of Venus and to make basic measurements of Venus's atmosphere.
The primary mission was to receive communications from the spacecraft in the vicinity of Venus and to perform radiometric temperature measurements of the planet. A second objective was to measure the interplanetary magnetic field and charged particle environment.
En route to Venus, Mariner 2 measured the solar wind, a constant stream of charged particles flowing outwards from the Sun, confirming the measurements by Luna 1 in 1959. It also measured interplanetary dust, which turned out to be scarcer than predicted. In addition, Mariner 2 detected high-energy charged particles coming from the Sun, including several brief solar flares, as well as cosmic rays from outside the Solar System. As it flew by Venus on December 14, 1962, Mariner 2 scanned the planet with its pair of radiometers, revealing that Venus has cool clouds and an extremely hot surface.
Mariner 1 was the first spacecraft of NASA's interplanetary Mariner program, built to conduct the first American planetary flyby of Venus. Developed by Jet Propulsion Laboratory and originally planned to be a purpose-built probe launched summer 1962, Mariner 1's design was changed when the Centaur proved unavailable at that early date. Mariner 1 and its sibling spacecraft Mariner 2 were then adapted from the lighter Ranger lunar spacecraft. Mariner 1 carried a suite of experiments to determine the temperature of Venus as well to measure magnetic fields and charged particles near the planet and in interplanetary space.
Mariner 1 was launched by an Atlas-Agena rocket from Cape Canaveral's Pad 12 on July 22, 1962. Shortly after liftoff, errors in communication between the rocket and its ground-based guidance systems caused the rocket to veer off course, and it had to be destroyed by range safety. The errors were traced to a mistake in a specification of the hand-written guidance equations which were then subsequently codified in the computer program.
Dawn is a retired space probe that was launched by NASA in September 2007 with the mission of studying two of the three known protoplanets of the asteroid belt: Vesta and Ceres. In the fulfillment of that mission—the ninth in NASA's Discovery Program—Dawn entered orbit around Vesta on July 16, 2011, and completed a 14-month survey mission before leaving for Ceres in late 2012. It entered orbit around Ceres on March 6, 2015. In 2017, NASA announced that the planned nine-year mission would be extended until the probe's hydrazine fuel supply was depleted. On November 1, 2018, NASA announced that Dawn had depleted its hydrazine, and the mission was ended. The derelict probe remains in a stable orbit around Ceres.
Dawn is the first spacecraft to have orbited two extraterrestrial bodies, the first spacecraft to have visited either Vesta or Ceres, and the first to have orbited a dwarf planet.
The Dawn mission was managed by NASA's Jet Propulsion Laboratory, with spacecraft components contributed by European partners from Italy, Germany, France, and the Netherlands. It was the first NASA exploratory mission to use ion propulsion, which enabled it to enter and leave the orbit of two celestial bodies. Previous multi-target missions using rockets powered by chemical engines, such as the Voyager program, were restricted to flybys.
The Mars Reconnaissance Orbiter (MRO) is a spacecraft designed to search for the existence of water on Mars and provide support for missions to Mars, as part of NASA's Mars Exploration Program. It was launched from Cape Canaveral on August 12, 2005, at 11:43 UTC and reached Mars on March 10, 2006, at 21:24 UTC. In November 2006, after six months of aerobraking, it entered its final science orbit and began its primary science phase.
Mission objectives include observing the climate of Mars, investigating geologic forces, providing reconnaissance of future landing sites, and relaying data from surface missions back to Earth. To support these objectives, the MRO carries different scientific instruments, including three cameras, two spectrometers and a subsurface radar. As of July 29, 2023, the MRO has returned over 450 terabits of data, helped choose safe landing sites for NASA's Mars landers, and discovered pure water ice in new craters and further evidence that water once flowed on the surface on Mars.
The spacecraft continues to operate at Mars, far beyond its intended design life. Due to its critical role as a high-speed data-relay for ground missions, NASA intends to continue the mission as long as possible, at least through the late 2020s. As of August 31, 2026, the MRO has been active at Mars for 7279 sols, or 20 years, 5 months and 21 days, and is the third longest-lived spacecraft to orbit Mars, after 2001 Mars Odyssey and Mars Express.
Mariner 10 was an American robotic space probe launched by NASA on 3 November 1973, to fly by the planets Mercury and Venus. It was the first spacecraft to visit Mercury and the first to perform a flyby of multiple planets.
Mariner 10 was launched approximately two years after Mariner 9 and was the last spacecraft in the Mariner program. (Mariner 11 and Mariner 12 were allocated to the Voyager program and redesignated Voyager 1 and Voyager 2.)
The mission objectives were to measure Mercury's environment, atmosphere, surface, and body characteristics and to make similar investigations of Venus. Secondary objectives were to perform experiments in the interplanetary medium and to obtain experience with a dual-planet gravity assist mission. Mariner 10's science team was led by Bruce C. Murray at the Jet Propulsion Laboratory.
First Gravity Assist: It was the first spacecraft to use the gravitational pull of one planet (Venus) to alter its path and reach another (Mercury).First Dual-Planet Mission: It successfully explored two planets in a single mission.First Mercury Visit: It returned the first-ever close-up images and detailed maps of Mercury's cratered, moon-like surface.Major Discoveries: It discovered a magnetic field at Mercury and observed high-speed rotation and cloud circulation patterns in Venus's upper atmosphere.
Venus Flyby: Reached Venus on February 5, 1974, passing about 5,000 km above the surface.First Mercury Flyby: Occurred on March 29, 1974.Second Mercury Flyby: Occurred on September 21, 1974.Third Mercury Flyby: Occurred on March 16, 1975.
The International Space Station (ISS)[c] is a space station in low Earth orbit (LEO). It is the product of the International Space Station program and is operated by five partner space agencies: NASA (United States), Roscosmos (Russia), ESA (Europe), JAXA (Japan), and CSA (Canada). It is the first space station built, maintained and crewed through international cooperation and the largest human spacecraft ever constructed. It is an orbital research station, where scientific experiments in microgravity are conducted and the space environment is studied. Since 2 November 2000, it has hosted the longest continuous human presence in space. Alongside China's Tiangong, it is one of two currently operational space stations.
The station orbits between 51.64° north and south, at about 400 kilometres (250 miles) above Earth, below the Van Allen radiation belts and most space debris. Its orbit takes it at 7.67 km/s (27,600 km/h; 17,200 mph) roughly every 93 minutes around Earth, 15.5 times a day. Measuring 109 m (358 ft) (with solar arrays) by 73 m (239 ft), it is as large as a full-sized football or soccer field, and has a pressurised internal volume of 1,005 m3 (35,491 ft3), comparable to a Boeing 747 airliner.
The station is a modular space station divided into two main sections: the Russian Orbital Segment (ROS), developed by Roscosmos, and the US Orbital Segment (USOS), built by NASA, ESA, JAXA, and CSA. The Integrated Truss Structure connects the station's vast system of solar panels and radiators to its 16 major pressurized modules. These modules support scientific research, crew habitation, storage, spacecraft control, and airlock operations. The ISS has eight docking and berthing ports for visiting spacecraft. In total, the station consists of 43 different modules and elements. Crews visit via the Soyuz and Crew Dragon spacecraft, and previously the Space Shuttle.[d] Cargo supply craft include Progress, Cargo Dragon, Cygnus, Automated Transfer Vehicle, and HTV-X.
The ISS is the political product of the development of international cooperation in space throughout the space age. The station combines two previously planned crewed Earth-orbiting stations: the United States' Space Station Freedom and the Soviet Union's Mir-2. The first ISS module was launched in 1998, with major components delivered by Proton, Soyuz and Space Shuttle launch vehicles. Long-term occupancy began with the arrival of the Expedition 1 crew on 2 November 2000. Since then, the ISS has remained continuously inhabited for 25 years and 301 days, the longest continuous human presence in space. As of August 2025, 290 individuals from 26 countries had visited the station.
Future plans for the ISS include the addition of at least one module, the Payload Power Thermal Module by Axiom Space, forming the commercial segment of the station. The station is expected to remain operational until the end of 2030, by which parts of it are to be used for Axiom Station and the Russian Orbital Service Station. After this the ISS is planned to be de-orbited using the US Deorbit Vehicle, but critique of this plan and the proposal of parking the station at a more stable higher orbit has gathered congressional support as of 2026.
