Solar wind

solar windslosessolar
The Project Apollo missions deployed passive aluminum collectors in an attempt to sample the solar wind, and lunar soil returned for study confirmed that the lunar regolith is enriched in atomic nuclei deposited from the solar wind. These elements may prove useful resources for lunar colonies. The solar wind "blows a bubble" in the interstellar medium (the rarefied hydrogen and helium gas that permeates the galaxy). The point where the solar wind's strength is no longer great enough to push back the interstellar medium is known as the heliopause and is often considered to be the outer border of the Solar System.

Astronomical unit

AUastronomical unitsAUs
He then measured the apparent sizes of the Sun and the Moon and concluded that the apparent diameter of the Sun was equal to the apparent diameter of the Moon at the Moon's greatest distance, and from records of lunar eclipses, he estimated this apparent diameter, as well as the apparent diameter of the shadow cone of Earth traversed by the Moon during a lunar eclipse. Given these data, the distance of the Sun from Earth can be trigonometrically computed to be 1210 Earth radii. This gives a ratio of solar to lunar distance of approximately 19, matching Aristarchus's figure.

Hubble Space Telescope

HubbleHSTHubble Telescope
HST has also been used to study objects in the outer reaches of the Solar System, including the dwarf planets Pluto and Eris. The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 was fortuitously timed for astronomers, coming just a few months after Servicing Mission1 had restored Hubble's optical performance. Hubble images of the planet were sharper than any taken since the passage of Voyager 2 in 1979, and were crucial in studying the dynamics of the collision of a comet with Jupiter, an event believed to occur once every few centuries. During June and July 2012, US astronomers using Hubble discovered a tiny fifth moon orbiting around Pluto.

Timeline of discovery of Solar System planets and their moons

Timeline of discovery of Solar System planets and their natural satellitesdiscoversSun, the Moon, the five known "wanderers" (planets)
Timeline of Solar System astronomy. Timeline of Solar System exploration. Solar System. City of Hudson's Natural Satellite Page. Scott Sheppard's Giant Planet Satellite Page. JPL Natural Satellite Discovery Data. James L. Hilton, When did the asteroids become minor planets?.

Planetary habitability

habitabilityhabitablehabitable planet
Early in the Solar System's history, Jupiter is accepted as having played an important role in the hydration of our planet: it increased the eccentricity of asteroid belt orbits and enabled many to cross Earth's orbit and supply the planet with important volatiles such as water and carbon dioxide. Before Earth reached half its present mass, icy bodies from the Jupiter–Saturn region and small bodies from the primordial asteroid belt supplied water to the Earth due to the gravitational scattering of Jupiter and, to a lesser extent, Saturn. Thus, while the gas giants are now helpful protectors, they were once suppliers of critical habitability material.

Name conflicts with minor planets

the same namesname conflictsName conflicts of solar system objects
Among Jupiter's moons, two are named after the goddess Ersa (the Greek goddess of dew) – Herse, a moon of Jupiter discovered 2003, and Ersa, a moon of Jupiter discovered 2018. There was also a potential conflict between Hades, an unofficial name for Sinope, a moon of Jupiter between 1955 and 1975, and Pluto, discovered 1930. Vulcan, a hypothetical planet once proposed to exist in an orbit between Mercury and the Sun, and 2212 Hephaistos, discovered 1978. 4 Vesta, discovered 1807 and 46 Hestia, discovered 1857. Hestia was also an unofficial name for Jupiter's moon Himalia 1955-1975.

Mantle (geology)

mantleEarth's mantlemantles
The silicate mantle of the Earth's moon is approximately 1300–1400 km thick, and is the source of mare basalts. The lunar mantle might possibly be exposed in the South Pole-Aitken basin and/or the Crisium basin. The lunar mantle contains a seismic discontinuity at ~500 km depth, most likely related to a change in composition. Titan and Triton each have a mantle made of ice or other solid volatile substances. Some of the largest asteroids have mantles; for example, Vesta has a silicate mantle similar in composition to diogenite meteorites. * Donald L.


orbitsorbital motionplanetary motion
Mercury, the smallest planet in the Solar System, has the most eccentric orbit. At the present epoch, Mars has the next largest eccentricity while the smallest orbital eccentricities are seen with Venus and Neptune. As two objects orbit each other, the periapsis is that point at which the two objects are closest to each other and the apoapsis is that point at which they are the farthest. (More specific terms are used for specific bodies. For example, perigee and apogee are the lowest and highest parts of an orbit around Earth, while perihelion and aphelion are the closest and farthest points of an orbit around the Sun.)

Retrograde and prograde motion

retrogradeprograderetrograde orbit
This is an irregular moon. In the Solar System, many of the asteroid-sized moons have retrograde orbits, whereas all the large moons except Triton (the largest of Neptune's moons) have prograde orbits. The particles in Saturn's Phoebe ring are thought to have a retrograde orbit because they originate from the irregular moon Phoebe. All retrograde satellites experience tidal deceleration to some degree. The only satellite in the Solar System for which this effect is non-negligible is Neptune's moon Triton. All the other retrograde satellites are on distant orbits and tidal forces between them and the planet are negligible.

Orders of magnitude (length)

mm100 nmGm
Proteus, the second largest moon of Neptune. 468 km – diameter of the asteroid 4 Vesta. 472 km – diameter of Miranda, one of Uranus' major moons. 974.6 km – greatest diameter of 1 Ceres, the largest solar system asteroid. 1 E+6 m (one million metres). approximately 621.37 miles.

Geology of solar terrestrial planets

AstrogeologyLobate ScarpsPlanetary geologists
Olympus Mons, the largest volcano in the known Universe, formed during this period along with lava flows elsewhere on Mars. Lunar soil. Martian soil. Water on terrestrial planets. International Astronomical Union. Solar System Live (an interactive orrery). Solar System Viewer (animation). Pictures of the Solar System. Renderings of the planets. NASA Planet Quest. Illustration comparing the sizes of the planets with each other, the sun, and other stars. Q&A: The IAU's Proposed Planet Definition. Q&A New planets proposal. Solar system – About Space. Atlas of Mercury – NASA. Nine Planets Information. NASA’s fact sheet. Planetary Science Research Discoveries.

List of uncrewed NASA missions

Galileo was an uncrewed spacecraft sent by NASA to study the planet Jupiter and its moons. It was launched on October 18, 1989, by the Space Shuttle Atlantis on the STS-34 mission. It arrived at Jupiter on December 7, 1995, via gravitational assist flybys of Venus and Earth. Despite antenna problems, Galileo conducted the first asteroid flyby, discovered the first asteroid moon, was the first spacecraft to orbit Jupiter, and launched the first probe into Jupiter's atmosphere. Galileo's prime mission was a two-year study of the Jovian system. The spacecraft traveled around Jupiter in elongated ellipses, each orbit lasting about two months.

Planetary mass

The distinction is very slight, as natural satellites are much smaller than their parent planets (as can be seen in the table above, where only the largest satellites are even listed). The Earth and the Moon form a case in point, partly because the Moon is unusually large (just over 1% of the mass of the Earth) in relation to its parent planet compared with other natural satellites. There are also very precise data available for the Earth–Moon system, particularly from the Lunar Laser Ranging Experiment (LLR).

2018 in science

This could imply that most planetary systems form differently from the Solar System. Analysis of the stone Hypatia shows it has a different origin than the planets and known asteroids. Parts of it could be older than the solar system.

Triton (moon)

TritonAndvari Triton
Triton is the largest natural satellite of the planet Neptune, and the first Neptunian moon to be discovered. The discovery was made on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planet's rotation. At 2710 km in diameter, it is the seventh-largest moon in the Solar System, the only satellite of Neptune massive enough to be in hydrostatic equilibrium and the second-largest planetary moon in relation to its primary, after Earth's Moon.


HH 2 hydrogen gas
This ion has also been observed in the upper atmosphere of the planet Jupiter. The ion is relatively stable in the environment of outer space due to the low temperature and density. is one of the most abundant ions in the Universe, and it plays a notable role in the chemistry of the interstellar medium. Neutral triatomic hydrogen H 3 can exist only in an excited form and is unstable.

Charon (moon)

CharonPluto I Charon(134340) Pluto I Charon
Had the draft proposal been accepted, even the Moon would be classified as a planet in billions of years when the tidal acceleration that is gradually moving the Moon away from Earth takes it far enough away that the center of mass of the system no longer lies within Earth. The other moons of Pluto–Nix, Hydra, Kerberos and Styx–orbit the same barycenter but they are not large enough to be spherical and they are simply considered to be satellites of Pluto (or of Pluto–Charon). *List of natural satellites * Charon Profile at NASA's Solar System Exploration site. Hubble reveals new map of Pluto, BBC News, September 12, 2005. Cryovolcanism on Charon and other Kuiper Belt Objects.


Ceres appears to be differentiated into a rocky core and icy mantle and may harbour a liquid-water ocean under its surface. Not enough is known of the larger trans-Neptunian objects to determine whether they are differentiated bodies capable of supporting oceans, although models of radioactive decay suggest that Pluto, Eris, Sedna, and Orcus have oceans beneath solid icy crusts approximately 100 to 180 km thick. Some planets and natural satellites outside the Solar System are likely to have oceans, including possible water ocean planets similar to Earth in the habitable zone or "liquid-water belt".


Enceladus – potential habitabilityEnceldausfountains of frozen particles erupting from Enceladus
Enceladus is the sixth-largest moon of Saturn. It is about 500 km in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System. Consequently, its surface temperature at noon only reaches -198 °C, far colder than a light-absorbing body would be. Despite its small size, Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrains.

Conjunction (astronomy)

conjunctionconjunctionsinferior conjunction
As a consequence, over the period 1–4 December 1899, the Moon reached conjunction with, in order, Jupiter, Uranus, the Sun, Mercury, Mars, Saturn and Venus. Most of these conjunctions were not visible because of the glare of the Sun. Over the period 4–6 February 1962, in a rare series of events, Mercury and Venus reached conjunction as observed from the Earth, followed by Venus and Jupiter, then by Mars and Saturn. Conjunctions took place between the Moon and, in turn, Mars, Saturn, the Sun, Mercury, Venus and Jupiter. Mercury also reached inferior conjunction with the Sun.


CassiniCassini-HuygensCassini spacecraft
Scientists have also determined that the storm witnessed was the largest, hottest stratospheric vortex ever detected in the Solar System, initially being larger than Jupiter's Great Red Spot. On December 21, 2012, Cassini observed a transit of Venus across the Sun. The VIMS instrument analyzed sunlight passing through the Venusian atmosphere. VIMS previously observed the transit of exoplanet HD 189733 b. On July 19, 2013, the probe was pointed towards Earth to capture an image of the Earth and the Moon, as part of a natural light, multi-image portrait of the entire Saturn system.

Titius–Bode law

Titius-Bode lawBode's lawTitius Bode
In it, he says "... supposing the distance of the Earth from the Sun to be divided into ten equal Parts, of these the distance of Mercury will be about four, of Venus seven, of Mars fifteen, of Jupiter fifty two, and that of Saturn ninety five." A similar sentence, likely paraphrased from Gregory, appears in a work published by Christian Wolff in 1724. In 1764, Charles Bonnet said in his Contemplation de la Nature that, "We know seventeen planets that enter into the composition of our solar system [that is, major planets and their satellites]; but we are not sure that there are no more."

4 Vesta

Vesta(4) VestaAsteroidal Gravity Optical and Radar Analysis
Vesta is currently one of only six identified Solar System bodies of which we have physical samples, coming from a number of meteorites suspected to be Vestan fragments. It is estimated that 1 out of 16 meteorites originated from Vesta. The other identified Solar System samples are from Earth itself, meteorites from Mars, meteorites from the Moon, and samples returned from the Moon, the comet Wild 2, and the asteroid 25143 Itokawa. In 1981, a proposal for an asteroid mission was submitted to the European Space Agency (ESA). Named the Asteroidal Gravity Optical and Radar Analysis (AGORA), this spacecraft was to launch some time in 1990–1994 and perform two flybys of large asteroids.


water iceicyglacier ice
In the Solar System, ice is abundant and occurs naturally from as close to the Sun as Mercury to as far away as the Oort cloud objects. Beyond the Solar System, it occurs as interstellar ice. It is abundant on Earth's surface – particularly in the polar regions and above the snow line – and, as a common form of precipitation and deposition, plays a key role in Earth's water cycle and climate. It falls as snowflakes and hail or occurs as frost, icicles or ice spikes. Ice molecules can exhibit eighteen or more different phases (packing geometries) that depend on temperature and pressure.

Rhea (moon)

Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System. It is the second smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium, after dwarf planet Ceres. It was discovered in 1672 by Giovanni Domenico Cassini. Rhea was discovered by Giovanni Domenico Cassini on 23 December 1672. It was the second moon of Saturn that Cassini discovered, and the third moon discovered around Saturn overall. Rhea is named after the Titan Rhea of Greek mythology, the "mother of the gods".