Outer planets
Encyclopedia
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
beyond the asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
, which typically refers to these gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
planets in order of their distance from the Sun:
- Jupiter is the largest planet in the Solar System, with distinct bands and four very large satellites.
- Saturn is the second largest planet, with a large and bright ring system.
- Uranus was discovered in 1781. It is tilted almost onto the plane of its orbit.
- Neptune was discovered in 1846 as a result of its perturbations of Uranus.
The gas giant outer planets all have ring systems, although all but Saturn's are faint.
Another aspect common to the gas giants is their many natural satellites, two of which are larger than the planet Mercury (Jupiter's Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...
and Saturn's Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
). That pair and Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....
, Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...
, Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...
, and Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
, are larger than either Pluto or Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...
.
This region of space is also occupied by centaurs, various fields of trojans
Trojan (astronomy)
In astronomy, a Trojan is a minor planet or natural satellite that shares an orbit with a larger planet or moon, but does not collide with it because it orbits around one of the two Lagrangian points of stability , and , which lie approximately 60° ahead of and behind the larger body,...
, and many comets.
Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
had been considered as an outer planet from its discovery in 1930 until its reclassification as a dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...
in 2006 (see also Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
).
Exploration
In situIn situ
In situ is a Latin phrase which translated literally as 'In position'. It is used in many different contexts.-Aerospace:In the aerospace industry, equipment on board aircraft must be tested in situ, or in place, to confirm everything functions properly as a system. Individually, each piece may...
exploration by spacecraft includes Pioneer 10
Pioneer 10
Pioneer 10 is a 258-kilogram robotic space probe that completed the first interplanetary mission to Jupiter, and became the first spacecraft to achieve escape velocity from the Solar System. The project was managed by the NASA Ames Research Center and the contract for the construction of the...
, Pioneer 11
Pioneer 11
Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere...
, Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
, Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
, Ulysses, Galileo, Cassini–Huygens, and New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...
. Planned missions include Juno Jupiter Orbiter
Juno (spacecraft)
Juno is a NASA New Frontiers mission to the planet Jupiter. Juno was launched from Cape Canaveral Air Force Station on August 5, 2011. The spacecraft is to be placed in a polar orbit to study the planet's composition, gravity field, magnetic field, and polar magnetosphere...
and possibly the Outer Planet Flagship Mission
Outer Planet Flagship Mission
The Outer Planet Flagship Mission is a joint mission between NASA and ESA to send a probe to study the icy satellites of the outer Solar System. There are two primary candidate missions under study: Europa Jupiter System Mission and Titan Saturn System Mission .On February 18, 2009 NASA announced...
s; there are various proposals too, such as Uranus orbiter and probe
Uranus orbiter and probe
A Uranus orbiter and probe mission to explore the planet Uranus, its atmosphere, rings, and moons, was recommended to NASA in 2011 by its . A mission study was conducted which also considered Neptune; however, for feasibility reasons Neptune was dropped in favor of Uranus. Two white papers on...
. Ongoing missions for the outer planets as of 2011 include Cassini probe, orbiting Saturn, New Horizons, headed for Pluto, and Juno, headed for Jupiter. Cassini and New Horizons also visited Jupiter with a flyby.
| Spacecraft | Launch Year | Jupiter | Saturn | Uranus | Neptune | End Year |
|---|---|---|---|---|---|---|
| Pioneer 10 Pioneer 10 Pioneer 10 is a 258-kilogram robotic space probe that completed the first interplanetary mission to Jupiter, and became the first spacecraft to achieve escape velocity from the Solar System. The project was managed by the NASA Ames Research Center and the contract for the construction of the... |
1972 | 2003 | ||||
| Pioneer 11 Pioneer 11 Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere... |
1973 | 1995 | ||||
| Voyager 1 Voyager 1 The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of... |
1977 | |||||
| Voyager 2 Voyager 2 The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space... |
1977 | |||||
| Galileo | 1989 | 2003 | ||||
| Galileo Probe | 1989 | 1995 | ||||
| Ulysses Ulysses probe Ulysses is a decommissioned robotic space probe that was designed to study the Sun as a joint venture of NASA and the European Space Agency . The spacecraft was originally named Odysseus, because of its lengthy and indirect trajectory to near Solar distance... |
1990 | 2009 | ||||
| Cassini | 1997 | |||||
| New Horizons New Horizons New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015... |
2006 | |||||
| Juno Juno (spacecraft) Juno is a NASA New Frontiers mission to the planet Jupiter. Juno was launched from Cape Canaveral Air Force Station on August 5, 2011. The spacecraft is to be placed in a polar orbit to study the planet's composition, gravity field, magnetic field, and polar magnetosphere... |
2011 |
See also
- Solar SystemSolar SystemThe Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
- Moons of Jupiter - Rings of JupiterRings of JupiterThe planet Jupiter has a system of rings, known as the rings of Jupiter or the Jovian ring system. It was the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. It was first observed in 1979 by the Voyager 1 space probe and thoroughly investigated in the 1990s...
- Exploration of JupiterExploration of JupiterThe exploration of Jupiter has to date been conducted via close observations by automated spacecraft. It began with the arrival of Pioneer 10 into the Jovian system in 1973, and, , has continued with seven further spacecraft missions... - Moons of Saturn - Rings of SaturnRings of SaturnThe rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn...
- Exploration of SaturnExploration of SaturnThe exploration of Saturn has been solely done by robotic probes. Like all gas giants, there is no solid surface for a solid probe to land on. Most missions therefore have been flybys, although the Cassini–Huygens spacecraft is currently in orbit.... - Moons of Uranus - Rings of UranusRings of UranusThe planet Uranus has a system of rings intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune. The rings of Uranus were discovered on March 10, 1977, by James L. Elliot, Edward W. Dunham, and Douglas J. Mink...
- Exploration of UranusExploration of UranusThe exploration of Uranus has been solely through NASA's Voyager 2 spacecraft, with no other visits currently planned. The closest approach to Uranus occurred on January 24, 1986. Voyager 2 discovered 10 previously unknown moons; studied the planet's cold atmosphere; and examined its ring system,... - Moons of Neptune - Rings of NeptuneRings of NeptuneThe rings of Neptune consist primarily of five principal rings predicted in 1984 by André Brahic and imaged in 1989 by the Voyager 2 spacecraft...
- Exploration of NeptuneExploration of NeptuneThe exploration of Neptune has only begun with one explorer, Voyager 2, which visited on August 25, 1989. The possibility of a Neptune Orbiter was discussed, yet other than that, no other missions have been given serious thought...
Further reading
- Scott W. Benson – Solar Power for Outer Planets Study (2007) – NASA Glenn Research Center
- Trautman and Bethke – Human Outer Planet Exploration (2003) – NASA Langley Research Center and Princeton University