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The outer planets are those planets in the Solar System beyond the asteroid belt, and hence refers to the gas giants, which are in order of their distance from the Sun: * Jupiter is the largest planet in the Solar System. It has four very large satellites. * Saturn is the second-largest planet, with a large and bright ring system. * Uranus is the third-largest planet and the least massive of the four outer planets. It is tilted almost onto the plane of its orbit. * Neptune is smallest of the four outer planets. It has one big retrograde moon and many small ones. The 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 and Saturn's Titan). That pair and Io, Callisto, Europa, and Triton, are larger than Pluto and Eris. This region of space is also occupied by centaurs, various fields of trojans, and many comets. Pluto was considered to be an outer planet from its discovery in 1930 until its reclassification as a dwarf planet in 2006 ''(see also Kuiper belt)''. == Exploration == ''In situ'' exploration by spacecraft includes ''Pioneer 10'', ''Pioneer 11'', ''Voyager 1'', ''Voyager 2'', ''Ulysses'', ''Galileo'', ''Cassini–Huygens'', and ''New Horizons''. Planned missions include ''Juno'' Jupiter Orbiter and possibly the Outer Planet Flagship Missions; there are various proposals too, such as Uranus orbiter and probe. 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. One of the breakthroughs that made exploration of the outer planets much easier, was the concept of the planetary gravity assist.〔(Basics of Spaceflight - Chapter 4. Interplanetary Trajectories )〕 Discovered in the 1960s, a spacecraft approaches a planet like Jupiter in such a way as to be accelerated to a higher speed. This allows a much smaller rocket to be used for a given launch.〔 Another promising technology, tested on ''Deep Space 1'', is the ion engine. Ion engines can make much more efficient use of propellant than existing chemical rockets.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Outer planets」の詳細全文を読む スポンサード リンク
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