Outer planets

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From top to bottom: Neptune, Uranus, Saturn, and Jupiter to approximate scale and color.

The outer planets are those planets in the Solar System beyond the asteroid belt, and hence refers to the gas giants and ice 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 (moons).
  • 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.[1] It is tilted almost onto the plane of its orbit.
  • Neptune is the fourth-largest planet, as smallest of the four outer planets, but third-most massive. It has one big retrograde moon and many small ones.

The outer planets all have ring systems, although all but Saturn's are faint when viewed from Earth.[1]

Another aspect common to the gas giants is their many natural satellites (moons), 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).


The Galileo Probe plunged deep into Jupiter in 1995. It was carried to the Jovian system by the Galileo spacecraft, where it was released and survived what was then the highest-velocity atmospheric entry yet attempted.

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.[2] 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.[2]

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.[2]

Spacecraft Exploration Summary
Spacecraft Launch
Jupiter Saturn Uranus Neptune End
Pioneer 10 1972 flyby 2003
Pioneer 11 1973 flyby flyby 1995
Voyager 1 1977 flyby flyby
Voyager 2 1977 flyby flyby flyby flyby
Galileo 1989 orbiter 2003
Galileo Probe 1989 entry 1995
Ulysses 1990 flyby 2009
Cassini 1997 flyby orbiter 2017
New Horizons 2006 flyby
Juno 2011 orbiter

Mission concepts[edit]

Real image of the northern hemisphere of Saturn, as taken by the Cassini–Huygens mission
Jupiter by Voyager 1 on February 25, 1979

List of mission concepts:

  • Argo[3] - Jupiter, Saturn, Neptune, Kuiper belt flyby
  • AVIATR (Titan Airplane)[4]
  • Chiron Orbiter Mission Concept Study[5]
  • Enceladus Flyby & Sample Return Concept Studies[5]
  • Enceladus Orbiter Concept Study[5]
  • Europa Flyby[6]
  • Europa Flyby Study[7]
  • Europa Lander[6]
  • Europa Lander Study[7]
  • Europa Orbiter[6]
  • Exploration of Jovian Atmosphere Using Nuclear Ramjet Flyer[8]
  • Ganymede Orbiter Concept Study[5]
  • Io Observer Concept Study[5]
  • Journey to Enceladus and Titan (JET)[9]
  • Jupiter Europa Orbiter Study[5]
  • Life Investigation For Enceladus (LIFE) - flyby sample return for Enceladus and Saturn's E-ring.[10]
  • Neptune-Triton-Kuiper Belt Objects Mission Concept Study[5]
  • Saturn Atmospheric Entry Probe Study[5]
  • Saturn Ring Observer Concept Study[5]
  • Titan Lake Probe Concept Study[5]
  • Titan Saturn System Mission[5]
  • Titan Rover[11]
  • Trojan Tour Concept Study[5]
  • Uranus and Neptune Orbiter and Probe Concept Studies[5]
  • Uranus orbiter and probe
  • Uranus Pathfinder[12]

See also[edit]


  1. ^ a b "How big is Neptune? - space.com". Space.com. 2015. Retrieved 2016-04-04. The 2nd-largest planet is Saturn, 3rd-largest is Uranus, 4th-largest is Neptune, while 2nd-most massive planet is Saturn, 3rd-most massive is Neptune, but 4th-most massive is Uranus.
  2. ^ a b c Basics of Spaceflight - Chapter 4. Interplanetary Trajectories
  3. ^ Argo - A Voyage Through the Outer Solar System
  4. ^ AVIATR
  5. ^ a b c d e f g h i j k l m Planetary Science Decadal Survey Mission & Technology Studies
  6. ^ a b c March 2012 OPAG Meeting
  7. ^ a b OPAG
  8. ^ G. Maise - Exploration of Jovian Atmosphere Using Nuclear Ramjet Flyer
  9. ^ JET - Cost Capped Titan-Enceladus Proposal
  10. ^ Tsou, et al. - LIFE link courtesy of Future Planetary Exploration
  11. ^ ARPS Enabled Titan Rover Concept with Inflatable Wheels Archived 2010-02-06 at the Wayback Machine
  12. ^ Uranus Pathfinder

Further reading[edit]

External links[edit]