- 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.
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. 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.
List of mission concepts:
- Argo - Jupiter, Saturn, Neptune, Kuiper belt flyby
- AVIATR (Titan Airplane)
- Chiron Orbiter Mission Concept Study
- Enceladus Flyby & Sample Return Concept Studies
- Enceladus Orbiter Concept Study
- Europa Flyby
- Europa Flyby Study
- Europa Lander
- Europa Lander Study
- Europa Orbiter
- Exploration of Jovian Atmosphere Using Nuclear Ramjet Flyer
- Ganymede Orbiter Concept Study
- Io Observer Concept Study
- Journey to Enceladus and Titan (JET)
- Jupiter Europa Orbiter Study
- Life Investigation For Enceladus (LIFE) - flyby sample return for Enceladus and Saturn's E-ring.
- Neptune-Triton-Kuiper Belt Objects Mission Concept Study
- Saturn Atmospheric Entry Probe Study
- Saturn Ring Observer Concept Study
- Titan Lake Probe Concept Study
- Titan Saturn System Mission
- Titan Rover
- Trojan Tour Concept Study
- Uranus and Neptune Orbiter and Probe Concept Studies
- Uranus orbiter and probe
- Uranus Pathfinder
- Discovery and exploration of the Solar System
- Giant planets of the Solar System:
- Kuiper belt
- List of missions to the outer planets
- Oort cloud
- Basics of Spaceflight - Chapter 4. Interplanetary Trajectories
- Argo - A Voyage Through the Outer Solar System
- Planetary Science Decadal Survey Mission & Technology Studies
- March 2012 OPAG Meeting
- G. Maise - Exploration of Jovian Atmosphere Using Nuclear Ramjet Flyer
- JET - Cost Capped Titan-Enceladus Proposal
- Tsou, et al. - LIFE link courtesy of Future Planetary Exploration
- ARPS Enabled Titan Rover Concept with Inflatable Wheels
- Uranus Pathfinder
- Solar Power for Outer Planets Study (2007) – NASA Glenn Research Center
- Human Outer Planet Exploration (2003) – NASA Langley Research Center and Princeton University
- Future Exploration of the Outer Planet Satellites: A Decadal Perspective (2009)