Exploration of Neptune
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The 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. Because Neptune is a gas giant and hence has no solid surface, a surface mission such as a lander or rover is impossible.
In Voyager 2's last planetary encounter, the spacecraft swooped only 3,000 miles above Neptune's north pole, the closest approach it made to any body since it left Earth. Voyager 2 studied Neptune's atmosphere, Neptune's rings, its magnetosphere, and Neptune's moons. Several discoveries were made, including the discovery of the Great Dark Spot and Triton's geysers.
Voyager 2 revealed that Neptune's atmosphere was very dynamic, even though it receives only 3% of the sunlight Jupiter receives. Voyager 2 discovered an anticyclone called the Great Dark Spot, similar to Jupiter's Great Red Spot and Little Red Spot. However, images taken by the Hubble Space Telescope revealed that the Great Dark Spot had disappeared. Also seen in Neptune's atmosphere at that time was an almond-shaped spot designated D2, and a bright, quickly moving cloud high above the cloud decks dubbed "Scooter".
Voyager 2 found four rings and evidence for ring arcs, or incomplete rings above Neptune. Neptune's magnetosphere was also studied by Voyager 2. The planetary radio astronomy instrument found that Neptune's day lasts sixteen hours, seven minutes. Voyager 2 also discovered auroras, like on Earth, but much more complex.
Voyager 2 discovered six moons orbiting Neptune, but only three were photographed in detail: Proteus, Nereid, and Triton. Proteus turned out to be an ellipsoid, as large as an ellipsoid could become without rounding in a sphere. Proteus is very dark in color, almost like soot.
Nereid, though discovered in 1949, still has very little known about it. Triton was flown by at about 25,000 miles away, and became the last solid world Voyager 2 would explore within the Solar System. Triton was revealed to have remarkable active geysers and polar caps. A very thin atmosphere was found, as well as thin clouds.
Planned future missions
It is known that any future mission will have radioisotope thermoelectric generators and similar instrumentation to Voyager craft, but with more cameras and storage capacity; and will need better error correction than Voyager.