Exploration of Neptune
The exploration of Neptune has only begun with one spacecraft, Voyager 2, which visited on August 25, 1989. Currently there are no approved future missions to visit the Neptunian system. NASA, ESA and also independent academic groups have proposed future scientific missions to visit Neptune. Some mission plans are still active, while others have been abandoned or put on hold.
In Voyager 2's last planetary encounter, the spacecraft swooped only 3,000 miles (4,800 km) 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 is very dynamic[clarification needed], even though it receives only 3% of the sunlight that Jupiter receives. Voyager 2 discovered an anticyclone called the Great Dark Spot, similar to Jupiter's Great 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".
The fly-by of the Neptunian system provided the first accurate measurement of Neptune's mass which was found to be 0.5 percent less than previously calculated. The new figure disproved the hypothesis that an undiscovered Planet X acted upon the orbits of Neptune and Uranus.
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 16 hours and 7 minutes. Voyager 2 also discovered auroras, like those 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 into a sphere. Proteus is very dark in color, almost like soot.
Nereid, though discovered in 1949, still has very little known about it. The probe flew past Triton about 25,000 miles (40,000 km) away, and was the last solid world Voyager 2 explored 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.
Currently there are no approved future missions to visit the Neptunian system. NASA, ESA and independent academic groups have proposed and worked on future scientific missions to visit Neptune. Some mission plans are still active, while others have been abandoned or put on hold. Examples are:
After the Voyager flyby, NASA's next step in scientific exploration of the Neptune system is considered to be a flagship orbiter mission. Such a hypothetical mission is envisioned to be possible in the late 2020s or early 2030s. Another one proposed for the 2040s is called the Neptune-Triton Explorer (NTE). NASA has researched several other project options for both flyby and orbiter missions (of similar design as the Cassini–Huygens mission to Saturn). These missions are often collectively called "RMA Neptune-Triton-KBO" missions, which also includes orbital missions that would not visit Kuiper belt objects (KBOs). Because of budgetary constraints, technological considerations, scientific priorities and other factors, none of these have been approved. Mission examples include:
- Neptune Orbiter mission — An orbiting mission concept focusing on Neptune and Triton.
- Argo — A mission concept New Frontiers flyby mission to visit Jupiter, Saturn, Neptune (with Triton) and the Kuiper belt with launch in 2019.
- ODINUS — An mission concept based on a twin spacecraft mission to investigate the Neptunian and Uranian systems. Launch date would be 2034.
- OSS mission — A proposed collaborative flyby mission by ESA and NASA. Its main focus would be to map the gravitational fields in deep space, including the Outer Solar System (up to 50 AU).
Direct flyby missions to Neptune are only preferable in window gaps with a 12-year interval, due to the complexity of planetary positions. There is currently a window open for launching a Neptune mission from 2014 to 2019, with the next opportunity occurring from 2031. These constraints are based on current rocket technology which relies on gravity assists from Jupiter and Saturn. With the new Space Launch System (SLS) technology in development at Boeing, deep space missions with heavier payloads can be propelled at much greater speeds (200 AU in 15 years) and missions to the outer planets could be launched independently of gravity assistance.
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