The exploration of Neptune has only begun with one spacecraft, Voyager 2 in 1989. 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. Neptune has also been scientifically studied from afar with telescopes, primarily since the mid 1990s. This includes the Hubble Space Telescope but most importantly ground-based telescopes using adaptive optics.
''Voyager 2''
After having visited Saturn successfully, it was decided to continue and fund further missions by Voyager 2 to Uranus and Neptune. These missions were conducted by the Jet Propulsion Laboratory and the Neptunian mission was dubbed "Voyager Neptune Interstellar Mission". Voyager 2 started taking navigation images of Neptune in May 1988. Voyager 2 observation phase proper of Neptune began 5 June 1989, the spacecraft officially reached the Neptunian system on 25 August and the data collection stopped on 2 October. Initially it was planned to use a trajectory that resulted in Voyager 2 passing around from Neptune and from Triton. The need to avoid ring material detected by stellar occultations resulted in this trajectory being abandoned and a trajectory that largely avoided the rings but resulted in more distant flybys of both targets. On 25 August, in Voyager 2 last planetary encounter, the spacecraft swooped only above Neptune's north pole, the closest approach it made to any body since it left Earth in 1977. When the spacecraft visited the Neptunian system, Neptune was the farthest known body in the Solar System. It was not until 1999, that Pluto was further from the Sun in its trajectory. Voyager 2 studied Neptune's atmosphere, Neptune's rings, its magnetosphere, and Neptune's moons. The Neptunian system had been studied scientifically for many years with telescopes and indirect methods, but the close inspection by the Voyager 2 probe settled many issues and revealed a plethora of information that could not have been obtained otherwise. The data from Voyager 2 are still the best data available on this planet in most cases. The exploration mission revealed that Neptune's atmosphere is very dynamic, even though it receives only 3% of the sunlight that Jupiter receives. Winds on Neptune were found to be the strongest in the Solar System, up to three times stronger than Jupiter's and nine times stronger than the strongest winds on Earth. Most winds blew westward, opposite the planets rotation. Separate cloud decks were discovered, with cloud systems emerging and dissolving within hours and giant storms circling the entire planet in 16–18 hours in the upper layers. Voyager 2 discovered an anticyclone dubbed the Great Dark Spot, similar to Jupiter's Great Red Spot. However, images taken by the Hubble Space Telescope in 1994, revealed that the Great Dark Spot had disappeared. Also seen in Neptune's upper 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. Neptune's magnetosphere was also studied by Voyager 2. The magnetic field was found to be highly tilted and largely offset from the planet's centre. The probe discovered auroras, but much weaker than those on Earth or other planets. The radio instruments on board found that Neptune's day lasts 16 hours and 6.7 minutes. Neptune's rings had been observed from Earth many years prior to Voyager 2 visit, but the close inspection revealed that the ring systems were full circle and intact, and a total of four rings were counted. Voyager 2 discovered six new small moons orbiting Neptune's equatorial plane, and they were named Naiad, Thalassa, Despina, Galatea, Larissa and Proteus. Only three of Neptune's moons were photographed in detail: Proteus, Nereid, and Triton; of which the last two were the only Neptunian moons known prior to the visit. Proteus turned out to be an ellipsoid, as large as gravity allows an ellipsoid body to become without rounding into a sphere. It appeared very dark in color, almost like soot. Nereid, though discovered in 1949, still has very little known about it. Triton was revealed as having a remarkably active past. Active geysers and polar caps were discovered and a very thin atmosphere as well, with thin clouds of what is thought to be nitrogen ice particles. At just, it is the coldest known planetary body in the Solar System. The closest approach to Triton was about and it was the last solid world that Voyager 2 explored close by. A list of previous and upcoming missions to the outer Solar System can be found at the List of missions to the outer planets article.
Possible future missions
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. After the Voyagerflyby, 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. NASA has researched several other project options for both flyby and orbiter missions. These missions are often collectively called "RMA Neptune-Triton-KBO" missions, which also includes orbital missions that would not visit Kuiper belt objects. Because of budgetary constraints, technological considerations, scientific priorities and other factors, none of these have been approved. Specific exploration missions to Neptune include:
Argo — A cancelled mission concept in the New Frontiers program, a flyby mission to visit Jupiter, Saturn, Neptune and the Kuiper belt with launch in 2019.
ODINUS — A 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.
Triton Hopper — A NIAC study of a mission to Neptune with the goal of landing, and flying from site to site, on Neptune's moon Triton.
Trident — A finalist in the Discovery program, would perform a single flyby of Neptune in 2038 and closely study its largest moon Triton.
The lowest energy trajectory for a launch from Earth to Neptune uses a Jupiter gravity assist, opening an optimal launch window with a 12-year interval, when Jupiter is in a favourable position relative to the Earth and Neptune. An optimal launch window was open for such a Neptune mission from 2014 to 2019, with the next opportunity occurring from 2031. These constraints are based on the requirement of a gravity assist from Jupiter. With the new Space Launch System technology in development at Boeing, deep space missions with heavier payloads could potentially be propelled at much greater speeds and missions to the outer planets could be launched independently of gravity assistance.
Space telescopes such as the Hubble Space Telescope have signified a new era of detailed observations of faint objects from afar, across the entire electromagnetic spectrum. This includes faint objects in the Solar system, such as Neptune. Since 1997, adaptive optics technology has also allowed for detailed scientific observations of Neptune and its atmosphere from ground-based telescopes. These image recordings now exceed the capability of HST by far and in some instances even the Voyager images, such as Uranus. Ground-based observations are however always limited in their registration of electromagnetic waves of certain wavelengths, due to the inevitable atmospheric absorption, in particular of high energy waves.
