With some recent news about Uranus and we made the case for exploring Uranus, it’s about time we talked about the other planet in the outer Solar System, Neptune! Voyager 2 is the only spacecraft to visit the planet as no other spacecraft have been sent out that far. This is important to note that with the recent discoveries from the Uranus data 34 years later after Voyager 2 came close to the planet, it is crucial to note that there is very much to learn about Neptune!
Q: How long would it take for a probe to get to Neptune?
A: It took the Voyager probe 12 years to get to Neptune after launch, and it took the New Horizons spacecraft 9.5 years to get to Pluto. Therefore, it is a safe bet to say 15 years or so for a spacecraft to go from Earth to Neptune and not just fly by.
Q: When was the last time Humanity visited Neptune?
A: It was in 1989 when Voyager 2 visited the planet.
Q: How long would it take to build a potential spacecraft?
A: It took approximately 5 years between awarding funding and launch for the New Horizons probe. Therefore, it can be expected to take a similar amount of time for the new spacecraft. But the time can differ depending on the technology used onboard and the timeframe for launch.
Q: What was learned last time humanity visited the planet?
A: When Voyager 2 visited Neptune in 1989, it discovered previously unknown Neptunian rings, and confirmed six new moons: Despina, Galatea, Larissa, Proteus, Naiad and Thalassa. Of these, the irregularly shaped Proteus is notable for being as large as a body of its density can be without being pulled into a spherical shape by its own gravity. While in the neighborhood of Neptune, Voyager 2 discovered the “Great Dark Spot”, which has since disappeared, according to observations by the Hubble Space Telescope. The Great Dark Spot was later hypothesized to be a region of clear gas, forming a window in the planet’s high-altitude methane cloud deck. Voyager 2 observed weather phenomena. This occultation had been attributed to ring arcs, but when Voyager 2 observed Neptune in 1989, Larissa was found to have caused it.
Q: What would we learn about Neptune?
A: Most likely the spacecraft would study the atmosphere, interior, moons, rings, and magnetosphere in some form. This could change depending on the mission, but there is so much that can be learned by visiting the planet.
Future missions to Neptune that have not yet been cancelled include, but are not limited to the following missions.
The ESA has proposed as part of the Cosmic Vision program the ODINUS, Origins, Dynamics, and Interiors of the Neptunian and Uranian Systems, space mission, which would provide two twin orbiters. These twin orbiters would be named Freyr and Freyja, in honor of the twin gods of the Norse pantheon, with a primary mission of studying Neptune and Uranus. If this mission is selected, ODINUS would launch in 2034.
Trident is a proposed mission that would perform a single fly-by of Neptune in 2038 and closely study its giant moon Triton. During this study, Trident would be able to obtain a near-complete map of the Neptune’s moon during its sole flyby and sample the atmosphere to determine measurements to determine information such as the potential existence of an internal ocean.
 = Stone, E.C.; Miner, E.D. (1989). “The Voyager 2 Encounter with the Neptunian System”. Science. 246 (4936): 1417–21. Bibcode:1989Sci…246.1417S. doi:10.1126/science.246.4936.1417. PMID 17755996.
= Brown, Michael E. “The Dwarf Planets”. California Institute of Technology, Department of Geological Sciences. Archived from the original on 19 July 2011.
=ODINUS Mission Information
 = https://earthsky.org/space/flyby-mission-trident-neptune-moon-triton