Voyager 2 is the only
space probe to have visited the
Neptune system, completing a
flyby on August 25, 1989. An
orbiter to Neptune was considered as part of the aborted
Mariner Mark II program in the 1990s, and several mission concepts for an orbiter were developed in the 2000s, including a concept by the
California Institute of Technology and a version by the
University of Idaho and
Boeing, though none were selected for further development. Some concepts included nuclear-electric propulsion to shorten travel times and increase payload to Neptune. The 2011-2022
Planetary Science Decadal Survey recommended a
Flagship-class orbiter mission to an
ice giant with priority behind what would become the
Mars 2020 rover and the
Europa Clipper. Ice giants are now appreciated as a common type of
exoplanet, precipitating the need for further study of ice giants in the
Solar System. The ice giants
Uranus and
Neptune were seen as unique yet equally compelling scientific targets, but a
Uranus Orbiter and Probe was given preference for logistical and cost reasons. A Uranus orbiter would logically follow Flagship-class orbiter missions undertaken at
Jupiter and
Saturn (
Galileo and
Cassini, respectively). A mission to Neptune is viewed by some to be of greater scientific merit because
Triton, likely a captured
Kuiper belt object and
ocean world, is a more compelling
astrobiology target than the
moons of Uranus (though
Ariel and
Miranda in particular are possible ocean worlds). Nevertheless, again due to cost and logistical considerations including launch vehicle availability and available launch windows, the 2023–2032
Planetary Science Decadal Survey recommended the Uranus Orbiter and Probe instead of
Neptune Odyssey. Another Triton-focused Neptune orbiter proposal called
Nautilus was developed by several graduate students at the
Jet Propulsion Laboratory in summer 2023; under the New Frontiers program, it would be launched in August 2042 and arrive in orbit in April 2057. The
Chinese Academy of Sciences, along with other institutions of the Chinese space program, have proposed a
nuclear electric probe which would fly past Jupiter on its way to orbit Neptune, where it could potentially deploy an atmospheric probe, or landers and surface penetrators for Triton. Two flyby mission proposals for Neptune and Triton also exist, both of which would conduct a flyby in 2038. The
China National Space Administration's
Interstellar Express, a trio of probes designed to explore the heliosphere, would have its second probe (IHP-2) explore Neptune and deliver an atmospheric probe before going off to explore a Kuiper Belt object and fly toward the tail of the heliosphere.
NASA's
Trident was a finalist for selection as a
Discovery mission in 2020, with a proposed launch in 2025 and an intent to focus on Triton. In June 2021, it was announced that
Trident was not selected for development and launch, while
Interstellar Express is yet to be approved by CNSA. ==Objectives==