Neptune Odyssey

Neptune Odyssey is a proposed NASA orbiter mission designed to explore Neptune and its moons, with a special focus on Triton. The spacecraft would enter a retrograde orbit around Neptune, allowing it to study Triton and the planet simultaneously, and would release an atmospheric probe to analyze Neptune's atmosphere. The concept is being led by the Johns Hopkins University Applied Physics Laboratory as a potential large strategic science mission. The current plan targets a 2033 launch using the Space Launch System, with arrival at Neptune in 2049, though Jupiter gravity-assist trajectories with a 2031 launch have also been considered.

The mission was not evaluated for recommendation in the 2023–2032 Planetary Science Decadal Survey; instead, the Decadal Survey evaluated a Uranus Orbiter and Probe and an Enceladus Orbilander as the two candidate flagship missions. The Uranus Orbiter and Probe was chosen as the ice giant orbiter priority, ahead of the Enceladus Orbilander.

**Background**

Only Voyager 2 has visited the Neptune system, flying by on August 25, 1989. An orbiter was previously considered under the canceled Mariner Mark II program in the 1990s, and various concepts emerged in the 2000s, including ones from Caltech and a collaboration between the University of Idaho and Boeing, but none advanced. Some designs used nuclear-electric propulsion to reduce travel time and increase payload. The 2011–2022 Planetary Science Decadal Survey recommended a Flagship-class ice giant orbiter, ranking it behind what became the Mars 2020 rover and Europa Clipper. Ice giants are now recognized as common exoplanet types, increasing the need for in-depth study. Uranus and Neptune were seen as equally compelling scientifically, but Uranus was favored for cost and logistical reasons, following the pattern of Galileo and Cassini at Jupiter and Saturn. Some scientists argue Neptune has greater scientific merit because Triton—likely a captured Kuiper belt object and ocean world—is a stronger astrobiology target than Uranus's moons (though Ariel and Miranda may also be ocean worlds). Still, the 2023–2032 Decadal Survey chose Uranus Orbiter and Probe over Neptune Odyssey due to launch vehicle availability, launch windows, and cost.