Only one spacecraft, Voyager 2, has ever visited the ice giants Neptune and Uranus. That could be set to change next decade, as NASA is considering sending an orbiter to one of these planets.
A report, called the Ice Giants Pre-Decadal Study, has been published by NASA looking at launching a mission to one of these two worlds between 2024 and 2037. The mission will be designed to cost no more than $2 billion.
Exploration of at least one ice giant system is critical to advance our understanding of the Solar System, exoplanetary systems, and to advance our understanding of planetary formation and evolution, a summary of the report states.
Four missions have been proposed. One is a Neptune orbiter that would carry an atmospheric probe. One is a Uranus orbiter with a probe, and another without. The fourth mission would be a flyby of Uranus, which would image the planet and its moons, and also carry an atmospheric probe.
Those missions with probes would carry only an additional three instruments a narrow angle camera, a doppler imager, and a magnetometer. The Uranus orbiter without a probe would have a larger suite of 15 instruments, which would allow for a much more extensive study of the planet without the benefit of learning about its inner atmosphere with a probe.
The orbiters would last for 10 to 15 years, powered by some form of radioisotope thermoelectric generator (RTG), which uses the decay of plutonium-238 to provide electrical power.
Getting to Uranus or Neptune requires using Jupiter for a gravity assist to lessen the amount of fuel needed. For Uranus, this makes 2030 to 2034 the optimal time to launch. For Neptune, its 2029 to 2030 with another suitable launch window not available until 2041. The journey time will be 11 years for Uranus and up to 15 years for Neptune.
The science goals are numerous. The missions would seek to work out what the planets are made of and monitor weather in their atmospheres. They would also attempt to study the ring systems around each planet, map the surface of major and minor moons, and monitor how the solar wind interacts with each planet.
For Neptune, scientists are particularly interested in its largest moon Triton, which may have been captured from the outer Solar System. This type of planet also seems to be fairly common in the universe, so were keen to find out more about how Neptune-like worlds form and evolve.
Only one of the missions is likely to be picked at most, so the scientists behind each proposal will now push their case for funding from NASA.