A new white paper by planetary scientists who specialize in outer solar system worlds proposes return missions to Uranus and Neptune during the late 2020s or early 2030s.
Submitted to Arxiv, the white paper is a proposal that typically constitutes the first step toward a new mission in time for NASA’s decadal study, which sets the priorities for the next decade of planetary exploration.
Written by Amy Simon and Mark Hoffstadter, NASA experts on Uranus and Neptune, and Alan Stern, principal investigator of the New Horizons mission to Pluto and the Kuiper Belt, the study proposes sending a flyby probe to Uranus and a separate Neptune orbiter.
The Uranus spacecraft would fly within the planet’s magnetic field, which strangely turns on and off for reasons not well understood. Scientists believe this happens because the planet orbits on its side, creating an unusual polar arrangement.
To study Uranus’s interior, the spacecraft would drop a probe into the giant planet’s atmosphere. It would then leave Uranus and head for the large dwarf planets Orcus and Varuna, then possibly travel on to Haumea, Makemake, and Sedna.
In contrast, the Neptune orbiter would concentrate solely on the planet and its large moon Triton, studying interactions between the two objects. Triton is the only solar system moon that orbits in the opposite direction of its parent planet, which scientists attribute to it being a captured Kuiper Belt Object that once orbited the Sun independently.
A geologically active world, Triton spews liquid geysers that likely come from a subsurface ocean.
Uranus and Neptune have each been visited just once, by the Voyager 2 spacecraft, in 1986 and 1989 respectively. Scientists have combined the Voyager data with observations of the planets by the Hubble Space Telescope (HST) in attempts to better understand these worlds, which are larger than terrestrial planets but significantly different from the gas giants Jupiter and Saturn.
While Jupiter and Saturn are both composed largely of hydrogen and helium, Uranus and Neptune have larger percentages of water ice.
According to the proposal, the cost of both missions would be capped at $3.5 billion.
“You want to fill in some of the gaps with some of the instruments we didn’t have last time,” Simon noted.
Because these worlds are so far from the Sun, solar panels cannot be used to generate power on the spacecraft. Instead, the probes are powered by RTGs, radioactive batteries whose primary source of fuel is plutonium-238.
Production of plutonium-238, halted for close to 30 years due to international nuclear proliferation treaties, resumed in 2015.