Hofstadter says an ideal flagship mission to Neptune would consist of a large spacecraft carrying at least 10 scientific instruments and an atmospheric probe. These instruments would be used to answer a number of fundamental questions about Neptune. At present, he notes that scientists think that the bulk of Neptune’s mass is water, but they’re far from certain. Furthermore, Neptune defies our best models of planetary formation. Based on these models, which accurately reproduce the formation of all the other planets, Neptune and Uranus should have ballooned in size like the gas giants Jupiter and Saturn. But they didn’t—and scientists are at a loss to explain why.

“Right now we're in a situation where we recognize that these ice giants are kind of weird, but we don't understand what they're made of, how they're put together, or why they even exist,” Hofstadter says. “Yet they are everywhere we look in our galaxy, so learning some of these fundamental things is really going to advance our big-picture understanding of how planets form and evolve.”

Hofstadter has hope that a return mission to Neptune is feasible in the next decade. In 2017, he coauthored a report that detailed various mission proposals to Neptune and Uranus. The report will help inform NASA’s next planetary science decadal survey, which determines the agency’s exploration priorities for the coming decade. Work on the decadal survey will begin next year and will likely be finished sometime in 2021 or 2022. But even if a flagship mission to Neptune is selected as a priority and receives the necessary funding, by the time the decadal survey is finished it would take a Herculean effort to pull the mission together in time to hit the gravity assist window.

In light of this dilemma, some planetary scientists have already started discussing what a flagship mission to the outer solar system might look like, so that if the decadal survey green-lights a mission to an ice giant, they can start working on it immediately. A particularly tantalizing plan, according to Hofstadter, involves a collaborative mission between NASA and the European Space Agency. In January, the ESA completed a study of ways it could contribute to a NASA-led mission to the ice giants, such as creating a probe, a sister spacecraft to enable the exploration of Neptune and Uranus, or a lander for Triton. “We’ve started drilling into the details,” Hofstadter says, but whether NASA ends up buying into the ESA’s plan will depend on the results of the decadal survey.

Given the time crunch, Hofstadter says it’s also worth considering smaller mission profiles. Louise Prockter, the director of the Lunar and Planetary Science Institute, couldn’t agree more. In March, Prockter and her colleagues unveiled their plans for Trident, a flyby mission to Neptune’s moon Triton that would launch in 2026 and fly by the moon in 2038.

Prockter describes Triton as the solar system’s “forgotten moon.” This is unfortunate, she says, because Triton is quite unlike any other planetary body in the solar system. Many scientists think that the moon is actually from the Kuiper belt, a massive field of objects from the early solar system that lies beyond Neptune, and became trapped in the planet’s orbit. Based on data from Voyager 2, it also seems to be geologically active, and there’s evidence it may support a vast ocean beneath its surface. Its ionosphere is also 10 times more intense than any other ionosphere in the solar system, which is hard to explain because ionospheric activity is usually correlated with a planet’s interaction with the solar wind, and Triton is rather far from the Sun.