“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.Trident would spend approximately 10 days flying through the area around Neptune, during which it would map nearly all of Triton, study its geysers, determine whether it harbored an ocean, and fly within 300 kilometers of the moon’s “bizarre” surface to study its ionosphere. She says the mission could be accomplished with about $500 million, well under the cost of flagship missions, which tend to start around $1 billion. “We're trying to do something bold that no one thought could be done,” Prockter says.In July, Prockter will submit the Trident proposal for consideration as part of NASA’s Discovery program. If it gets approved, Trident’s arrival at Triton will almost perfectly coincide with the 50th anniversary of Voyager’s visit.
Justifying large planetary missions is always tough, and the timescales involved with missions to the outer solar system only increase the burden on the scientists who make the case for them. The thing about space exploration, though, is that the most exciting discoveries are rarely anticipated in advance. There is plenty of known science to be done on Neptune, but we’ll never know what we’re missing until we get there.
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