Early this year, NASA announced the four potential exploration missions currently competing for selection for its Discovery Program. Among them is Trident, a mission to go and study Triton, the largest moon of Neptune, the farthest planet from the Sun, and definitely among the most peculiar worlds in the Solar System. Understanding Triton will help us understand planetary formation and evolution a lot better.
There are many good arguments for why we should revisit Neptune's moon, three decades after Voyager 2 flew past. Triton has many peculiarities, including the fact it did not form around Neptune. Instead, it's a captured object from the Kuiper Belt, the region of the Solar System beyond the planet’s orbit where Pluto, Eris, and other dwarf planets live. We know Triton comes from there because it orbits against the rotation of the planet and with a 23-degree orbit compared to Neptune's equator.
But there’s more. Triton has one of the youngest surfaces of the Solar System. A mechanism must exist that rejuvenates the exterior of the moon since Triton’s surface that has been studied so far doesn’t appear to be older than 10 million years.
There are peculiarities above and below the surface, too. The moon's atmosphere and ionosphere are a lot more active than they should be and beneath the icy exterior, there could exist an ocean surrounding a rocky core.
"Triton has always been one of the most exciting and intriguing bodies in the Solar System," says Trident Principal Investigator Louise Prockter, director of the Lunar and Planetary Institute/Universities Space Research Association in Houston, in a statement. "I've always loved the Voyager 2 images and their tantalizing glimpses of this bizarre, crazy moon that no one understands."
If approved, Trident is set to launch in October 2025, with a backup date a year later. This would be to take advantage of a once-in-a-13-year gravitational window when Earth and Jupiter are aligned, which will help the craft reach Triton and Neptune for an extended 13-day encounter in 2038.
Once there, the craft will study the plumes of Triton first seen by Voyager 2 over 30 years ago. It will map the 60 percent of the surface not observed by Voyager, which will hopefully help us understand how the moon's surface is rejuvenated.
"Triton is weird, but yet relevantly weird, because of the science we can do there," said Karl Mitchell, Trident project scientist at JPL. "We know the surface has all these features we've never seen before, which motivates us to want to know 'How does this world work?'
"As we said to NASA in our mission proposal, Triton isn't just a key to Solar System science – it's a whole keyring: a captured Kuiper Belt object that evolved, a potential ocean world with active plumes, an energetic ionosphere and a young, unique surface."
NASA will announce which mission will move forward in summer 2021, so we've got just over a year to wait to find out if Trident will fly.