NASA scientists mull sending a spacecraft on a 13-year mission to visit Neptune's 'bizarre' moon, Triton

Triton may be a satellite now, but it was probably once a dwarf planet in its past

NASA is considering sending a spacecraft to fly by Neptune's largest moon, Triton, in a bid to study its random spurts of ice and strange atmosphere filled with charged particles.

The proposed mission named Trident, after the spear carried by Neptune, the Roman god of the sea, is currently undergoing review as a potential candidate in NASA's Discovery Program. If Trident is selected, the space agency hopes to send out a spacecraft to Triton in 2025.

In 1989, Voyager 2 became the first and only probe to whizz past Neptune. As it passed up to 40,000km away from Triton, cameras on board snapped images that revealed a fantastic world. Plumes of clouds - measuring some 8km (5 miles) high - of fine, icy, dark particles fired from its surface of nitrogen snow.

"How could an ancient moon six times farther from the Sun than Jupiter still be active?," scientists at NASA's Jet Propulsion Laboratory (JPL) asked. "Is there something in its interior that is still warm enough to drive this activity?" Scientists believe that these active geysers indicate complex geological processes are at work on the satellite.

The team have proposed a launch date in October 2025 ("with a backup in October 2026"), and hope to use a "once-in-every-13-years" window, when Earth is "properly" aligned with Jupiter. The JPL explained that the "spacecraft would use the gravitational pull of Jupiter as a slingshot straight to Triton for an extended 13-day encounter in 2038".

Triton has other odd features too; it's the only largest moon in the Solar System to have a retrograde orbit, meaning it travels in an opposite direction to its parent planet. The satellite is also pretty wonky, and is tilted at 23˚ away from Neptune's equator. Scientists believe that these traits show that it was probably a dwarf planet that was captured as it wandered from the Kuiper Belt.

Its atmosphere contains ions that are probably brought over from Neptune. No one knows what process is keeping its ionosphere charged, however. In most cases, the energy supplied to ionospheres is supplied by solar rays. But Triton is too far away to be receiving enough solar energy, leading astronomers to believe that its ionosphere must be powered by some other mysterious process.

"Triton has always been one of the most exciting and intriguing bodies in the Solar System," said Louise Prockter, director of the Lunar and Planetary Institute, which is part of the Universities Space Research Association. Prockter would lead the Trident and act as its principal if the mission is chosen. "I've always loved the Voyager 2 images and their tantalizing glimpses of this bizarre, crazy moon that no one understands," Prockter added.

Trident would explore uncharted territory to help scientists understand where they might find water in the Solar System and what internal processes are driving its landscape. "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," said Karl Mitchell, a project scientist working on the mission at JPL.

Triton is at the outer edge of the Solar System and it'd take about 13 years for a spacecraft to finally reach it for a flyby. ®