In a nutshell: NASA said the mission would have three main goals: determining if an underground ocean is the source of the icy plumes, mapping the remainder of the moon's surface (Voyager 2 only charted roughly 40 percent of the satellite's surface) and unraveling the mystery of the moon's remarkably young surface.

When Voyager 2 whizzed by Neptune's largest moon, Triton, more than three decades ago, the craft arguably supplied scientists with more questions than answers.

Images beamed back to Earth revealed dark plumes of icy material erupting from the moon's surface and a landscape that had been resurfaced multiple times with fresh material. But what was causing the icy plumes to shoot upwards, and what was the composition and origin of the material resurfacing the satellite?

A mission competing for selection as part of NASA's Discovery Program could answer these questions and more. One of four finalists, the mission, dubbed Trident, would involve sending a second spacecraft to study the unusual moon with modern instruments.

NASA will select up to two proposed missions by the summer of 2021 to become full-on missions that would launch later in the decade. The other missions vying for NASA approval include Veritas, which would map the surface of Venus to learn more about the planet's geologic history, Davinci+, a mission to analyze Venus' atmosphere to understand how it formed, and Io Volcano Observer to explore Jupiter's moon Io, the most volcanically active body in our solar system.

If selected, Trident would look to get off the launch pad in October 2025 in order to take advantage of a once-in-13-year window where Earth is aligned with Jupiter in such a way that the spacecraft could use the gravitational pull of the gas giant to slingshot itself towards Triton. If all goes according to plan, the rendezvous would take place in 2038.

Which missions would you like to see NASA ultimately select?