The only time a spacecraft visited Neptune and its large moon, Triton, was during August 1989 when Voyager 2 flew by. One of the greatest discoveries was the nitrogen geysers dotting the surface of Triton with plumes towering up to five miles being blown downwind over 100 miles. Triton turned out to be an active world in spite of it being as cold as -392ºF. Scientists wanted to return as soon as possible. A Neptune orbiter was designed to orbit Neptune at least 45 times over the course of four years and come within several hundred miles of Triton. It would have launched in July 2002 and arrive this May, but with a prohibitive price tag, a return mission to investigate Triton’s geysers close up would have to wait.

Triton, 1680 miles in diameter, is looked at in a different light these days. It has always been suspected that it was captured by Neptune because it orbits Neptune in the opposite direction from all other large moons in the Solar System. It is thought to be a relic of the Kuiper Belt, which was not yet known in 1989. The intense heat from the capture would have melted and deformed the surface possibly creating huge lakes of liquid nitrogen. While Voyager 2 disproved the lakes, it did confirm that Triton is an active world with a warm interior. In this current era, Triton may fall into a class known as Ocean Worlds where a vast ocean of water lies beneath the surface. Europa and Enceladus are members of this class, with Callisto, Ganymede, and possibly Pluto also qualifying. This new branch of planetary science demands further exploration of these worlds.

Technology has advanced since the Voyagers with miniaturized components and digital technology. Back in that era, the imaging system was essentially a television camera attached to a fax machine. Although the images were fantastic, the current technology is far superior as New Horizons proved at Pluto. A spacecraft costing less than New Horizons (under $500 million) can be designed and built using proven hardware and technology from other planetary spacecraft already flown or being built, such as the European Space Agency’s JUICE Jupiter/Ganymede mission, without having to be designed from scratch, which saves money.

Trident, named after Neptune’s 3-pronged spear, would be a nuclear-powered spacecraft since solar power is ineffective beyond Jupiter. It would carry a narrow angle camera that works like a telescope and can image Triton from a distance both on approach and departure. It would also include a wide-angle camera that can see in low light for the flyby itself, to more carefully image the surface in fine detail. It will be able to photograph all of Triton since it will be sensitive enough to photograph the night side dimly illuminated from light reflected from Neptune. Trident would also carry a spectrometer to study the composition of Triton and any plumes. It would fly only a couple of hundred miles from the surface so it can analyze the tenuous atmosphere. Trident will also have a magnetometer instrument similar to what the Galileo spacecraft used to detect the underground oceans of Europa, Ganymede, and Callisto.

Trident could launch between April 15 and May 5, 2026 to take advantage of Jupiter for a gravity assist on June 28, 2032 after multiple assists from Earth and Venus so that it can fly past Triton on June 28, 2038. It would compliment a future mission to orbit a spacecraft around Neptune, which may not occur until 2045 at the earliest. The time to launch Trident is critical because the portion of Triton with the huge nitrogen ice cap containing the geysers will soon slip into the darkness of a long polar winter by 2040, which will last over 80 years. Trident faces stiff competition for selection this year among a mission planned for Io and two missions for Venus, but the idea of having a mission to an exciting world that is part of a new class of worlds on the outer edge of the Solar System using proven components to keep cost down will make it a serious contender for selection. It has been too long since Triton was explored. Now is the time to go back, take a better look, and learn about the evolution and dynamics of Ocean Worlds.