Back to Neptune’s icy moon Triton. It is the largest natural satellite of planet Neptune. It is the first Neptunian moon to be found. The discovery was made on October 10, 1846, by English astronomer William Lassell. It is the only huge moon in the Solar System whose rotation is inverse to its planet’s rotation i.e., it has a retrograde orbit. At 2,710 kilometers (1,680 mi) in distance across, it is the seventh-biggest moon in the Solar System, the only satellite of Neptune sufficiently monstrous to be in hydrostatic equilibrium and the second-largest planetary moon in connection to its primary, after Earth’s Moon. As a result of its retrograde circle and composition like Pluto’s, Triton is thought to have been a dwarf planet caught from the Kuiper belt.
This is a picture of Triton, one of Neptune’s moons. It was taken by a spacecraft nearly 30 years ago and is part of a limited batch of photos we have of this icy world. Scientists think this moon might have an underground ocean that could be hospitable for life, among other intriguing science questions. So, they want to go back for a closer look. But there’s a race against time and orbital mechanics. They want to launch soon so they can take advantage of a gravity slingshot from Jupiter.
Triton is a very large moon. It’s the seventh largest moon in the solar system. Triton itself is larger than every other smaller moon combined. But it is a captured world around Neptune. Something happened, we don’t necessarily know what, that caused it to be captured into orbit around Neptune.
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When we first encountered Triton when Voyager 2 passed by in 1989, astronomers saw a world that was very unlike what we expected. They saw one of the youngest surfaces in the solar system. Astronomers thought it maybe fifteen million years, but perhaps as young as ten million years. And so that might sound a lot to us as mere humans who were here for a blink of an eye. But in geological terms, it was essentially born yesterday.
Astronomers date the ages of surfaces in the solar system based on the number of craters they have experienced. There were almost no craters on the surface of Triton. So that was the first clue something weird is going on. The second clue was when they started looking even closer at the surface. They realized there were all these weird features that they could interpret as being the result of the movement of materials through an ice shelf. And that was particularly exciting because that suggests there’s an energy source. And an ocean provides a really nice energy source.
They also found that Triton had plumes of active material firing off from the surface in some way. These dark columns going up to about eight kilometers above the surface. When they first saw plumes erupting from an icy world it was an evidence of a subsurface ocean which might support alien life. Astronomers and researchers saw them on other distant moons too, like Enceladus.
There’s been a lot more interest in some of these icy bodies. They are very unusual and alien looking at bizarre landscapes. But they are also now possibly places where life might have evolved or might evolve in the future. And all of these intriguing hints are what make Triton a prime target for a new flyby mission.
Trident is a mission concept that astronomers and researchers are proposing to NASA to get back to Neptune’s icy moon Triton in the next Discovery round. They want to launch in October of 2025 so that they could arrive at Triton in 2038.
One of the kinds of ingenious things about this is that 30 years after Voyager the technology has come so far that they can do nearly global mapping of the surface just from a flyby. They are also flying much lower. Voyager flew by at about 40,000 kilometers. They are flying by at about 300 kilometers so that they could go very close to the surface. There are a lot of cases where we can do a huge amount of really fundamentally important science just from the flyby. The New Horizons spacecraft flew by Pluto a few years ago and showed what an incredible world Pluto is.
How Trident will work?
Trident is carrying a fairly complex suite of instruments. The first one is a combined narrow-angle camera and infrared spectrometer. The next one is a magnetometer and that has one real purpose which is to look for the magnetic signature of an ocean as they fly past Triton. Then they have a plasma spectrometer. This is an instrument that samples particles along the trajectory of a spacecraft. Then they are carrying a wide-angle camera. This is specifically intended to image the Neptune-facing hemisphere of Triton during the eclipse. Finally, they are using radio science. So that will give them a lot of understanding about the atmosphere and will also help them to detect more or less where the bottom of the water layer is.
But to get to Triton under the Discovery budget, they will want to hitch a ride on a smaller rocket and use radioisotope thermoelectric generators as a power source. They want to launch soon so they can take advantage of a uniquely suited flight path. In terms of propulsion, the trick is that they are using what they call a ballistic trajectory. Trident is being thrown into space, it’s going to do a few flybys of Venus and Earth, then it’s going to shoot out to Jupiter and then get what they call a gravity assist, where Jupiter will both deflect the trajectory and accelerate it straight at Neptune.
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This is one of those wonderful examples of a very pure application of physics. They should be able to hit exactly the right point relative to Triton within 20 kilometers. At JPL this is what they do. They do these crazy missions to far-out places and bullseye their target decades out at the extremes. So, at this point, they are very close to the end of getting the proposal ready. They are not getting a lot of sleep. It’s a very exciting and exhausting, but overall exhilarating time to be doing this kind of work.
One thing about this mission is that it is unusually long. If you are going to the moon it’s three days away. If you are going to Mars, you can get there in 9 months. It took them 13 years to get to Triton. It’s very rewarding, and it’s worth putting in all the extra time and sweat. You don’t just want to look at the familiar. You learn a lot more about studying the unfamiliar, the things at the extremes. Seeing the surface of the planet for the first time is, it’s hard to describe what that feels like. It’s almost spiritual, you are seeing something sometimes that no one else has ever seen. It’s a pretty incredible moment and they don’t happen very often in a lifetime. But when they happen, they are pretty magical. It’s worth the wait. Hope their mission to get back to Neptune’s icy moon Triton succeeds.