Trident: a new mission to Neptune’s moon Triton

Trident is a mission under consideration by NASA that would send a spacecraft to explore Neptune's largest moon Triton.

Voyager 2's view of Triton. Credit: NASA/JPL/USGS

Trident is a mission concept under consideration by NASA that would see a spacecraft sent to explore Triton, Neptune’s largest moon and a body thought to have a liquid ocean beneath the surface.

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If Earth has taught us anything, it’s that where you find water you often find life. So if we are to stand any chance of finding life elsewhere in the Solar System, somewhere with liquid water is a good place to start.

We spoke to Louise Procktor, Director of the Lunar and Planetary Science Institute in Houston, Texas, and principal investigator of the Trident mission, to find out more about what the spacecraft might discover and how it will get there.

Read more about Solar System exploration:

Louise Prockter is leading the Trident mission to Neptune's moon Triton. Credit: NASA

What are the Trident mission’s main science objectives?

We’re still in the study phase right now and we have not yet been selected for flight. We hope that’s going to happen about a year from now, but we’re in competition with several other mission proposals.

However, if we are selected, we would focus on Neptune’s largest moon, Triton, as one of the largest bodies in the Solar System. It’s very little explored but it’s a very fascinating place.

What makes Triton so ripe for exploration?

Triton is one of the largest moons in the Solar System. It’s an icy, wild, rocky interior with a thick ice covering.

It seems very unusual, even amongst icy bodies. The surface of Triton is extremely young, possibly the second youngest surface in the Solar System. The surface also has activity upon it.

Voyager 2 is the only spacecraft to have visited the Neptune system. It flew past Neptune and Triton in 1989, and at that time it spotted plumes of dark material coming off the surface, rising about 8 kilometres up and then getting dragged downwind for about 150 kilometres.

So we know there’s some activity on the surface. We don’t know what causes that or why the surface is so young, or how that surface is refreshed.

We think that Triton might have an ocean beneath its icy surface, which would make it very exciting and a possible candidate to be a habitable world in the Solar System.

And it’s got some other interesting characteristics, too. It has an ionosphere, a region of charged particles around it, just like we have around Earth.

But that ionosphere is 10 times more intense than around any other moon in the Solar System.

Usually ionospheres around icy bodies are driven by the Sun, and yet Triton is very far away, around 30 times father than Earth is from the Sun.

So that’s a bit of a mystery. There are a lot of mysteries about Triton.

A NASA infographic highlighting some of the main science objectives of the Trident mission. Credit: NASA/JPL-Caltech
A NASA infographic highlighting some of the main science objectives of the Trident mission. Credit: NASA/JPL-Caltech

Descriptions of the plumes make it sound like those the Cassini mission saw at Saturn’s moon Enceladus. Could Triton and Enceladus be similar?

Triton’s plumes are interesting. They were of course the first plumes that we saw coming off an icy body, long before Enceladus’s plumes were discovered.

And now many think that there could be plumes at Europa as well, which is another icy moon around Jupiter.

At the time Triton’s plumes were discovered in 1989, it was thought that they were the result of a sort of greenhouse effect where dark material or possibly rocky or dusty material lying below several metres of transparent nitrogen ice was being heated up by the very weak Sun, and eventually became over-pressurised and exploded through the ice above it, like a geyser-like process.

But since then we’ve discovered the plumes on Enceladus and have looked at the Triton plumes again.

They’re very massive compared to what we might predict for sublimation-driven or solar-driven plumes, and now we’re starting to re-evaluate whether they could cryovolcanism.

Could they actually be originating from some subsurface liquid reservoir in the same way that the Enceladus plumes are originating from an ocean?

We don’t know for sure, but one of our primary objectives is to try and figure out what drives the plumes. Are they a kind of greenhouse process or are they actually an icy vulcanism process?

Plumes of water ice and vapour spray from the south polar region of Saturn's moon Enceladus. The presence of water on the moon means that scientists cannot risk the Cassini orbiter contaminating its surface. Credits: NASA/JPL/Space Science Institute
Plumes of water ice and vapour spraying from the south polar region of Saturn’s moon Enceladus, as seen by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute

Is it unusual that an icy moon so far from the Sun could be so active?

You might think it’s unusual for something that’s 4.5 billion years old. Shouldn’t it be very old and dead right now?

But of course there are a lot of bodies in the Solar System that still have a heat source. A great example is the large moons of Jupiter, particularly Io and possibly Europa, that we think are active today.

Io is the most volcanically active body in the Solar System, and the reason is because it’s going around Jupiter in a slightly elliptical orbit.

It gets gravitationally tugged one way and another by its sibling moons, by Europa and to a lesser extent Ganymede, and this tidal heat creates the volcanism on Io.

So in same way that we get tides on Earth, there are tides on Io. With Triton it’s a little bit different because we still think tidal heating is occurring.

We’ll be able to see what happens when you take a Kuiper Belt Object and leave it somewhere else for 4.5 billion and see how it evolves

But Triton is in a very peculiar orbit. It’s in a highly inclined orbit and it’s actually travelling backwards.

We think it got captured early in its history. We think it might have originated in the Kuiper Belt around the same sort of area where Pluto and other objects are, and it somehow got captured around Neptune into this very bleak, highly inclined orbit.

Because of that highly inclined orbit, we think it’s tidally heated every time it goes above and below the equatorial plane of Neptune.

And so it may have an ocean beneath the surface, because we think the heating is sufficient to create an ocean.

What is unusual about Triton is that its ocean is created by a different kind of tidal heating than we see elsewhere.

So yes, it’s very surprising that something so far away from the Sun and so old in its interior could have such a young exterior and could still have liquid water beneath its surface today.

Voyager 2's view of Triton. Credit: NASA/JPL/USGS
Voyager 2’s view of Triton. Credit: NASA/JPL/USGS

Would the Trident mission search for signs of life beyond Earth?

We are hoping to take the first step. There are a lot of things involved in searching for life, but one of the primary things that we need for life is liquid water.

And so if we can determine for certain whether there is liquid water beneath the surface of Titan, then that immediately makes it a potentially habitable world.

We also need to understand whether it has the right ingredients for life. Does its composition contain the chemical ingredients, the kinds of things that you need for life?

We know it has a long-lived energy source, so it certainly becomes a whole lot more interesting if we find that it has liquid water beneath the surface.

Of course, there are a lot of other things to consider. Is that water beneath the surface? Does it have the right sort of pH? If it’s too acid or too alkaline, then probably life can’t exist.

Answers to those questions are way beyond what we can do with the Trident mission, but we can certainly take the first step to exploring a world that is so far away from the Sun.

If Triton was captured, then that ocean didn’t form at the beginning of the Solar System. It was probably formed when it got captured and has persisted.

Has its ocean been created? Was it nurtured into existence? It certainly does make it a much more interesting target for a future mission to determine its habitability.

Triton in the distance, as seen by the Voyager 2 spacecraft. Credit: NASA/JPL-Caltech/Lunar & Planetary Institute
Triton in the distance, as seen by the Voyager 2 spacecraft. Credit: NASA/JPL-Caltech/Lunar & Planetary Institute

What did Voyager 2 discover at Triton and how will Trident build upon that?

Many of the things that we know about Triton, we learnt from Voyager. For example, the ionosphere, the fact that it’s unusually intense, we learnt that from Voyager.

We also took images of about 40% of the surface, and most of them were very low resolution compared to kind of images that we’re used to now, when we see images of the Moon or Mars, for example.

The Triton images showed not only the plumes, but also a very unusual alien landscape: really bizarre landforms of the kinds that we hadn’t seen on any other moon before.

But Voyager didn’t carry an instrument that could measure their composition. So most of what we know about the composition of Triton, we know from ground-based or near-Earth telescopic observations.

Because Triton is so far away, we can’t learn a lot about it from the ground, which is one reason why we want to go back.

What we think we know about the interior, the prediction that it has an ocean, that’s all been done from modelling over the last sort of 10 or 20 years. So those results are quite recent.

Voyager was fantastic and we learnt a lot about this very unusual moon from it, but a lot more has been learnt since then just from ground-based telescopes and modelling efforts.

A view of Triton's South Pole, as seen by Voyager 2. About 50 dark plumes could be ice volcanoes erupting. Credit: NASA/JPL
A view of Triton’s South Pole, as seen by Voyager 2. About 50 dark plumes could be ice volcanoes erupting. Credit: NASA/JPL

Have you decided what science instruments the spacecraft will have?

We scientists are very greedy! We always want more science. This is relatively low-cost mission, so we can only put a limited number of instruments on the spacecraft.

Nevertheless, we have chosen our instruments with care and we have chosen them to answer our science objectives.

We have a magnetometer, which is very important because we are going to be able to determine whether or not Triton has an ocean. We won’t be able to tell much about it. But we have to ask “is there an ocean or not?” This is a key measurement.

We have two cameras. One is a narrow angle camera, which is almost like a telescope, so we can take images from far away and we can take very high resolution, close up images of the surface.

And we have a wide angle camera, which we will use to image part of Triton’s surface in reflected light from Neptune. So it will be almost like a shadow, but we will still be able to see a lot of detail.

The reason for that is so that we can image some of the surface that Voyager saw and compare the two sets of data.

Once we fly by, we can see the limbs of Triton with the Sun shining from behind.

Another instrument that we are carrying is an infrared spectrometer. This will enable us to see for the first time the composition of different landforms and different regions on the surface.

We’ll be able to compare the two sets of data that we have gathered with telescopes here on Earth and near the Earth, so that’s going to be very exciting.

We can start to look at whether there are things like organic compounds on the surface of Triton.

And we are also carrying a plasma spectrometer. This will be able to measure the particle environment as we fly through the environment around Triton.

We’re actually going down to about 300 kilometres above the surface, so we will be able to measure the particle environment as we go from far away, right close to the surface and then back out again. This is also very exciting.

All of these measurements will allow us to compare Voyager data and our models and and learn new things about how Triton works and operates.

A global map of Triton produced using data from the Voyager 2 spacecraft. Credit: NASA/JPL-Caltech/Lunar & Planetary Institute
A global map of Triton produced using data from the Voyager 2 spacecraft. Credit: NASA/JPL-Caltech/Lunar & Planetary Institute

There must also be the opportunity to use technology that didn’t exist when Voyager was at Neptune.

That’s exactly right. Voyager was an incredible mission and really sophisticated. It did the grand tour of the outer Solar System: something that we’d never seen before.

With Trident we’re using instruments that are tried and tested, but they are, of course, very much more advanced than the Voyager instruments.

What we’re able to do with Trident is take instruments that have flown before and just make little tweaks to them. We don’t need any new major technology.

We’re just using them in a new way to do this encounter with Triton and learn so much more about it.

What will the journey to Triton be like?

You can get from A to B in many different ways. If you have a very large rocket, a very large launch vehicle, that gives you a lot of power. You can go pretty fast and you can go pretty far.

In this case, we are trying to fit under a cost cap for a certain type of mission class called Discovery.

The Discovery Mission cap is about 500 million US dollars which, of course, sounds like an awful lot to you and me, but in planetary exploration terms it’s considered a relatively small, low class mission.

Because of that, we’re trying to do everything in the most cost-effective way so we’re going to use a small-to-medium-class launch vehicle. It’s not certainly not one of the larger ones like those that would take humans to Mars in the future, for example.

We will have to use Jupiter for a gravity assist and we swing round Jupiter on the way, nicely into a position where we can encounter Triton. But it does take a long time when you go on a smaller launch vehicle.

In our case, we would launch at the end of 2025 and we would arrive at the Neptune Triton system in 2038. You trade speed for cost, basically.

We have a lot of patience. We will get there and we can still do exactly the same science. It just takes a little bit longer.

A natural colour image of Triton's limb captured by Voyager 2 on 25 August 1989 from a distance of 210,000 km. Credit: NASA/JPL
A natural colour image of Triton’s limb captured by Voyager 2 on 25 August 1989 from a distance of 210,000 km. Credit: NASA/JPL

Will Trident be an orbiter or is it a fly-by mission?

We like to call it an encounter. We fly by Triton, but the beautiful thing about Triton is that it rotates around in its orbit.

It rotates completely about once every 5.9 days. It is tidally locked to Neptune, so it keeps the same face pointed to Neptune just as our Moon is tidally locked to Earth.

Because we have this fantastic camera, these other instruments, we can start our encounter sequence before we get there.

The whole thing takes about 9 days, so several days away from Triton, we start imaging it. And as we fly towards it and then past it, it rotates almost beneath us, or next to us.

We can actually get near global coverage just flying by. This is another wonderful example of orbital mechanics, the way the Solar System is cooperating with us in this case.

These missions are so few and far between. We want to demonstrate that we're going to do what we set out to do.

And because we have these instruments, we can start to take data from the surface from very far away.

Once we fly by, we turn around and look back and we can do things while Triton is in eclipse, so we can see the limbs of Triton with the Sun shining from behind.

If there are plumes, things like that, we can actually look at them on the limb. We can tell the shape of Triton.

We can do some some quite sophisticated science, building on what we’ve learnt in planetary exploration over the last 50 years.

Will it be similar to New Horizons’ Kuiper Belt flyby?

Yes, and New Horizons was, of course, a wonderful mission. We saw so much, learnt so much about Pluto because we’d never seen it before with a spacecraft. We are doing something very similar. It’s a very good analogy.

The other thing that’s great is that because we think Triton started in the Kuiper Belt, and Pluto’s still in the Kuiper Belt, there are some similarities, we think, in the surface.

Certainly the ices and other materials that we think are on the surface seem similar to Pluto, so this is a great example of comparing the two.

What happens if you have one body that originates in the Kuiper Belt and just stays there and evolves in place, in situ, and another body that somehow gets kicked out of the Kuiper Belt and gets trapped into orbit around this ice giant planet?

Then an ocean forms beneath the surface. What happens when you evolve a Kuiper Belt Object?

They might have been twin-like in the past, and we’ll be able to compare what happens when you take a Kuiper Belt Object and then leave it somewhere else for 4.5 billion and see how it evolves.

New Horizons flew by Kuiper Belt Object 486958 Arrokoth on 1 January 2019. Credit: NASA/JPL-Caltech
New Horizons flew by Kuiper Belt Object 486958 Arrokoth on 1 January 2019. Credit: NASA/JPL-Caltech

Is there a chance that Trident could do Kuiper Belt flyby?

There certainly is. And we’re looking right now at what other science can we do en route.

For example, we need Jupiter’s gravity to tweak us in the right direction, so we’re hoping to do some observations of the larger Jovian satellites as we go by.

After Triton, we don’t know where the Kuiper Belt Objects will be at that time, but there’s no reason why we couldn’t do a flyby.

And of course, we’ll look at Neptune, too. At this time we’re considering what Neptune science can be done, but a lot of our instruments can also do great science at Neptune.

How close is the mission to being approved and how do you get the go-ahead from NASA?

Well, first of all, we’re not supposed to put any pressure on NASA! We’re not supposed to influence the reviewers in any way.

But we were originally one of 18 mission concepts that was proposed to NASA. We put in our first proposal in July 2019.

We were one of 4 mission concepts that were selected in February 2020, and we are now working like crazy to produce what’s called a concept study report.

This is a huge, dense proposal. If you think of an old style phone book, that’s what it looks like when it’s printed out. We’re trying to retire any risks, to address any weaknesses.

We turn that in in November 2020 and then we start preparing for what is called a site visit and usually a large number of reviews.

Many tens of people come to your site where you’re maybe building the spacecraft or managing the spacecraft.

In this case, the Jet Propulsion Laboratory in Pasadena, California, is managing the mission, and we were planning for the reviewers all to come to JPL. It would have been in probably March 2020

Then they would quiz us very rigorously for at least a day, and NASA goes away and makes a final decision based on what these reviewers come up with.

Because we’re in the middle of this COVID pandemic, it looks like that site visit will actually be a virtual site visit.

As we understand it right now, we’re going to have about 2 days of virtual back and forth where we will present to these reviewers and they will ask us some very challenging questions, because we’re spending $500 million of taxpayers money. They want to make sure that everything is going to work.

We’re really going to address the science objectives and that it’s worthwhile.

These missions are so few and far between. We want to demonstrate that we’re going to do what we set out to do.

The ultimate decision we expect will be made probably in April or May 2021. It’s quite a long, arduous process, but the payoff, of course, is its phenomenal science and discovery.

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Find out more about the Trident mission via NASA’s webpage.