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Trident: Keeping an Eye on the Triton Flyby

38 K, which translates to -235 Celsius or -390 Fahrenheit, is cold enough to allow atmospheric nitrogen to condense as surface frost, which appears to be what is happening on Neptune’s large moon Triton. This is an intriguing place, with pinkish deposits at the enormous south polar cap that are thought to contain methane ice — the color would derive from reactions with sunlight to form a variety of pink or red compounds. Moreover, there are geyser-like plumes here that leave dark streaks over the ices, some of them active when Voyager 2 flew past.

All this and Triton’s odd ‘cantaloupe’ terrain, still mysterious, and what appear to be landscape features produced by liquid eruptions from Triton’s interior. Absorbed by Triton and its mysteries for decades now, I’m all in on a Discovery Program mission concept called Trident, now under discussion at NASA (see Firming Up the Triton Flyby for my initial take on this one). It has been 31 years since Voyager’s August 25, 1989 flyby. I still have TV coverage of the event, hours of it, preserved in the now archaic VHS video tape format.

There was a bit of the New-Horizons-at-Pluto feel to the Neptune encounter, because what Triton was showing us was a young surface obviously resurfaced again and again with fresh material. Thus even then we were forced to consider what processes could drive this activity at the outer rim of the Solar System. What might be under the ice? A surviving ocean? Let’s take another look at the mission and its possibilities.

Louise Prockter (Lunar and Planetary Institute/Universities Space Research Association, Houston) is principal investigator for Trident, which would be managed at JPL:

“Triton has always been one of the most exciting and intriguing bodies in the solar system. I’ve always loved the Voyager 2 images and their tantalizing glimpses of this bizarre, crazy moon that no one understands.”

And Trident project scientist Karl Mitchell (JPL) gives a nod to the science prospects:

“Triton is weird, but yet relevantly weird, because of the science we can do there. We know the surface has all these features we’ve never seen before, which motivates us to want to know ‘How does this world work?’ 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.”

Image: A new Discovery mission proposal, Trident would explore Neptune’s largest moon, Triton, which is potentially an ocean world with liquid water under its icy crust. Trident aims to answer the questions outlined in the graphic illustration above. Credit: NASA/JPL-Caltech.

Remember, Trident is still no more than a mission concept, which means it’s in a competition, in this case with three other entrants in NASA’s Discovery Program, which complements larger flagship missions like Cassini with smaller missions having shorter development times. The other concepts: VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy); DAVINCI+ (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus); and Io Volcano Observer (IVO). Each of the nine-month studies receives $3 million to develop a Concept Study Report, with up to two missions selected for further development.

Given its likely origins, Triton gives us another look at a Kuiper Belt object, one that found its way into its current retrograde orbit around Neptune at an extreme tilt of 23 degrees from the planet’s equator. The moon’s ionosphere is 10 times more active than that of any other moon, an oddity given how far Triton is from the Sun. Nitrogen snows keep up a climatic churn here, while energy in the interior is produced by gravitational interactions with the parent planet. The matter is considered in this mission summary from the 50th Lunar and Planetary Science Conference (2019):

The possibility of an endogenic heat source is considered more likely over the past few years, given recent studies that have suggested sufficient heat to maintain an internal ocean. Radiogenic heating alone may play an important role, possibly providing sufficient heat to sustain an ocean over ~4.5 Ga [8]. Capture into orbit around Neptune [9, 10 and references therein] would have almost certainly resulted in substantial heating [11]; the time of capture is not constrained, but if sufficiently recent some of that heat may be preserved. Finally, despite having a highly circular orbit, Triton’s high inclination also results in significant obliquity, which should be sufficient to maintain an internal ocean if sufficient “antifreeze” such as NH3 is present [12]. Confirmation of the presence of an ocean would establish Triton as arguably the most exotic and probably the most distant ocean world in the solar system, potentially expanding the habitable zone to 30 AU.

There’s a lot of territory to explore at Triton, given that Voyager 2 data yielded a view of no more than 40 percent of its surface. If Trident flies, we’ll capture more views of the area where Voyager 2 found plumes, making for useful comparisons. The young surface may be no more than 10 million years old — note the lack of craters in the now familiar image below.

Image: Global color mosaic of Triton, taken in 1989 by Voyager 2 during its flyby of the Neptune system. Color was synthesized by combining high-resolution images taken through orange, violet, and ultraviolet filters; these images were displayed as red, green, and blue images and combined to create this color version. Credit: NASA/JPL/USGS.

Interior oceans may be the key to understanding many outer system objects, from Europa and Enceladus to Triton and perhaps Pluto itself. Trident would carry a magnetometer to probe the moon’s magnetic field to firm up or refute the ocean theory; a high-resolution mapping and compositional infrared spectrometer for characterizing surface materials; a narrow-angle camera for the largely unseen anti-Neptune hemisphere; a wide-angle camera for imaging the sub-Neptune hemisphere to look for signs of change over time; a gravity and atmospheric occultation radio system; and a plasma spectrometer to sample Triton’s outer atmosphere.

Trident would pass within 500 kilometers of Triton, actually inside its atmosphere, and close enough for high-quality magnetometer data, while passage through a total eclipse will allow atmospheric occultations. A proposed launch date is October of 2025, with a gravitational assist at Jupiter for a 2038 encounter at Triton. It would be good to make these dates, as the gravitational slingshot window with Jupiter opens only once every 13 years. This New Horizons-like mission — the concept uses heritage components and builds on New Horizons concepts of operation — has the potential to yield a similarly useful trove of data.

The mission summary I referred to above is Prockter, et al., “Exploring Triton with Trident: A Discovery-Class Mission,” 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132), available here.

{ 14 comments… add one }
  • Geoffrey Hillend June 17, 2020, 17:05

    From what I recall reading is that although there is more radiogenic heating than tidal heating, the tidal heating is what keeps the ice from freezing since the tides heat the bottom of the ice shell and stops it from growing creating like a tidally heated blanket . Doyle 2012., Astrobiology Magazine. https://www.astrobio.net/news-exclusive/does-triton-have-a-subsurface-ocean/

  • Mike Serfas June 17, 2020, 20:13

    A technology seems to be missing for the exploration of extraterrestrial oceans: the ability to burrow cheaply through ice. In theory, a probe moving laterally and tracelessly through a block of ice should not need to expend energy at all, since there is no potential energy difference from beginning to end. In practice, we expect great energy costs for it to cut, drill, or melt even a short distance. Biology appears to have made some innovations involving salts and polymers: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3960889/ What if there were a way to melt ice ahead of a probe with brine, or convert it directly to a supercooled liquid state with some sort of cleverly tuned electromagnetic radiation, or break the bonds between microscopic blocks in some reversible way that can be profitably undone? What if you could transform the ice and move it to the far side of the probe without increasing the entropy very much, so that most of the free energy could be recovered and recycled?

  • Andrew LePage June 17, 2020, 23:49

    For those interested in our first reconnaissance of Triton in August 1989 (almost 31 years ago!) check out the following article:


    We’re loooong overdue for missions to the ice giants and their moons.

  • Simon van Ede June 18, 2020, 2:19

    Couldn’t it be an orbiter around Neptunus? I understand that an extra rocket for deceleration would cost a lot more, but I think it will pay off

    • Ashley Baldwin June 18, 2020, 13:37

      Trident is a Discovery mission with a paltry budget of just $450 million . The NASA OPAG group have modelled numerous Orbiter mission options to Neptune . All of which were flagship class with a minimum cost ( with little more than a straw man payload ) of $3 billion . As we all know NASA tend to optimism when it comes to cost modelling flagship missions. Didn’t JWST originally come with a $ 1 billion tag ?

      Trident offers fantastic science at bargain basement cost . Takes advantage of a unique Triton orbital position in 2038 that allows imaging of most of BOTH hemispheres over a ten hour encounter (thanks to ‘Neptune-shine’) . With an ultra efficient ballistic trajectory to Neptune requiring a smaller iteration Atlas EELV combined with a ‘free lunch ‘Jupiter gravity assist ( providing a bonus close up Io encounter en passant en route ) .

      Twelve years in transit but mostly in efficient hibernation. All of which helps the chances of TWO Discovery missions on the shoestring budget NASA has left after JWST – far from a shoo in.

      My only regret is that Trident wasn’t a New Frontiers mission with the near double budget offering options for Neptune science too. Such a long way to go it’s a shame not to take advantage .

      Uranus’ superior proximity and diverse Moon system is likely to make it favourite for any Ice Giant flagship if this is finally prioritised in the next Decadal Review . I think Louise Procter’s team know this and are boxing clever here in anticipating this eventuality .

      NASA have just two MMRTG power sources left after Dragonfly and if Trident or IVO don’t use them then they go to waste.

      Trident and DaVinci offer the most science of any of the short listed missions – At the lowest cost – with the latter especially offering up totally new ( overdue ) Venus discovery space . Atmospheric assessment beneath the cloud deck all the way down the surface , Wow. Rather than simply refining topography alone like VERITAS, grandson of Magellan. All done and dusted before Trident reaches Jupiter even so nicely spreading the mission costs – and the science return.

      The lowest dual cost leading to their hopeful twin selection.

  • Jens Knudsen June 18, 2020, 15:45

    If this is going to be a flyby, I wish the craft would be built to last longer, to possibly provide for more flybys after the Triton one, deeper into the Kuiper belt. If I remember right, it is evisaged as using a modified type of RTG that will fizzle out after this one job is done! It stretches my patience to see a spacecraft fly for thirteen years for a one and only encounter, simply to be written off thereafter :)

    • Ashley Baldwin June 23, 2020, 14:08

      Trident’s two MMRTGs have a defined operational lifetime of seventeen years , but in practice will last longer -indeed they are designed to have more “end of mission” power than older RTGs. So it’s likely Trident will enjoy a mission extension – possibly with a KBO target picked by the much larger and sensitive JWST (versus Hubble) , in size and IR bandwidth. It’s also likely to have a close scientifically productive encounter with Io during its flyby. About halfway mission time . So don’t despair.

      • Jens Knudsen June 23, 2020, 16:41

        Thanks :D

  • torque_xtr June 18, 2020, 18:07

    While Neptune system is tantalizing, i would be actually more glad to see Venus missions winning. Space exploration is gaining momentum, and there is non-zero chance that some future mission which hasn’t even been conceived by now, will get near Neptune before 2038. Maybe some nano-scout with magsail and laser link, maybe even something bigger. 2038 is too far to plan for lightweight missions. Flagships – yes… But Venus atmosphere/surface probes do not risk much from being outrun by private companies, because of extreme environment, and they will bring fascinating results just in few years.

  • Raj Pillai June 19, 2020, 11:14

    When is the announcement date of the final two missions..

    • Ashley Baldwin June 23, 2020, 14:38

      Circa March 2021.

      No guarantee of two missions either – though that is the hope. There are significant cost variations between the submissions . Trident and DaVinci the cheapest in terms of operations costs ( Trident due to most of its twelve year odyssey being in powered down hibernation and DaVinci due to the short – hour’s lifespan – of its atmospheric entry probe. IVO and VERITAS have mission operations lives running into years – so much more costly.

  • ljk June 24, 2020, 19:42

    JUNE 24, 2020

    This Hopping Robot Could Explore the Solar System’s Icy Moons

    Steam locomotion may sound like an antiquated way to get around, but it might be getting a science fiction makeover as we expand our reach into the solar system.

    A novel robotic concept being investigated at NASA’s Jet Propulsion Laboratory in Southern California would use steam propulsion to hop across the sort of icy terrains found on Jupiter’s moon Europa and Saturn’s moon Enceladus. Both are thought to host vast subsurface oceans of salty water under a thick ice crust. But while that makes them fascinating destinations for scientific study, the little we know about their surfaces could also make navigating them especially challenging.

    That’s where the Steam Propelled Autonomous Retrieval Robot for Ocean Worlds, or SPARROW, comes in. About the size of a soccer ball, the robot consists of a system of thrusters, avionics and instruments encased in a protective spherical cage. To keep the environment pristine for study, SPARROW would run not on rocket fuel but on steam produced from melted ice, traveling primarily through the air via short thrusts.

    In the sort of low-gravity environment found on those distant icy moons, there’d be no atmospheric drag to slow it down, enabling hops of many miles over landscapes that other robots would have difficulty navigating.


  • Richard M June 26, 2020, 16:37

    Hi Paul,

    Thanks for the great writeup on my favorite Discovery finalist this round.

    One thing I *am* curious about is the launch vehicle. Now Discovery proposals get specced on using an Atlas V, via information provided by LSP. But with Falcon Heavy now being available and certified, I am wondering how that might change things for Trident.

    Now, Trident’s window is very time sensitive in multiple ways – the need for the Jupiter gravity assist, the positioning that will allow imaging of both hemisphere of Triton, etc. – but I am wondering if FH will open up more possibilities for reduced flight time, especially if a Star 48 kick stage is added. Even just eliminating the Venus flyby would at least reduce the need for additional thermal shielding, with all the risk and cost associated with that.

    Of course, if nothing else, it would reduce the cost of the whole thing…

  • ljk June 30, 2020, 11:59

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