What would be our next step in the exploration of the outer system once New Horizons has visited one or more Kuiper Belt objects (KBOs)? One intriguing target with a nearby ice giant to recommend it is Triton, Neptune’s unusual moon, which was imaged up close only once, by Voyager 2 in 1989. The views were spectacular but at the time of the encounter, most of Triton’s northern hemisphere remained unseen because it was in darkness. Only one hemisphere showed up clearly as the spacecraft passed the moon at a distance of 40,000 kilometers.
Our next visit should tell us much more, but we’re still working out the concept. Thus Steven Oleson’s Phase II grant from NASA’s Innovative Advanced Concepts (NIAC) office. Oleson (NASA GRC) calls the idea Triton Hopper. In his Phase I study, he identified the various risks of the mission, analyzing its performance and its ability to collect propellant. For Triton Hopper — moving from point to point — would rely on a radioisotope engine that would collect nitrogen ice and use it for propellant, mining the moon’s surface to keep the mission viable.
Image: Graphic depiction of Triton Hopper: Exploring Neptune’s Captured Kuiper Belt Object. Credit: S. Oleson.
Triton is interesting on a number of levels, one of which has received recent examination As with other outer system moons, we’re learning that there may be a liquid ocean beneath the crust. Let me quote a short presentation from Terry Hurford (NASA GSFC) on this:
There is compelling evidence that Triton should be considered an ocean world. Fractures observed on Triton’s surface are consistent in location and orientation with tidal stresses produced by the decay of Triton’s orbit as it migrates toward Neptune. Tidal stresses can only reach levels to fracture the surface if a subsurface ocean exists; a solid interior will result in smaller tidal stress and likely no tectonic activity. Tidal stresses therefore provide a mechanism for fracturing and volcanism analogous to activity observed on Enceladus and, possibly, Europa. Given that Triton’s interior has dissipated a tremendous amount of energy as heat, which likely drove differentiation, and that this heat may remain until the present day, an energy source likely exists to drive geologically recent activity. Moreover, it is possible that tidal volcanism has facilitated, if not dictated, the expression of this activity on Triton’s surface.
Triton’s surface seems to be in geological motion, given how few craters show up in the Voyager views. We can also factor in that this is the only large moon in the Solar System with a retrograde orbit, leading to the view that it is a captured dwarf planet from the Kuiper Belt. That nitrogen that Steven Oleson wants to use should be abundant at the surface, with a mostly water-ice crust to be found below. Also of considerable interest: Triton’s surface deposits of tholins, organic compounds that may be precursor chemicals to the origin of life.
Image: Triton’s south polar terrain photographed by the Voyager 2 spacecraft. About 50 dark plumes mark what may be ice volcanoes. This version has been rotated 90 degrees counterclockwise and artificially colorized based on another Voyager 2 image. Credit: NASA/JPL.
Geologically active places like Triton are intriguing — think of Io and Europa, Enceladus and Titan — and we can add Triton’s nitrogen gas geysers into the mix, along with its tenuous nitrogen atmosphere. No question a lander here would offer abundant science return. Oleson proposes heating that surface nitrogen ice under pressure and using it as a propellant that would allow a continuing series of ‘hops’ as high as 1 kilometer and 5 kilometers downrange. Thus we would get images and videos while aloft, and surface analysis while on the ground.
Intriguing. The thin atmosphere and even the geysers could be sampled by a Triton Hopper in the same way we have looked at the Enceladus plumes, by passing directly through them.
Working with GRC colleague Geoffrey Landis, Oleson presented Triton Hopper last year at the Planetary Science Vision 2050 Workshop in Washington DC. The thinking is to land near the south pole in 2040, in the area where geysers have already been detected. The surface can then be explored in as many as 60 hops, covering some 300 kilometers. Using in situ ices as propellants offers a uniquely renewable potential for mobility.
Oleson’s Phase II work will cover, in addition to mission options to reach Triton and descend to the surface in about 15 years, details of safe landing and takeoff of the hopper. Propellant gathering is obviously a major issue, one that will be explored through a bevameter experiment on frozen nitrogen (a bevameter can measure the properties of a surface in terms of interaction with wheeled or tracked vehicles). Also in play: How to collect and heat the nitrogen propellant and find ways to increase hop distance, solutions that could play into other icy moon missions.
Be aware, too, of a Phase II grant to Michael VanWoerkom (ExoTerra Resource), who will be studying in situ resource utilization (ISRU) and miniaturization. VanWoerkom’s NIMPH project (Nano Icy Moons Propellant Harvester) will deepen his investigation into mission refueling at destination, producing return propellant on site. The work thus complements Triton Hopper and deepens our catalog of strategies for sample return from a variety of surfaces.
The NASA precis for Oleson’s Phase II study is here. The NIMPH precis is here. The Hurford presentation is available as Hurford et al., “Triton’s Fractures as Evidence for a Subsurface Ocean,” Lunar and Planetary Science XLVIII (2017) (full text), but see as well Should we reconsider our view on Neptune’s largest moon?, which ran at Astronomy.com.
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Great summary thanks; Triton is indeed exciting! Regarding launch vehicles for such projects, it seems like a good candidate for SLS.
When choosing I prefer Neptune over Uranus as main target, we get two worlds at the same time.
You are correct that SLS or an equal launcher would be needed to lift the spacecraft, it would have to carry a lot of fuel to break and enter orbit at the destination. Unless aerobreaking is used in Neptune atmosphere, but that would be risking the entire mission.
And a hopper that can get more fuel and do more work sound like a very good idea, it must carry some fuel in case it end up in one area with less or no useful material.
The best would be to make the rocket engine able to heat and use any kind of available material, meaning material we think of as a gas on Earth.
Organic material will be of great interest if found, Triton is perhaps similar to a deep frozen Titan type of world. The big problem to learn more is a very long mission time, where advanced instruments might break down and even if working, will have to operate in extreme environment.
Uranus seems to have ended up with the “boring” bunch of satellites, unless Voyager 2 got really unlucky and all the activity is on the hemispheres that we didn’t get to see because they were hidden in winter darkness.
Preparing a mission to Uranus now would likely observe the moons near the opposite solstice (given the time required to build the mission and get it out to Uranus) and therefore be useful for completing the mapping. On the other hand I think it would probably be better to visit Uranus during an equinox. Observations have indicated that conditions on the planet are very different to those seen during Voyager 2‘s encounter. Something to look forward to in the 2040s, perhaps?
I wouldn’t call Uranus’ moon Miranda–which has many randomly-juxtaposed types of terrain, on the collision fragments that re-cemented themselves together–“boring” in the least. It would be an exciting target for a hopper lander, which could–in effect–collect data and samples from many different moons (older and younger adjacent fragments of Miranda’s “pre-fragmentation, precursor satellites”), simply by hopping from one location to another on Miranda.
Here in particular is where I think a gravity assist (if the planets cooperate!) would be particularly applicable. Depending on what’s what in the article configuration you could use a gravity assist to actually slow down the probe to Neptune enough such that you might be able to perform a ballistic capture by Neptune and at the same time put it into an orbit that would allow perhaps another ballistic capture by Triton with perhaps using virtually no fuel in the process. The beauty of all this is that any fuel that you do not use within the process can be used to do station keeping in your arbiter above the moon Triton , and this would allow you to obtain tremendous flexibility.
I agree. Also, a small subsatellite, which separated before final approach and used a Triton flyby to brake into an eccentric prograde orbit, could conduct repeated flybys of Neptune’s other moons, including Nereid (which Voyager 2 only glimpsed dimly, from a great distance), like Galileo’s and Cassini’s multiple flybys of the Jovian and Saturnian moons. Such a subsatellite (using the term for the Apollo 15 and 16 lunar orbit-released particles & fields subsatellites), since its between-flybys intervals could range from days to months, might use batteries that would be charged by a small RTG during its “inter-flyby coasting periods.”
subsatellite ?? define
Charley, “subsatellite” has two–that I know of–meanings, depending on the context (a third, unrelated term, “subsatellite point,” is where a straight line drawn from a satellite to the center of the Earth intersects the Earth’s surface); they are:
 Astronomically speaking, a subsatellite is a natural satellite of another, larger, natural satellite. Dr. Clyde Tombaugh (the discoverer of Pluto) conducted a long, diligent search for lunar subsatellites in the 1950s, and concluded–after failing to find any–that our Moon could have no natural satellite of its own that was more than about 15 feet across. (It is possible that temporarily-captured, asteroidal subsatellites of the Moon could exist [like the Earth’s distant, temporarily-captured asteroidal moons, which orbit a million or more miles out], which could either eventually impact on the Moon, or escape from eccentric orbits around it.) Also:
 A subsatellite (an artificial one, in astronautical parlance) is a satellite that is launched from another (usually larger) satellite. The small, TRW-built, hexagonal prism-shaped (with deployable instrument booms on one end) Apollo 15 and 16 P & F (Particles and Fields) subsatellites, which were ejected from the SIM bays of those Apollo spacecraft’s Service Modules while they were in lunar orbit, were in this category of subsatellite, as was the NanoSail-D2 solar sail (which was launched from the FASTSAT satellite), and:
Earlier subsatellites included a 6″ sphere with polar xenon strobe lights (launched from Gordon Cooper’s Faith 7 Mercury capsule on May 15, 1963–this might have been the first-ever subsatellite [or at least the first launched from a manned satellite], not counting multiple-satellite rocket launches) and the REP (Radar Evaluation Pod) launched from Gordon Cooper’s and Pete Conrad’s Gemini 5 spacecraft on August 21, 1965.
I have found what look like a presentation for the proposed mission.
They do suggest Solar electric propulsion and aerocapture as best alternative and 12 years to reach Neptune! That’s fast.
It seem best to get the N2 from atmosphere after all.
Not by pumping, that take 2KW and a 400 Kg pump, but freezing out N2 with ‘cryopumping’ 50W & 10Kg.
Power: Stirling radioisotope generator. – Wasn’t that one cancelled?
Fifth post on this page, and it’s from 2016 so the proposal can have changed: https://forum.nasaspaceflight.com/index.php?topic=33971.380
I really like the idea of using local N2 as a propellant. Collecting it conceptually seems “easy” – just warm some ground, capture the emitted gas and let it coll again to liquid temperatures in a tank. No doubt the devil is in the details.
The same approach could be used on Mars, for a hopper at the poles, collecting CO2. On other bodies like Pluto and Charon, indeed anywhere where there is a liquid or frozen gas that can be used as a propellant and surface gravity is low enough. Titan seems particularly relevant. What mechanism is proposed to heat the gas to a sufficient temperature to create the thrust – microwaves or some other heating mechanism?
I thought it was clear: Waste heat from the radioisotope generator.
Where does it say that in the text?
Triton’s surface gravity is about 0.08g. The thrust has to lift the lander and propel it some distance and gently land it again on a surface where the N2 can be extracted again. 2 bursts of gas with a non-vertical ascent and descent.
Would waste heat be sufficient? I’m guessing that the Isp is of the same order as a steam rocket, but I really don’t know.
It will be like a cold gas thruster so for nitrogen it is about 76 isp. Depending on the mass of the craft a few watts of heat can propel it quite far. I would think we only need one charge of liquid in a closed system using a source of nuclear heat and then the cold to return it to a liquid and then use a piston effect to hop about.
“Where does it say that in the text?”
Sorry, it doesn’t actually say that in the text, just seems like an obvious way to do it, as converting to electricity first would considerably increase the size of the radioisotope generator needed.
With such low ISP and thrust requirements (due to the low Tritonian surface gravity), a modified RTG might be sufficient to power such a low-performance nuclear thermal rocket for a hopper lander, *and* generate electricity for its electronic systems.
I’m really wondering how useful this particular method of heating would be technologically feasible. The mall you start talking about having to ‘mine’ the requisite nitrogen ice to replenish your propellant tanks, I begin to see doubt creeping in. We have seen that for our example, helium three is contained distributed among the lunar dust and would require that the lunar regolith would be scooped up and heated to release the requisite material.
As a result, I wonder whether or not the same thing will be true here on this moon; if it requires a scooping mechanism such that the soil can be processed for the nitrogen. This this would be an energy intensive process and it might cripple the prospects of being able to refuel the tanks to make your hops between landing destinations. Just a thought.
That’s kind of my concern. If you’re tapping the atmosphere for your N2 source, sure, you need some additional equipment, but you can be darned sure it will work. If your plan is shoveling N2 snow into a pressure cooker, sealing it, and heating it, that works great if you actually pick up pure N2 with your shovel, but, will you every time?
“If you’re tapping the atmosphere for your N2 source, sure”
not so sure, if N2 atmosphere is near vacuum
Perhaps the best solution would be to put heating coils at the bottom of the rocket bell, and refrigeration coils inside the gas storage tank. Set down, lower until the bell is flush, and evaporate gas from the surface, and condense it in the chamber.
No chance of loading particulates into the chamber that way, which might compromise the seal on your shutoff valve.
There is no need to dig just freeze it out of the very, very thin atmosphere, it will take a while but it is doable.
I don’t think you need to do any shoveling into a chamber. If you can warm the surface of some snow precisely, N2 can be fractionated off directly. An open vessel placed onto the snow would be sufficient. Perhaps membranes to filter the gases so that only N2 is collected would further improve performance.
That doesn’t solve the problem of whether N2 exists in the snow in the first place, but it does make collection simpler.
Yes, that is a very clever idea that could work on Mars as you suggested. Even water, sufficiently heated over a period of time by a radioisotope in a well insulated pressure vessel, could make a dandy steam rocket. What a sight that would be!
Collecting the water would be a challenge, though.
I can see the advantage in terms of efficiency of re”fueling” from solid N2.
But it adds a lot of risk, as that likely won’t be present everywhere, the way the gaseous N2 will be. Better keep enough N2 for several hops on hand at all times.
Hmmm…a SNAP-10A- or TOPAZ- type electric power reactor might double as a low-ISP (for a solid-core nuclear rocket engine) nuclear thermal rocket. On low-gravity, airless (or nearly airless) bodies such as Triton, Pluto, Charon, and the satellites of Uranus, Saturn, and Jupiter, even an RTG with the extra “ice collection & rocket plumbing” might suffice as a low-ISP nuclear thermal rocket for hopper landers (and possibly even for surface-to-orbit sample return vehicles, on the smaller moons), and:
On Titan, the low gravity (17% of Earth’s) and dense atmosphere (whose surface pressure is 50% greater than ours) would make reactor- or RTG-powered, nuclear-heated turbine-powered (or electrically-driven) lander/cruiser aircraft attractive, perhaps even more so than rocket-powered hopper landers (since the Titanian air need not be liquefied and stored aboard the aircraft–and nuclear turbojet [and even ramjet] engines were successfully tested in the 1950s). Aerostatic vehicles (balloons and airships [especially non-rigid ones–blimps]) would also be advantageous on Titan. Also:
Heavier-than-air VTOL (Vertical Take-Off and Landing) Titanian flying vehicles could be helicopters, compound (wings-and-rotor) helicopters, or turboprop-, turbofan-, turbojet-, or motorjet (ducted fan)-powered fixed-wing VTOL aircraft. (Ducted fans could be powered by electric motors or by nuclear-heated Stirling engines, ordinary reciprocating engines, Wankel rotary engines, or turbines.) The aircraft’s wings could also be quite small and stubby by terrestrial aircraft standards–yet produce high lift even at low airspeeds–due to the low gravity and high atmospheric density (some aeronautical engineers have said that Titan is a world made for flying over [Venus also has these advantages for aerostatic and aerodynamic flight, as long as the sulfuric acid cloud decks are avoided]).
That is a good idea. A cold relatively dense atmosphere, combined with low gravity, could make a simple turbo jet (or fan jet) feasible using a radioactive heat source in lieu of combustion. Of course, the numbers have to work but conceptually, it seems very attractive. As a SWAG, perhaps 50 kW thermal could be enough (equivalent to the heat of combustion in a smallest RC jet engine. Thrust could be in the 5-10 lbf range. I don’t know if this is a reasonable value for a radioactive heat source.
The 1950s-vintage atomic turbojet (which used a modified General Electric J47 engine, if I recall correctly) was rather cumbersome, but the newer, compact reactors (or even RTGs, for small atomic turbojets or turbofans), and the new heat pipes, could probably enable such planetary aerobot (or crew/cargo transport aircraft) jet engines to be self-contained, unlike the 1950s one, and:
In comparison, the J47-based atomic turbojet, which was intended to power bombers or transports (with multiple engines running off a single reactor, if memory serves), had a separate reactor outside the engine(s), which would have been mounted inside the aircraft’s fuselage. The engine(s)’ combustor burner cans were replaced with heat exchanger tubing, through which hot liquid reactor coolant was circulated, to simultaneously heat the air passing through each jet engine (instead of burning jet fuel to do this) while cooling the reactor. Also:
Modern atomic jet engines for use in oxygen-less planetary and satellite atmospheres could probably have the reactor (or RTG “heat pile”) mounted inside the engine, in place of the combustor in an ordinary jet engine, with air passages to heat the air while cooling the reactor. Larger, higher-thrust engines might require a larger, external reactor, but solid-state heat pipes (instead of the atomic J47’s hot liquid coolant heat exchanger tubing) could transfer the reactor heat into the engine, with a smaller and lighter arrangement, plus:
On Titan, particularly for supporting manned exploration expeditions (after some local gas liquefying & storage and equipment maintenance infrastructure was set up), non-nuclear jet engines would work just fine, since Titan’s atmosphere contains ample fuel (methane, a little hydrogen, and trace amounts of various other alkane [and alkene] hydrocarbons, including ethane, propane, and acetylene [Air Liquide may claim Titan someday… :-) ]). Such aircraft could carry liquid oxygen (which is easily storable at ambient Titanian conditions) or–perhaps for special applications–nitrous oxide, nitric acid, or dinitrogen tetroxide (Titan has plenty of atmospheric nitrogen and surface water ice, from which to prepare all of these oxidizers), which the gas turbine, piston, or Wankel rotary engines of Titanian aircraft and surface vehicles could burn with the atmospheric methane, hydrogen, and other hydrocarbons.
If I remember correctly a narrow optical window was discovered for Titan’s atmosphere after the Cassini was launched. There were even some low resolution pictures taken from Earth of the surface before the Cassini radar images.
The hopper mission would require an orbiter for efficient communications. Perhaps a simple optical imaging system using a narrow band filter could be added to the mission. One would then have high resolution pictures of the area near the hopper landing site.
Exciting discussion. The nitrogen hopper seems a brilliant idea but a good point about the possibility that nitrogen will probably not be evenly distributed so make provisions for several hops between reloads. Long distance communication seems a likely problem and therefore surely an orbiter is essential as stated previously. Quite a challenge but robotics has always seemed to be NASA’s real strength. I would prefer to prioritize a sample collection mission to Europa if costs become prohibitive. I’m not sure which round of funding these two missions are likely to end up in. Has the Europa mission received funding yet?
Yes it did, big time. To quote from the first linked TPS article below:
“And then there’s Europa, the mission The Planetary Society and its members have worked so hard to support over the years. It stands to receive $595 million in 2018, not just for the Clipper spacecraft, but for work on a lander as well. The legislation reiterates that the mission launch in 2022 on a Space Launch System rocket. NASA has stated it wants to launch the Clipper in 2025 on a commercial rocket.”
” The legislation reiterates that the mission launch in 2022 on a Space Launch System rocket. ”
So, the mission has been constructively banned? In as much as there’s about zero chance of a working SLS by then…
Did you see the next sentence after the one about the SLS, which I quoted here:
“NASA has stated it wants to launch the Clipper in 2025 on a commercial rocket.”
The SLS needs Europa Clipper for a reason to exist far more than the probe needs that rocket. I think we know which one will still be around in the 2020s.