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Game-Changer: A Pluto Orbiter and Beyond

To say that the Space Science and Engineering Division at Southwest Research Institute has been busy of late is quite an understatement. Alan Stern, principal investigator for New Horizons, has been leading an SwRI study examining just how we might operate an orbiter at Pluto/Charon, with results that are surprising and encouraging for the future of such a project.

Working with spaceflight engineer and mission designer Mark Tapley and planetary scientist Amanda Zangari, as well as project manager John Scherrer and software lead Tiffany Finley, Stern has been looking at an orbital tour of Pluto built around a series of gravity assist maneuvers involving Charon, its large moon. The mission would use the kind of electric propulsion system we saw in the Dawn mission to Vesta and Ceres, and by clever use of gravity assists, would pull off another Dawn feat by leaving Pluto once its orbital operations were concluded and moving into the Kuiper Belt for encounters with further objects.

Image: NASA’s New Horizons mission made the first exploration of Pluto-Charon and its system of small moons in 2015. The scientific discoveries led the science community to push for a return to Pluto with an orbiter or to explore other dwarf planets in the Kuiper Belt. Credit: NASA/JHUAPL/SwRI.

The news comes out of a workshop on Pluto and Kuiper Belt exploration held at the American Astronomical Society’s Division for Planetary Sciences meeting in Knoxville, Tennessee. Gravity assists from Charon save propellant, making it possible to continually change the orbiter’s track over Pluto while studying its surface and atmosphere, as well as its system of moons. Several years in such an adjustable orbit would allow us to home in on major discoveries from New Horizons while also uncovering numerous other features that spacecraft did not see.

But it’s also the move into the Kuiper Belt that has Stern’s attention. It makes a Pluto orbiter of this design a multi-purpose spacecraft and leverages our growing experience with ion propulsion. Says Stern:

“This is groundbreaking. Previously, NASA and the planetary science community thought the next step in Kuiper Belt exploration would be to choose between ‘going deep’ in the study of Pluto and its moons or ‘going broad’ by examining smaller Kuiper Belt objects and another dwarf planet for comparison to Pluto. The planetary science community debated which was the right next step. Our studies show you can do both in a single mission: it’s a game changer.”

As analyzed by Tiffany Finley, the orbital tour, still being optimized, would allow dozens of gravity assists at Charon, offering a series of flybys of Pluto’s smaller moons as well as close encounters with Charon itself. Finley talks about studying Pluto’s polar and equatorial regions using plane changes enabled by these maneuvers, including sampling the dwarf planet’s atmosphere. A final Charon gravity assist would take the orbiter into the Kuiper Belt.

And Mark Tapley’s work on electric propulsion ups the ante. He’s arguing that not only would this craft be able to perform flybys of Kuiper Belt objects, but that it could even enter orbit around a second dwarf planet in the Kuiper Belt, just as Ceres followed Vesta for Dawn.

Image: The SwRI-developed Pluto orbital tour design shows scientific objectives can be met with only tiny fuel resources and numerous gravity assists using Pluto’s giant moon Charon. Credit: Southwest Research Institute.

What sort of missions are we talking about? Amanda Zangari’s separate study for this group has looked at the 45 largest Kuiper Belt objects and dwarf planets (covering “the most desirable and scientifically interesting targets”) with launches possible between 2025 and 2040. How these trajectories might be accomplished is presented in a paper that has been accepted by the Journal of Spacecraft and Rockets. From the paper, this is a bit of an eye-opener:

There is a KBO mission possible for every Earth-Jupiter launch window throughout a Jupiter revolution, thus Pluto and every one of the selected 45 KBOs are accessible via Jupiter gravity assist with a flight time of under 25 years and a C3 [excess launch energy] less than 140 km2/s2. Many, but not all objects can be reached via Saturn flyby, and a smaller list still can be compatible with a visit to an ice giant, though it does not necessarily provide a TOF [time of flight] advantage.

Which leads to this:

We found that all five of the non-Pluto KBOs studied by McGranaghan et al [23] can be reached by giant planet swingby— (136199) Eris and (90377) Sedna with Neptune, and (50000) Quaoar, (136472) Makemake and (136108) Haumea via Jupiter-Saturn. Fast-rotator (20000) Varuna is reachable after a Uranus encounter.

Working out the best targets will be critical, for as the paper goes on to note:

We have not considered multiple KBO flybys. It is worth noting that the prospect of an additional flyby of a KBO after Pluto played a large role in New Horizons’ selection, and that it was possible because of Pluto’s proximity at the time to the plane of the Solar System. Many of our KBOs have highly inclined orbits and are often far outside the plane of the Solar System at the time of flyby, thus no additional flybys will be possible en route owing to the vast separations between objects. To probe a second KBO typically requires one to fly through the classical belt.

Image: This figure depicts an electric propulsion spacecraft leaving Pluto orbit, flying by other small Kuiper Belt objects and then reaching the dwarf planet 2002 MS4 as just one example of the “gold standard” Pluto orbiter-Kuiper Belt explorer mission discovered by the SwRI study team. Credit: Southwest Research Institute.

We’ll need a good name for the Pluto orbiter/Kuiper Belt explorer mission, so start thinking about it now as we begin hoping that such a mission will be funded. Back to Stern for the conclusion:

“Who would have thought that a single mission using already available electric propulsion engines could do all this? Now that our team has shown that the planetary science community doesn’t have to choose between a Pluto orbiter or flybys of other bodies in the Kuiper Belt, but can have both, I call this combined mission the ‘gold standard’ for future Pluto and Kuiper Belt exploration.”

The team’s paper on spacecraft trajectories at Pluto/Charon and beyond is Zangari et al., “Return to the Kuiper Belt: launch opportunities from 2025 to 2040,” accepted at the Journal of Spacecraft & Rockets (preprint).


{ 29 comments… add one }
  • Alex Tolley October 25, 2018, 12:38

    The tables of targets and mission times based on assumed C3 energy values is impressive. The mission time cutoff is 25 years, which seems acceptable, although sadly I doubt I will be around to see all but a few targets close up starting at the earliest mission dates and shortest mission times.

    I applaud the assumption of existing technology to do these missions, rather than starting a long program of development.

    What I would like to know is what could be achieved assuming far higher delta Vs at launch, using different technology, like beamed sails. While we don’t have that technology [yet], I do wonder what could be achieved in a shorter time frame when it is available. As usual, I am thinking of CubeSat (3-6U configurations) platforms, an appropriate size propulsion system – sails (photon or electric) with beaming. Lower cost per craft, and more of them. (I also anticipate the beaming would supply power rather than RTGs for the craft to store, operate off, and communicate back to base).

    Suppose we can accelerate such a craft at 1 g for a little over a day, reaching 1000 km/s by about the orbit of Mars. It would reach Pluto within 55 days. If we can also slow the craft down by beaming (more complex, but possible) to allow for orbital insertion, we could get results within a couple of months. This doesn’t require Breakthrough Starshot accelerations, although the energy required might be comparable. If the beamer could also power a low-cost launcher, so much the better.

  • Geoffrey Hillend October 25, 2018, 16:02

    A flyby of all the large Kuiper objects is impressive, so no separate missions are needed.

  • Antonio October 25, 2018, 16:26

    Using Charon gravity assists to flyby other KBO seems plausible, but being able to enter Pluto orbit simply by using Jupiter assists is quite surprising. I’ll have to look at the paper in more detail.

    • Antonio October 26, 2018, 2:06

      I just read the paper and there is no orbit insertion anywhere, only flybys of gas giants and KBOs. There is no flyby of Charon either. Wrong link?

      • Paul Gilster October 26, 2018, 11:21

        No, the link is correct, but the paper cited doesn’t provide the orbital insertion and capture information. I’m told that this will appear in a second paper, to be presented at the DPS today. I hope to have more on this soon.

  • ljk October 25, 2018, 17:09

    The Grave Farthest From Earth Is on a Spacecraft Headed Out of Our Solar System

    Clyde Tombaugh discovered Pluto, and then hitched a ride toward it.

    by Jessica Leigh Hester

    October 25, 2018


  • Ashley Baldwin October 25, 2018, 18:58

    Novel , but such missions were considered previously and rejected as far too costly . But as NASA seem content to prioritise visiting assorted debris about the solar system for their small to medium mission programmes , ( actively EXCLUDING exoplanet science in the process post Kepler ) then flagship missions atleast should be devoted to bone fide planetary science.

    I rather think that orbiters to each of the ice giant, planets, ( with a large Kuiper belt body thrown in up close to boot with Triton ) are somewhat higher priorities . Both, planets, have similar Jupiter and Saturn gravity assist windows and are reachable in about a third of the transfer time. The NASA Outer Planets Assessment Group, OPAG, have worked up multiple mission scenarios for both, planets , and found they are reachable for reasonable budgets of between $2 -2.5 billion . For info search NASA OPAG and look at the comprehensive presentations in the recent meeting archives. A good read. More so if you’re a fan of the SLS.

    Alan Stern has played a blinder getting Pluto covered so well , and onwards . But if he wants to look at more Kuiper Belt Objects after New Horizons he would be better off putting his prodigious energies and enthusiasm into Neptune and with it Triton. Most of its outer moons are likely captured KBOs too.

    • Patient Observer October 28, 2018, 19:59

      …visiting assorted debris about the solar system…

      Thank you for the strong points that were well said.

  • Tim Kyger October 26, 2018, 9:27

    A name for this mission? Orpheus. Going into the Underworld, looking for his lost love….

    • Patient Observer October 28, 2018, 20:02

      Better than my proposal: PLUtoKuiperExploreR

  • Charley October 26, 2018, 18:27

    Not wishing to play the part of Debbie Downer here, but I’ve often had a thought here concerning this blog and that is: what is the ultimate game that is attempting to be achieved?

    We see these endless plans to visit the asteroids in the asteroid belt; the KBO objects in the outer solar system; the moons of Jupiter and Saturn, Mars, Venus, Mercury so forth and so on.
    So that begs the question, are we just exploring everything just so that we can say we’re exploring everything? It’s an important question because we are seeing a crunch in this nation (let’s be realistic here, it’s mostly a US effort) and quite frankly the pot of money is not bottomless.

    I’m often quite amazed that so many people who contribute to this particular blog and post in it never bring up the simple question of what is and is not financially viable and what should or should not be emphasized given limited resources. I guess I just expected better because I feel that we are coming up against a hard limit in a period of time that shorter than most people think, and it’s going to impact not only something is esoteric as the space program, but the nation as a whole. This is not any kind of original thought that was present in the 1960s. But it’s no less valid, we can’t do everything just because simply it’s out there-so anybody here agree or disagree?

    • Michael C. Fidler October 26, 2018, 22:15

      Speaking of downsizing, the original New Horizens spacecraft was a marvel of miniaturizing and long life tech. Just combine the Breakthrough Starshot development with a completely new miniature spacecraft that has a solar or laser sail and a miniature ion engine. A total integrated vehicle could be as small as the size of a silver dollar with current tech and probably as small as a nickle by the time of launch. Instead of one spacecraft – ten thousand!!!

    • Alex Tolley October 26, 2018, 23:54

      Resource issues do raise their heads on this blog in several ways. We argue for our spending choices, e.g. a Titan vs Mars mission. We also understand the need for an economy to support goals, e.g. when and how an economy can support an interstellar mission.

      Regarding the resources pot, that is a larger policy issue. The US science budget has been shrinking relative to GDP for decades, partially propped up by short-term commercial R&D. Economies need to spend on ways to increase growth, of which technology is an important leg. In the US, the military budget dwarfs our spending on science and space activities and represents the bulk of discretionary spending. That is a policy decision, and if it was to change as the Pax Americana retreats, it would make sense to focus more on science and technology spending to enhance growth. As the US is eclipsed, you will see a greater share of space activities by other nations, like China. We may also see more commercially orientated space activities as “New Space” companies try to exploit profitable opportunities. Many people feel that NASA should focus on “pathfinding”, the search for life and technology development, rather than consuming the bulk of its funding on human spaceflight operations. One can hardly read any space blog without a chorus of people suggesting NASA should stop wasting funds building the SLS. If those funds were redirected to mostly robotic science missions, there would be a huge increase in those activities.

      As an expat Brit, I have lived through the rapid postwar decline of British technology spending. The nation retreated from aerospace and became a follower and buyer of other nation’s technology. That was partially offset by joint funding of projects with the EU, now about to end with Brexit. With the ending of the Pax Britannica, Britain largely retreated from all but a few areas of R&D. The US is following suit, but I have confidence that the desire to explore and eventually expand our economy off the planet will be a logical move for the next global hegemon, assuming we can meet the existential threats facing our global civilization. If not, then I would hope to leave a lasting legacy that a future civilization can take up (c.f. A Canticle for Leibowitz).

      • Charley October 28, 2018, 19:03

        All right-I appreciate where you’re coming from, and it’s nice to get a sense of whom you are in the sense of being a former Brit, so you know that what economies are occurring under the Briexit goings-on.
        As I said before, I’m for a much more limited world population for the simple reason that a smaller population creates less friction, but ultimately more opportunity for each individual. That’s directly connected to economics, efficiency, and stretching the resources rather than pushing them up against a limit. You make good arguments with regards to technology, but that is not the entire answer, in itself. As for the idea that we can satisfy our exploration and economy urges by moving off world, I would have to think further on that although I’m not at all certain that would do the trick (but I can’t be concrete as to why I think that way, just chalk it up to feelings).

        All these issues as to how money is currently spent to me boils down to a issue of last wastefulness and less greed on the part of current government within the realms of what is realistic and practical-you have to be realistic to realize there will always be waste in any human enterprise.

        My main argument is that I am not against the space program, but I do feel that exploration should look at targets that give maximum scientific payback for the buck. What does that mean? That’s kind of nebulous but it obviously must ring true, rather than throw money at the wall and see where it sticks.
        Perhaps confining ourselves for the time to the inner solar system and building an infrastructure for science might be the best way to proceed and then gradually step out ward further and further keeping some kind of proportionality with the world’s economy. There are endless ways to argue these issues, and none is perfect, but throwing money at every particular planetary and small body target does not seem to be wise. Just personal opinion.

      • Patient Observer October 28, 2018, 20:12

        Insightful comments even if difficult for us Americans to accept.

    • Antonio October 27, 2018, 2:34

      As for the money, we don’t bring the topic because it has not much sense. There are a lot of things more expensive than space exploration. Current NASA budget is around 1/200 of the total federal budget. Roskosmos budget is around 105 % of Spanish soccer league. Etc.

      What’s so important about space? For me, it’s a quite strange question. It’s like people in a small tribe in the savannah asking what’s so important about the rest of the world.

      For me personally, short term interests are like this:


      A more general answer, explaining why fund science, not only space exploration, is this:


      • Charley October 28, 2018, 19:09

        no one is dismissing the importance of space exploration; but individual urges and wishes to see the cosmos will not stand scrutiny from the general populace if they are in dire straits and if the cost of living is such that any additional burden will be looked upon as a distasteful imposition rather than in improvement in the general well-being of the populace. People are already feeling stretched in democratic societies because of the inescapable inflation and the lack of buying power with their personal finances. These are just indisputable facts and cannot be argued away by appeals to our better nature. I think that space will enlighten people, but at the same time at the end of the day, a large percentage of the population will have to have their attention be drawn to much more immediate concerns.
        It’s true that man does not live by bread alone, but that’s not to say that you can leave out the bread entirely and expect everything to go right from that point on. Focus on planetary science spending must be achieved or otherwise they’ll be a point where people will say why are we doing this? That’s what I’m trying to get to here.

        • Antonio October 31, 2018, 16:39

          “These are just indisputable facts”

          No, these are myths.

    • Gary Wilson October 27, 2018, 12:37

      I’ve asked a similar question on here as well Charlie. I just don’t understand the US priorities in space. Are there any? Or is the goal just to hand out the money to every state with some NASA related infrastructure to make their congressional delegates look good? I hate to say it but it certainly looks that way. What proportion of the total budget should the manned program get? What are the main priorities of the unmanned programs? Who decides? Is it strictly the responsibility of congressional oversight (they proved the peoples’ money after all)? And if so what expertise other than NASA do they call on to make informed decisions? Or does it just go back to getting a piece of the pie for your state? What are the answers to these questions?

      • Charley October 27, 2018, 15:27

        Thank you Mr. Wilson, you said everything I wanted to say, but in a far more succinct way and with better questions than I could ever do. Unfortunately, I often believe that I am conveying to others what I want to say in a complete manner. But unfortunately (hubris?), I often find that I am falling far short of the mark in doing so. You asked extremely good questions and they were on point all the way!
        Yes- my questions were more to the point of our we shotgunning money everywhere to hit any planetary target that simply strikes someone’s fancy or scientific program that some guy at a university things will be an interesting subject to study.
        And many times there is a complete failure to realize that the money (for many, many complex reasons) is not available and while I don’t possess a crystal ball I do believe is going to get quite a bit tighter in the very near future and probably for a considerable time in the future.

        I’m in no way attacking science basic or applied, nor do I think that space exploration is a ‘waste of time or money’ but I do see that certain more focused applications might be received by the taxpaying public at large as perhaps desirable. We all wish we could have unlimited funds to do whatever we wish, but I do see a tremendous crunch in the coming years and now I wonder if perhaps this would be the time to address it.

        • Antonio October 28, 2018, 16:17

          So your question was really about manned/unmanned budget rate, instead of the whole budget as I thought.

          As I see it, the unmanned program is expending money very well, working with a goal-oriented philosophy. OTOH, the manned program, after Apollo, has been a complete disaster, doing things in order to expend great amounts of money instead of expending money in order to do great things.

          • Charley October 28, 2018, 22:39

            “So your question was really about manned/unmanned budget rate, instead of the whole budget as I thought.”
            Yes, but also too, among the unmanned part of the budget for NASA how much of that is carefully planned so that ‘meaningful’ targets are chosen?

        • Michael C. Fidler October 29, 2018, 12:07

          I just realized what you have been missing and that is man’s urge to explore and discover. Our species has become so powerful and productive because of our curiosity, some time in the past we developed the urge to find what was over the next mountain, where most animals have a specific area or range, we do not. Robotic planetary exploration is very cheap compared to manned flights and we have finally reached the golden age when the amount of money put in has given a huge return in information. This also includes the telescopes that have been launched, in that it has extended man’s vision a thousand fold. I can remember when the US and Russia would be launching 100’s of satellites each year with very little info released on what there uses were for. Now you have all sorts of commercial satellites being launched and used for the public good, the technology has advanced to where very rarely are the missions lost. Just in the last 20 years there has been a complete change in the attitude of the public toward space exploration. Maybe the taxpaying public is finding some satisfaction and fulfillment in the results, it has taken a lot of time and money to get to this stage but the depth of the information and the ease of access is the reason funding even in times of economic crisis will last. As they say the government has deep pockets and also information that none of public is really aware of… ;-}

  • P October 26, 2018, 20:34

    I’m with Ashley I’m afraid.
    The ice giants very badly need some love before we go back to Pluto/Charon, no matter how novel this mission design is.
    Maybe JAXA or ESA could step up and expand their interplanetary envelopes if NASA missions don’t eventuate.

    In the current budget environments I actually favour well planned/timed flybys to both ice giants to be followed by any large KBO targets of opportunity within gravity assist cones. I know many will object to this, but seriously, are ice giant orbiters going to materialise in the next ten years with the NASA flagship budget as it is? It’s going to have to be done as a mid-sized mission if at all.


  • wdk October 28, 2018, 12:50

    There was brief mention here and in the paper of electric or ion propulsion, but no specification about how it would be applied.
    Granted solar electric works well enough to get a spacecraft to Ceres, but I don’t see how it will be possible to apply it much past Jupiter. Even there the magnetosphere’s ionizing radiation would likely do considerable damage to solar panels. The underlying assumption for such missions is probably nuclear electric. And should it narrow down to that, it might be difficult to convince the committee on decade aims for astronomy and astrophysics considering spaceflight missions.
    “We” would have to figure out what we are trying to do out there for the cost and resistance to launching plutonium or other active isotopes into space.
    One possible alternate consideration for a dwarf planet would be a dwarf spacecraft – and a lot of progress has been made in that area.
    But afterall, Pioneer spacecraft preceded Voyager, etc.

    • Antonio November 2, 2018, 16:04

      Yeah, good point! What energy source you use is paramount. About dwarf spacecrafts… they have the same delta-V requirements than big spacecrafts, so that’s no solution.

      One way or another, you probably need a nuclear reactor if you want to orbit a KBO. In that case, maybe electric propulsion is not the best option, because it requires energy conversion (thermal->electric) and heavy radiators. Maybe nuclear thermal propulsion is better.

  • Charley October 28, 2018, 19:19

    Returning to the issue of the science behind this proposed plan to explore the outer solar system. I did read through the entire paper that was connected with this article and I can summarize it in the following manner. Essentially, the author is saying that many gravity assist opportunities will exist among several of the planets to permit chances to explore bodies beyond the planet Pluto.

    But here’s where they get into a bit of an issue as far as I can see and that is boiled down to the simple idea that: you can get there fast, but do not expect is spend a great deal of time being there because you are moving so fast when you actually arrive.
    So you get there fast, but don’t spend a lot of time there seeing the object you went to go see. This particular paper as was previously stated does not include anything concerning rendezvous with the targets; there’s an expectation that a future paper will deal with that issue. But let me just say that if there is the viewpoint that electrical propulsion will permit a quick but at the same time, an easy rendezvous means, then it does not appear to be in the cards. This is what I think other people may view this as being possible. I could be wrong with that view, because and I have not seen the second paper, but it seems that where were your thinking that this is a semi-rapid method using electrical propulsion and gravity assist to reach targeted beyond Pluto bodies, you might be mistaken.
    The paper is essentially a compilation of all these launch dates, targeted planetary flybys, and the expected arrival times and energies involved for this entire set of missions. It’s more of a paper involving tabular information rather than detailed scientific ideas.

  • Gary Wilson October 29, 2018, 15:24

    I think this has been a very useful discussion of the complex issue of funding space exploration of all kinds. There are no easy answers unfortunately. NASA is a very powerful organization and the main recipient of public funding for space exploration in the United States. It does seem there is some agreement on here that unmanned exploration has in large part been very successful and has been a good use of the people’s money, whereas the manned program seems lacking in direction but still absorbs a large part of NASA’s overall budget. Americans can and should try to change this. The main approach as I see it would be to contact your congressional representatives and make your concerns clear. The other effective tool is to vote appropriately of course. If people want a change in public policy it’s incumbent on them to make their voices heard.

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