≡ Menu

Hitchhiker to the Outer System

At the Jet Propulsion Laboratory in Pasadena, Masahiro Ono has been using supercomputer simulations to model a new way of moving between small bodies in the Solar System. We’ve had a demonstration in the last few years of what ion propulsion can do to enable orbital operations at one asteroid (Vesta) followed by a journey to another (Ceres) and orbital insertion there. But Ono is looking at ways to simplify the process of asteroid and comet rendezvous that replaces the need for propellant during the orbital insertion and landing phases.

Call it Comet Hitchhiker. “Hitchhiking a celestial body is not as simple as sticking out your thumb, because it flies at an astronomical speed and it won’t stop to pick you up. Instead of a thumb, our idea is to use a harpoon and a tether,” says Ono, who presents the idea today at the American Institute of Aeronautics and Astronautics SPACE conference in Pasadena. The work has intriguing implications for our investigations of the Kuiper Belt and outer system.

Ono’s method uses reusable tethers that, as this JPL news release explains, are best understood by means of a deep sea fishing analogy. As when a fisherman, having hooked a fish, releases more line under moderate tension, letting the line play out but gradually braking, so a spacecraft would sling an extendable tether toward a comet or asteroid, with a harpoon attached to the tether acting as the ‘hook.’ The spacecraft then releases tether even as it applies a regenerative brake to harvest kinetic energy as the craft accelerates.


Image: Comet Hitchhiker, shown in this artist rendering, is a concept for orbiting and landing on small bodies. Credit: NASA/JPL-Caltech/Cornelius Dammrich.

According to Ono’s simulations, a long enough line allows Comet Hitchhiker to match velocity with the target gradually, after which it begins to reel in the tether for a gentle descent. The idea, Ono believes, works in both directions. After conducting its investigations at the object in question, the spacecraft uses harvested energy to retrieve the tether and accelerate away to another object. The potential for studying five to ten asteroids in a single mission is on the table.


Image: The Comet Hitchhiker concept. Credit: Masahiro Ono.

The question, of course, is what kind of tethers could handle both the harpoon impact on the surface and the demands of the subsequent maneuver. To study the prospects, Ono and colleagues have been using a mathematical formulation they call the Space Hitchhike Equation, relating the specific strength of the tether, the mass ratio between spacecraft and tether, and the needed velocity change to make the operation work. Enormous tension will be placed on the tether and heat will be generated by the rapid decrease in speed for orbit and landing.

The hitchhike maneuver would require a tether anywhere from 100 to 1000 kilometers long. Ono is reporting that a velocity change of 1.5 kilometers per second can be managed with existing materials like Zylon and Kevlar, but a 10 kilometer per second velocity change would be possible with future technologies like carbon nanotubes and a diamond harpoon. The latter would give us sufficient delta-V to land on or orbit long-period comets or Kuiper Belt Objects, objects for which current methods only offer us flyby options.

Consider, too, the prospects for non-gravitational slingshots around small bodies, as Ono reports in a description of this work:

A comet hitchhiker can obtain ~5 km/s of additional delta-V by utilizing just 25% of the harvested energy for reeling in the tether and/or driving electric propulsion engines. The tether is detached from the target after the desired delta-V is obtained. Our concept enables to design a fast trajectory to a wide range of destinations in the Solar System by taking full advantage of the high relative velocity, abundance, and orbital diversity of small bodies. For example, by hitching a comet with q=0.5 AU, a comet hitchhiker can reach the current orbital distance of Pluto (32.6 AU) in 5.6 years and that of Haumea (50.8 AU) in 8.8 years.

With the European Space Agency’s Rosetta spacecraft continuing operations around Comet 67P/Churyumov–Gerasimenko and ongoing work with Dawn at Ceres, will Comet Hitchhiker open up future options for studying multiple small bodies in a single mission? Much depends on what happens next. Ono and team plan to do further modeling and experiments using materials simulating a comet or asteroid surface. Their work is being supported by a Phase 1 study from the NASA Innovative Advanced Concepts (NIAC) program. Ono’s description of Comet Hitchhiker is available at the NIAC site.


Comments on this entry are closed.

  • Tulse September 2, 2015, 12:03

    This approach is really interesting, but seems a pretty risky method of transportation to me. It depends on ensuring well in advance that the surface of the body is well-enough understood and mapped to guarantee that at the point of impact a harpoon will a) penetrate the surface, and b) not come loose in material that is too light, brittle, weak, and/or unconsolidated to hold the harpoon. If the purpose of such missions is exploration, I’m not sure we’ll actually know enough to understand in advance how well a harpoon might “stick”. It also offers only one shot at hooking onto the body, which means if you miss you’re out of luck (i.e., the recovery modes are far more limited than using conventional propulsion). We have direct evidence of how problematic this can be with Philae, which presumably we would still be in contact with if its harpoons had fired correctly on its initial touchdown.

  • Alex Tolley September 2, 2015, 12:14

    Isn’t this a case of life imitating art? I read a SF story about maneuvers in the Jupiter/Saturn system. I think it was written by Forward or Benford?

    Whatever the harpoon material, doesn’t it also depend on the structure of the asteroid or comet? The harpoon could pull out of compacted ice comet or rubble pile asteroid rather than anchor the tether under the huge loads.

    I think there is also a chapter on using this technique in a technical book on space tethers. Good to see this being worked on seriously as a possible propellantless technique.

    I would also like to get some sense of the breakeven point between using such a tether (and powering the harpoon shot) (mass of tether and power system) vs using an ion engine.

  • Ron S September 2, 2015, 13:04

    To gain the delta-V requires that the difference between Vtarget and Vcraft is of similar order. That is, up to 10 km/s, or thereabout (and considering the velocities are vectors, not scalars). The harpoon therefore must achieve this velocity, plus an amount to allow surface penetration.

    Where does the fuel for that come from? Since the harpoon weighs less than the spacecraft the fuel requirement is less than a conventional maneuver, but still quite substantial. The harpoon should carry the fuel and engine since if explosively launched from the spacecraft there will an unwanted Newtonian reaction force.

    Will the harpoon hold? There will no chance of choosing the precise spot on the comet (due to the large distance involved and no mid-course correction possible), so what will it hit? Can it hold enough to support the high tension of spacecraft acceleration (up to 5 g)? Comets are little better than rubble piles with frozen bits that will never support this “pull out” force.

    Since the comet and spacecraft velocities have different directions the force on the harpoon will be significantly side-loaded most of the time, not axial. The harpoon is very likely to break (yield strength) or disturb the anchor and pull out.

    100 or 1,000 km of cable? Really? We can add a host of deployment and reliability problems to our experience of non-success to date with low-velocity harpoons.

    If the harpoon fails, and it is very likely will do so, the spacecraft will be lost. You could keep sufficient propellant on the spacecraft as insurance, which would defeat the entire purpose of this technology.

    My conclusion: interesting technology that will never be used.

  • FrankH September 2, 2015, 15:13

    While this is a great idea, space tethers have had a mediocre success rate so far (“interesting failures”) and penetrometers and harpoons on spacecraft have had a spectacular failure rate, approaching 100% (only if you count the Huygen’s probe penetrometer as a success). It would be nice to see either method work repeatedly on say, the Moon or LEO.

  • Tom Baty September 2, 2015, 15:34

    Alex… yes you are correct..in a SF novel astronauts used harpoons and tethers amongst the moons of Saturn. The book was “Saturn Rukh” by Robert Forward…..IMHO one of Forward’s better SF novels. I have it on my bookshelf and I am looking at it right now.

    As an aside.. off subject…Forward also describes metastable helium as a possible future rocket propellant in this book. Kind of a way out concept, but rather interesting replacement for nuclear powered rockets. Robert Forward (RIP) was always good at coming up with different ideas in his novels which always made them entertaining and thought provoking.

  • ljk September 2, 2015, 17:52

    I hope they test the harpoon on this probe BEFORE it is sent into space, unlike the one on Philae, which they did not test until a decade AFTER its launch from Earth.

  • Joy September 2, 2015, 18:49

    Agree with all the skeptics. Each of the components of the system would have a high failure rate. Chances of success of the whole Rube Goldberg device would approach zero. Agree with Ron S – will never be used.

  • J. Jason Wentworth September 2, 2015, 23:43

    I’m inclined to think the same way–that this is a method of momentum transfer which, although it’s allowed by the laws of physics, is so tricky to implement as hardware (which would have the mass and strength limitations mentioned above) that it’ll likely never be used. Perhaps wrapping the end of the tether *around* the objects could avoid the problems with the harpoon pulling out, but the tether would still be quite massive and would be no less subject to breaking under tension (plus, the portion that wrapped around the objects could be frayed or cut by surface rocks or ice fragments [out past the “snow line,” water ice is as hard as stone!]).

  • kamal ali September 3, 2015, 2:05

    what about a hybrid approach? put the harpoon system on a (projectile) spacecraft so the delta-v is minimized? such a hybrid may be more cost efficient? the spacecraft trajectory could be crafted so as to align the harpoon system and comet so as to minimize side loading.

  • Christopher L. Bennett September 3, 2015, 8:19

    I quite like this idea, because I used a similar method in my novel ONLY SUPERHUMAN in 2012. In that case, it was a team of transhuman heroes using “grappling gun” tethers to latch onto a small asteroid at the conclusion of a stealth approach without a ship.

  • ljk September 3, 2015, 11:33

    Why am I reminded of this satellite model regarding the Comet Hitchhiker design:


  • Michael September 3, 2015, 13:30

    I would think it would be easier to just land on the comet or asteroid, then ride it out to what ever region of the solar system you want to explore, then blast off for a journey to the target body when it is in range.

  • Ron S September 3, 2015, 17:06

    Michael: “I would think it would be easier to just land on the comet or asteroid, then ride it out to what ever region of the solar system you want to explore…”

    This is redundant. The spacecraft would already have achieved the same velocity as that body.

  • Peter Popov September 4, 2015, 6:03

    This is novel! I really like the idea.

  • Eniac September 5, 2015, 15:11

    As many have noted, the reliability of the harpooning process is clearly an issue. Also, most of these bodies rotate, which would complicate the process, to say the least. Perhaps a less ambitious approach would be preferable: Land on the target the conventional way, by matching velocity using propulsion, but then take off from it by anchoring a tether and using the rotation of the body to gain delta-v for reaching the next target.

    While sitting on the surface, you’d deploy an anchor, with plenty of time to ensure it will hold, and try again if need be. You’d then jump off the surface and reel out the tether to the point where centrifugal force exceeds gravity. You the essentially hang off the target in a stable, forced, “geostationary” orbit, like a rock being twirled at the end of a string. Adjust the length of the tether until the radial velocity matches the desired delta-V, then dislodge the anchor at the right time. Many details to be resolved about keeping the anchor without getting hit by it, but in principle it should work.

    If you are using a plasma thruster that works with water, you might even be able to scrape enough ice off each target to make it to the next, for an indefinite number of hops (until something fails, that is).

  • Eniac September 5, 2015, 15:17

    Ugh, that should have been “tangential velocity” instead of “radial velocity”, above.

  • James Swarowski February 7, 2016, 15:45

    I tried working out the so called Hitchhikers equation they used here and comparing it to Tsiolkovsi equation. I was really surprised to see that it scales a lot worse with the mass fraction. But you never loose the tether which is good. You can read about it here: https://easterneuropeanbarbarian.wordpress.com/2016/02/07/spiderman-spacecraft-concept-why-not-harpoon-a-comet/