Gravity to the rescue

By Sean Carroll | November 14, 2005 11:19 am

Apparently nuclear bombs and Bruce Willis are two things that are just too hard to control. So if a massive asteroid appears to be on a collision course with Earth, a couple of astronauts have invented a new way to save us from this cosmic menace: a gravity tractor.

gravity tractor It’s a simple enough idea: instead of blowing up the asteroid, just use gravity to gently deflect it from its path. If you have plenty of warning, you can send up a spaceship that is as heavy as you can manage, and simply park it next to the asteroid. The gravitational pull of the ship will gradually tug the asteroid off course; a tiny force, indeed, but if you let it accumulate for a few years you might be able to do the job.

I confess to a certain amount of skepticism. The gravitational field of such a ship will be incredibly tiny, and even if you plug in the numbers and it seems to work, I would worry that other trace effects (e.g. outgassing or radiation from the ship) won’t be equally important and work in the opposite direction. And when I heard a report about the idea on NPR, there was a curious statement from one of the idea’s supporters, that it would work well for asteroids of such-and-such a mass. Where I was taught about gravity, the acceleration is independent of the mass, so that was a little confusing. It may be that the size of the thing is important — if the asteroid center of mass is too far away from the tractor, you’re in trouble, since gravity falls of as 1/r2.

But it’s certainly a more sensible idea than the one mentioned by John in an earlier comment, and again at the bottom of the gravity-tractor article: a space vehicle propelled by the pressure of the inflationary vacuum state, recently granted U.S. patent 6,960,975. That’s just completely crazy.

  • Bob Munck

    Sounds like a good use for Project Orion with its massive pusher plate. Of course, this is assuming that the asteroid isn’t cohesive enough to actually be pushed by nestling the vehicle against it and setting off a few more nukes.

    And while there are problems with Orion — things like fallout and EMP, it’s probably preferable to being hit by an asteroid.

  • bittergradstudent

    Like Sean said, Wouldn’t the radiation pressure from shining a series of lasers on the asteroid be more than the gravitational attraction from the spacecraft? And have the added benefit of being workable from the Earth?

  • Plato

    well most certainly a lot of references

    Love, Stanley G. 1998 17 Mission Specialist
    Lu, Edward T. 1995 15 Mission Specialist, ISS Flight Engineer

    I was thinking of IMpact as a reality to possible encounters. So if we can get there? What would work?

  • Elliot

    I will repeat my absolute astonishment that this patent was granted. Having filed a number of patents this should have gotten bounced early in the process. This is the intellectual property equivalent of Sokals article.


  • Count Iblis

    Some time ago I read somewhere that one could neutralize threat posed by supervolcanoes by crashing a kilometer sized asteroid into it, thereby triggering the eruption of the supervolcano. By storing food to feed the world’s population and evacuating people who live in the neighborhood you could neutralize the danger posed by the eruption.

  • plato


    Let me remind you: Alan Sokal from NYU became famous because he was able to submit a paper on “Hermeneutics of Quantum Gravity” to a renowned journal called “Social Text” and published by the postmodern social science experts at Duke University. The paper was a continuous flow of nonsense: for example it argued that the value of PI changes with the amount of political pressure and discrimination.

    I gave some light on this issue. But more importantly I have to agree with you, about what you are saying, even if Lubos thinks it was the right thing to do by Sokal.

    The travesty here was how new the era of quantum gravity is, and the decisive attempt to highlight how easily people can be fooled.

    If with good conscience you knew what you were doing to disrupt the publics views and encourage the failibilty of good science minds in the bunch, then the history, has been sourly infected.

    Why? Because good scientists were folded up into the bunch. The public mislead. This directed behaviors, around the issues of quantum gravity. Some would even quote Sokal article as to the reasons why “quantum gravity” was a failure? Da:)

    So now you know why honesty is important. Why “sokalitis,” is not avery good thing? So I place it along side of the infamous remarks on stringevangelism. It filters, to the public. Some science tabloids take hold of it. See?

  • Brad Holden

    You can read about the gravity tractor in an article that appeared in Nature and on astro-ph/0509595. The gravitational acceleration that the authors calculate for a canonical space rock is tiny, and assumes a 20 ton spacecraft. The authors expect 20 or more years to move the rock any reasonable distance.

    My favorite solution so far has been to dump a bunch of silver paint on the rock and let the sun do the work.

  • PB

    If I may point something out: the spacecraft must exert a force on the asteroid, which will equal the force of the asteroid on the spacecraft. Unless the mass of the spacecraft is on the same order as the mass of the asteroid, then the spacecraft will simply be drawn into the asteroid without appreciably changing its path (unless it uses rocket fuel to keeps its distance).

    That being said, if the asteroid is roughly the same size as the spacecraft, then there’s no point in using gavitational effects—it would be at least as effective just to push the asteroid out of the way.

  • Elliot

    I’m sure Pat Robertson wouldn’t let an asteroid hit earth. Having Pat take care of it would be at least as effective as the inflation powered space vehicle.

  • bittergradstudent

    Yeah, PB, I was thinking about that, too… in essence, the problem is exactly the same as it would be if you tethered the asteroid to to the spaceship, and had the latter fire it’s thrusters. It’s just that here, you have the tether provided by gravity.

  • Alex R

    The statement about it working for asteroids of a certain size is straightforward: “Parking” the ship near the asteroid requires thrust to keep the ship from falling into or orbiting the asteroid. The more massive the asteroid, the larger the thrust required…

  • k1ng_keenan

    this is interesting like MMH he is the coolest.

    but that would never work cause you dont have enough time to send the spaceship know what i mean, know what i mean??

    what is a cheerios worst nightmare??

    a serial killer

    hahaha get it? like a cereal killer or someone who smashes them cheerios

    dam you cheerio smaher!!

    i love you mom!

  • Matt McIrvin

    PB got the right reason why the asteroid’s mass matters. Mass won’t matter in a one-body problem with a static background field, but this is a two-body problem (well, actually more if you count the sun and planets, but if you just think of it as delta-vee delivery it’s two).

    However, the reason you might want to use gravity as a tether rather than simply pushing the asteroid is that asteroids rotate. Doing it this way means you don’t have to waste time, fuel and effort spinning down the asteroid so it can be pushed safely.

  • James

    Why not mate two rockets “nose to nose,” and push on the asteroid with the exhaust plume from one of them? Should be fairly gentle, and broad enough to avoid cracking the thing. The other rocket keeps the package in place, of course.


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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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