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NASA Space Science

Gravity Tractor Could Deflect Asteroids 372

Posted by ScuttleMonkey
from the massive-undertakings dept.
Hugh Pickens writes "A new study at the Jet Propulsion Labs shows that weak gravitational pull of a "gravity tractor" could deflect an Earth-threatening asteroid if it was deployed when the asteroid was at least one orbit away from potential impact with Earth. First a spacecraft would be crashed directly into the asteroid, similar to the Deep Impact mission that impacted a comet in 2005. This would provide a big change of direction, but in a less controllable fashion that could push the path of the asteroid into a dangerous keyhole. But then a second spacecraft, the gravity tractor, would come into play, hovering about 150 meters away from the asteroid, to exert a gentle gravitational force, changing the asteroid's velocity by only 0.22 microns per second each day. Over a long enough time, that could steer it away from the keyhole. In the simulation, a simple control system kept the spacecraft in position, and a transponder on the asteroid helped monitor its position and thus determine its trajectory more precisely than would be possible otherwise. 'The gravity tractor is a wimp, but it's a precise wimp,' said astronaut Jack Schweickart. 'It can make very small, precise changes in orbit, and that's what you need to avoid a keyhole.'"
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Gravity Tractor Could Deflect Asteroids

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  • by faloi (738831) on Monday August 04, 2008 @12:56PM (#24468855)
    Do I get a nifty green hat to wear while I'm on it?
  • I hope their simulations use doubles, not floats!
    • Re: (Score:2, Insightful)

      by Z_A_Commando (991404)
      Actually, I would rather they use a consistent measurement for distance at all times. No more use of meters and feet in the same device!
    • by 4D6963 (933028) on Monday August 04, 2008 @01:08PM (#24469113)

      I hope their simulations use doubles, not floats!

      I know you're joking but for just the speed values if they used time increments in the order of the second then the speed differences would be in the order of e-18, which is too small for a double's mantissa. I'd rather go with long doubles, or better (I think you can achieve something like that by using a number to store the closest representable value and another one to represent the tiny difference from what it should be).

      • Re: (Score:3, Interesting)

        by jandrese (485)
        Isn't the whole point of floating point that you store the exponent separately so you don't run into that problem (at least not as fast)?
        • by JesseMcDonald (536341) on Monday August 04, 2008 @01:57PM (#24469929) Homepage

          Yes, but if you add, say, 1.0e0 and 1.0e-18, the result may get rounded down to 1.0e0 (no change) as the real result, 1.000000000000000001, is not distinctly representable in the number of bits available. To avoid losing precision all the bits after the leading '1' (which is assumed) have to fit within the mantissa, which is of finite size. This applies regardless of the exponent, except for the special cases of zero, NaN, and infinity.

          For a problem like this one tends to be better off using fixed-point notation, with a word size large enough to represent both the smallest increment and the largest magnitude you expect to work with. That way you don't run into the case where a small increment may or may not get rounded off depending on the magnitude of the other operand.

      • by HairyCanary (688865) on Monday August 04, 2008 @01:31PM (#24469495)

        Why do they have to be limited to the precision of built-in data types? If dc can support unlimited precision calculations, the JPL can probably figure out too.

      • by DrYak (748999) on Monday August 04, 2008 @01:58PM (#24469945) Homepage

        (I think you can achieve something like that by using a number to store the closest representable value and another one to represent the tiny difference from what it should be).

        Yup. It's the Kahan summation algorithm [wikipedia.org]. It works as you describe it and it used to compensate the error that happens when doing very big sums of very small numbers (exactly the situation in the gravity tractor's problem)

        • Re: (Score:3, Funny)

          by PaganRitual (551879)
          Yup. It's the Kahan summation algorithm. It works as you describe it and it used to compensate the error that happens when doing very big sums of very small numbers

          Ah yes. It's a very useful algorithm, although you only touch on the second stage of dealing with the error, which is the usage of the algorithm in compensating for the error.

          There is a first, less commonly mentioned stage that the algorithm helps with, which is dealing with the frustration on encountering the error in the first place.

          You
  • by Anonymous Coward

    In a sense, you could apply the same approach, except try to modify earth's orbit, which might actually be easier...

    • by 4D6963 (933028) on Monday August 04, 2008 @01:11PM (#24469183)

      In a sense, you could apply the same approach, except try to modify earth's orbit, which might actually be easier...

      You realise of course that the Earth is pretty much a trillion times heavier than a mankind-threatening asteroid, right? And what would you want to modify Earth's orbit for anyways?

      • You realise of course that the Earth is pretty much a trillion times heavier than a mankind-threatening asteroid, right?

        Says you. You'll be proved wrong when an asteroid the size of Venus crosses our path. Then who'll be so smug? (Me!)

        And what would you want to modify Earth's orbit for anyways?

        To avoid the asteroid, of course.

        Then, we move the orbits of the other 7 (well, 7+, I should say) planets.

        Finally, it will be trivial to move the sun relative to all the planet(oids) to put them all back in their

      • by kat_skan (5219)

        And what would you want to modify Earth's orbit for anyways?

        Martian: My people worked themselves to extinction converting their planet into a navigatable space vessel, using similar technology tested and proven on another nearby planet.

        Zim: Another planet? Why would you do all that?

        Martian: Because it's cool.

    • by Drakin020 (980931)

      For starters, why would you want to change the earth's orbit?

      On top of that, if you change the earth's orbit then you change the impact risks of other asteroids.

      Perhaps if things started getting a tad hot, you could always push the earth a tad away from the sun to cool things off, and likewise when things get a bit cold, but again I'd think that modifying the earth's orbit might not be the best idea.

    • by sm62704 (957197)

      In Larry Niven's A World Out of Time [wikipedia.org] they do indeed move the Earth out to Jupiter's orbit, making it a satellite of that planet.

      It would be a lot easier to move a mountain than an entire world, and moving a world would undoubtedly lead to ecological destruction.

    • In a sense, you could apply the same approach, except try to modify earth's orbit, which might actually be easier...

      Didn't I see a low-budget japanese movie about this 20 years ago? Yes, they moved the Earth. With rocket engines. *rolls eyes*

    • Oh sure. What could possibly go wrong?

  • by FlyingSquidStudios (1031284) on Monday August 04, 2008 @12:58PM (#24468905) Homepage
    Bruce Willis hovers over an asteroid for two action-packed hours!
  • by kwabbles (259554) on Monday August 04, 2008 @01:02PM (#24468977)

    Will be done by holding monthly Gravity Tractor Pulls at the local fairgrounds, with free beer.

  • If the first spacecraft moves it significantly why not just keep throwing those at it. If you are planning on moving this thing by the micron then the first can't be all that innacurate because you'd never be able to nudge out any drastic error. Just keep throwing the firt at it until we're OK.
    • Wouldn't it be easier (and more effective) to have something land on it once, latch on, and fire rocket boosters to move it rather than to drive next to it for a long time?

      • Re: (Score:3, Interesting)

        by zippthorne (748122)

        You are presuming that landing is possible. The object in question might be a loosly-conglomerated gravitationally bound pile of rocks and dust.

        • Re: (Score:3, Informative)

          by Chyeld (713439)

          Even if the asteroid is solid, and there is some 'miracle' way of anchoring the rocket to the asteroid: Landing and pushing requires the assumption that the center of gravity and the shape of the asteroid is such that you can position the rocket push in a productive manner and not just cause the rock to pinwheel or split in two.

          • Re: (Score:3, Interesting)

            by zippthorne (748122)

            True, however in the case of a gravity tug, your maximum useful thrust is limited by the gravitational interaction, which is limited by how close you can get. Landing takes one of the variables away, and trades it for the ones you mentioned. If it's solid, your maximum effective thrust can be very high, even if you can only use it a shorter percent of the time.

            In specific cases, landers probably would be more effective. But gravity tugs are a much better general solution, and mass-production favors gener

    • by Chyeld (713439)

      Because it'd be highly unlikely that the asteroid is a perfect sphere. And since it's likely to be some weirdly shaped object with a wildly offset center of gravity, thowing things at it isn't a particularly effective means of control. Especially if you only have a limited number of things to throw at it.

      At best, you hit it spot on and it moves the direction you want.

      More than likely, you hit it off center and instead of budging it you've just caused it to pinwheel. Which has the added effect of causing it

  • Could someone with the proper knowledge submit a Wikipedia article for keyhole? The word is used in the summary three times and seven times in tfa. I guess the term is here to stay.

    Thank you.

  • Darn (Score:3, Funny)

    by corychristison (951993) on Monday August 04, 2008 @01:08PM (#24469119)

    ... and I thought it would be more like this [wikia.com].

  • Coaxing vs Pushing (Score:4, Insightful)

    by flaming error (1041742) on Monday August 04, 2008 @01:10PM (#24469153) Journal

    How is gently pulling the asteroid with a weak gravity string more efficient than just landing the same "tractor" on the asteroid and pushing it gently but directly?

    • by Dr. Eggman (932300) on Monday August 04, 2008 @01:14PM (#24469239)
      maintaining an object near the asteroid would require less energy than actively pushing it away. Not to mention the possibility of asteroids that aren't solid enough to support something on its surface pushing it.
      • by MyLongNickName (822545) on Monday August 04, 2008 @01:30PM (#24469485) Journal

        expending all of the energy carried by the probe

        You have to obey the laws of physics. There is no way you will expend less energy holding your position than using all of your fuel to build up speed and crash into the sucker.

        In fact, if there is any elasticity in the collision, it it far more efficient to crash. And this does not take into account the fuel you will waste simply by having to angle your exhaust to not hit the asteroid.

        Now, the down sides to crashing are that you cannot accurately know just how much you will move the meteorite. You cannot make midcourse adjustments as you learn more about its trajectory, and as you mention, not all asteroids will be landable. Soft surface or rocky surace, and you will have wasted the lander.

        • You have to obey the laws of physics. There is no way you will expend less energy holding your position than using all of your fuel to build up speed and crash into the sucker.

          I think the GP means that it takes less energy to maintain position above an asteroid then to attach it to an asteroid and then have some sort of propulsion system push it off it's original course.

        • by Bemopolis (698691) on Monday August 04, 2008 @02:08PM (#24470099)
          Another problem with direct impact vs. hovering is that the collision might not take place at the center of mass of the asteroid. Thus, some of the impact energy would be wasted adding or subtracting spin of the asteroid, which would not affect its trajectory. Additionally, some asteroids are more rockpile than solid rock. Hovering avoids the first problem and obviates the second.
      • I'm not sure that's true.... It seems to me that if the two objects are just hangin out in space, any force exerted, gravitational or otherwise, would require energy from the tractor craft to maintain. If there is a constant pull between the tractor and the asteroid, that should take just as much energy to maintain as pulling it with a cable would (or pushing it, which isn't as good for the analogy, but more realistic).

        Remember, for every action there is an equal and opposite reaction, even if it's somethin

      • by MythoBeast (54294)

        This is not true. The small object near the asteroid would be pulled towards the asteroid. The energy required to keep the small object away from the asteroid would be mathematically identical to the deflection energy imparted onto the asteroid. This is the first law of thermodynamics at work.

        The only advantage that you get is that you don't have to expend energy to stop the asteroid from spinning so you have a stable platform to thrust from. The disadvantage is that you have to move the secondary objec

      • by Moraelin (679338) on Monday August 04, 2008 @02:12PM (#24470163) Journal

        Actually, there is no free meal there. If you exert a force F on the asteroid, you get -F exerted upon the tractor. (Imagine a cute little vector mark above the F, to be completely true.) There is no known way to escape that.

        If you always stay X metres in front of the asteroid, then effectively you can treat the whole system as one body. You're not just accelerating the asteroid (with mass m1), you're also equally accelerating the tractor (let's call its mass m2) with the same acceleration, or they'll collide or drift apart. So effectively you're accelerating the sum of their masses, m=m1+m2.

        The force to do that is still F=m*a, or F=(m1+m2)*a. There is no free lunch. You're still accelerating the same m1+m2, and if done at the same a, you must apply the same force F. I.e., if the same rocket engine is used, you get to burn the same amount of fuel, regardless of whether they're physically in contact or weakly pulled by gravity. Using gravity there just puts a (very low) upper bound on F.

        But wait, that was assuming the ideal case, where you magically apply _exactly_ the amount of force to stay always at X metres drom the asteroid. Reality is much less ideal. Such a tractor would probably have to fire rocket engines back and forth, just to stay anywhere near the prescribed distance. I.e., it would use extra fuel for positioning and maneuvering, whereas a lander with a big jet pointed "upwards" would have no such worries.

        Just about the only reason I see there, is if you have to essentially rotate the system, to execute some complex maneuver with the asteroids (over aeons, mind you.) Then it's probably less waste to just move the tractor around the asteroid, than to rotate the asteroid with your thruster embedded in it.

        Still, I'm kind of at a loss as to when or why you'd need that, or have the luxury of enough time for such infinitesimal accelerations to do the job. More realistically, you'd just want the asteroid's orbit changed enough that it doesn't collide with Earth. And you'll likely not have that awfully much time. So you just want to push it out of the way, hard enough to make a difference, but not hard enough for it to shatter into a MIRV of death and destruction. Probably the safest bet being to push it upwards or downwards, in regard to Earth's orbit, so it becomes a lot more inclined than the orbit which threatened to collide. You have a lot more margin for error in the calculation there. You don't need to rotate and maneuver it accurately, you just want it out of the way.

        So basically while I'll agree that their method could work, I'm kinda at a loss as to why would you want to do it that way.

        • by Mattsson (105422) on Monday August 04, 2008 @05:29PM (#24473089) Homepage Journal

          or have the luxury of enough time for such infinitesimal accelerations to do the job.

          The article said that they'd use one probe to crash into the asteroid to make it miss the Earth, then use a second one to use gravitation over a very long period of time to make sure that the new orbit won't cross Earth orbit later.

          One possible reason why they wouldn't want to land it and then push it in order to fine tune the new orbit, which would take the exact same amount of fuel, is that they might have to change the thrust vector at a later date.
          This is hard to do with something standing in a certain place on the asteroid.
          Something keeping position a bit to the side is easier to move to a new position.
          And if the asteroid is spinning, which is not entirely unlikely, anything situated on it would have a hard time exerting a force at an exact vector relative to the orbit.

    • Re: (Score:3, Interesting)

      by Zocalo (252965)
      The tractor possesses mass, and therefore has a constant gravitational effect upon the asteroid (and visa versa) and the only fuel it needs to expend are the minuscule amounts necessary to keep it tracking the asteroid. The simulation shows that, given enough time, the cumulative effect of the gravitational tug can exceed that of expending all of the energy carried by the probe, whether kinetic (impact) or potential (engine burn). It's kind of like an ion engine; it's slower at first but over time it's go
      • Re: (Score:3, Insightful)

        The simulation shows that, given enough time, the cumulative effect of the gravitational tug can exceed that of expending all of the energy carried by the probe,

        So, basically, you are somehow getting more energy out of the system than you put in. This is basically the definition of a perpetual motion machine.

        Ion engines give better long-term acceleration than chemical engines because they can spit the exhaust out faster, not because there is some magical reason that slow acceleration is more efficient than

      • Re: (Score:2, Insightful)

        by caywen (942955)
        Doesn't the tractor have to expend energy to maintain a fixed distance from the asteroid? Also, wouldn't the tractor gain more benefit from maintaining as small a distance from the asteroid as possible to maximize its gravitation effect? In that case, the amount of energy it would have to expend would be proportional to inverse distance squared to maintain that distance, right? Is that really less energy than landing on the thing and using one of those ion propulsion engines? Or, perhaps the benefit is in
      • Re: (Score:3, Insightful)

        by ceoyoyo (59147)

        You've forgotten that you have to expend fuel to avoid being pulled into contact with the asteroid. The amount of movement (under ideal conditions) is the same. The tug method has the advantage that it works with piles of gravel just as well as a hunk of iron, and oblong asteroids just as well as spherical ones.

    • by LSD-OBS (183415)

      Thank you for saving me the time of having to remind people of Newton's Second. I'm fairly amazed that this isn't by now completely obvious to people.

      • by LSD-OBS (183415)

        Whoops, retract that, I had this figured out last week at the pub -- it's because of the objects's spin that you need to use a gravitational tether. Damn you, alcohol-impaired hippocampus!

    • by CastrTroy (595695)
      I imagine it's much easier to stay a certain distance from the asteroid and maintain altitude then to land on the asteroid and risk crashing, or something getting damaged in the process. If you stay above the asteroid, you don't have to worry about finding a suitable landing location.
    • Spin.

      Most asteroids spin, and thus using a thruster will, in the long run, be negated by the spin of the asteroid. Unless the thrusters are timed to coincide with the spin, etc -- and that's assuming there are no problems with landing (well, attaching, really) to the asteroid.

      Using gravity would, almost definitely, be more precise than trying to use thrusters -- and so we could avoid the possibility of massive failure (by, say, kicking the asteroid into a different keyhole, or by causing the asteroid to
    • Re: (Score:3, Insightful)

      by AlecC (512609)

      The asteroid is almost certainly spinning to some extent, and not about an axis that you wnat to push on. For a "one big thump" impact that doesn't matter, but for a steady push you would have to kill the rotation, which would probably take much more energy. The gravity tractor hangs just out of the reach of any flailing "mountains" and pulls in a constant direction.

      Th falling apart problem probably wouldn't matter for the tiny impulse suggested, but would for the "one bigh thump", whose consequences would

  • by smooth wombat (796938) on Monday August 04, 2008 @01:12PM (#24469199) Homepage Journal

    If all they're trying to do is move the orbit of the asteroid by a fraction or a millimeter per second, wouldn't it be easier to just, you know, harpoon the asteroid and use ion engines to gently pull on it rather than trying to keep a second spacecraft hovering over the non-smoking crater of the first spacecraft? Or, if harpooning isn't viable (cue 'Whalers on the Moon'), just have the spacecraft rest on the asteroid's surface and, using ion engines again, push on the thing.

    Can someone more well versed in orbital mechanics and the motion of bodies in space please provide some information as to why these are not viable options.

    • by Colonel Korn (1258968) on Monday August 04, 2008 @01:23PM (#24469389)

      If all they're trying to do is move the orbit of the asteroid by a fraction or a millimeter per second, wouldn't it be easier to just, you know, harpoon the asteroid and use ion engines to gently pull on it rather than trying to keep a second spacecraft hovering over the non-smoking crater of the first spacecraft? Or, if harpooning isn't viable (cue 'Whalers on the Moon'), just have the spacecraft rest on the asteroid's surface and, using ion engines again, push on the thing.

      Can someone more well versed in orbital mechanics and the motion of bodies in space please provide some information as to why these are not viable options.

      Orbital mechanics aren't the problem with your suggestion. Consider getting a craft to gently land on an asteroid. That's probably equivalent in difficulty to having a craft maintain its position 150 meters from the asteroid, as suggested in TFA. Already the lander has had about as much complexity as the "hoverer."

      Now consider that the object must pull or push the asteroid along a very specific and consistent trajectory to safely move it out of danger. Remember that the asteroid is certainly spinning about two axes, so an object stuck to the surface would not be able to simply face in one direction and push. The craft hanging out 150 meters from the asteroid ignores the spinning and does its job, while the craft on the surface of the asteroid has to either push really hard every once in awhile, when its trajectory happens to be lined up well, or it has to constantly push and angle its exhaust while continuously calculating the correct direction to maneuver the spinning object correctly. Or it could cease the asteroid's rotation, which itself is a difficult problem.

    • by the_other_chewey (1119125) on Monday August 04, 2008 @01:26PM (#24469421)

      Or, if harpooning isn't viable (cue 'Whalers on the Moon'), just have the spacecraft rest on the asteroid's surface and, using ion engines again, push on the thing.

      One word: Rotation.

      If you put an engine on the asteroid, you cannot use it about half of the time (very roughly, probably way less)
      because it would be pushing in the wrong direction.
      Hovering decouples your applied force from the rotational movements of the asteroid, so as long as you manage
      to hold your position on the right side of the asteroid, the force is applied constantly.

      So a gravity tractor can apply the same delta-v faster than a "ground based" solution.

    • Re: (Score:2, Funny)

      by verbamour (1308787)

      It's not orbital mechanics, but asteroid mechanics that come into play. An asteroid may be a loose gravel pile, so an engine placed on the surface may dig in, or even push all the way through. Gravity traction is a very low-stress way to impart momentum.

      I'm not a rocket scientist, but my hot girlfriend's little sister is...

  • I'm glad I already read this story and understand the metaphor, because the summary is rather unintentionally surreal otherwise.
    • by pilgrim23 (716938)

      Keyhole was the code name of the KH series of spy satellites.

      As to the concept itself: what of the Enviornmental Impact statement (if it hits)? What of its carbon footprint (If it hits)?

      Also, as I recall, the last body that could have posed an issue was discovered on its way AWAY from Earth after its close encounter. There are so many of these in orbits never before tracked that our first clue of a problem may be after a rock lands where India used to be...

  • by Jhan (542783) on Monday August 04, 2008 @01:16PM (#24469263) Homepage
    TFA spells it out very nicely. Get there one orbit early (a year or a little longer) then gently tug. Of course that's for a small asteroid, for a dinosaur killer maybe five years. If you wait until the object is a few weeks away you are toast. Cindered toast. The entire "nuclear might" of the planet launched at the intruder will do diddley squat, Bruce Willis or no Bruce Willis. That's why NEAR should get lots more funding.
    • Barman: Do you really think the world's about to end?
      Ford: Yes, in just over 3 minutes and 5 seconds.
      Barman: Well, isn't there anything we can do?
      Ford: No. Nothing.
      Barman: I thought we were supposed to lie down, put a paper bag over our head or something...?
      Ford: Yes, if you like.
      Barman: Will that help?
      Ford: No. Excuse me, I've got to go.
      Barman: Ah, well. Last orders please!

  • by Burz (138833) on Monday August 04, 2008 @01:16PM (#24469269) Journal

    Its probably a good time to remind people that the distributed computing project to search for dangerous NEOs [psi.edu] is soon to get under way. Test workunits have already been sent out and the news is that they ran very well.

  • That's a very small asteroid they're modeling, wake me up when they can do something about the mile-wide ones

    a few megatons TNT of impact energy won't do much on most of the earth's surface, likely hit nothing important

  • Dupe (Score:4, Informative)

    by Thelasko (1196535) on Monday August 04, 2008 @01:25PM (#24469411) Journal
    This article is sooo 2007. http://science.slashdot.org/article.pl?sid=07/03/17/0538220 [slashdot.org]
  • Next thing you know, bugs from Klendathu will throw rocks at us too.

  • by Yvan256 (722131) on Monday August 04, 2008 @01:33PM (#24469521) Homepage Journal

    We should be watching to see if the dolphins leave the Earth by their own means.

  • If it can be guided away from the keyhole, could it not also be guided towards? I think this presents a wonderful opportunity for extortion. If I only had the resources I would shave my head, get a cool chair, and become adept at holding my pinky to the corner of my mouth in an evil fashion.
  • A micron is a unit of length, not velocity. Do they mean microns per second?

  • by Lord Apathy (584315) on Monday August 04, 2008 @01:51PM (#24469833)

    Gravity Tractor? You know I love these sky high fantasy ideas to deflect asteroids as much as anyone else but shouldn't we be concentrating on what is real? If an asteroid does threaten Earth in the next few years we will use nuclear demolitions on it. We will not use a gravity tractor, laser beams, or giant snow balls. Nor will we attach plasma engines or mass drivers to it. We will use nuclear demolitions because that is, simply, all we have.

    We will not send a robot to do it nor will we send some type of futuristic space ship driven by plasma/ion engines. It will be a manned ship with old style chemical rockets right out of the '60. Why? Because we have over 60 years experience with them and they will get the job done. We'll send men and not a robot because the mission is to important to have place in the hands on questionable technology. A robot breaks down and the mission is over. With men at least you have some hope they can fix it. Yes, it will probably be one way but the pilots will know that. They will go anyway.

    Yes, we will break it up in to smaller pieces because that is best. Don't give me that shotgun crap about it scattering the damage over a wider area. We will think of that and cover it. If we let a huge honking rock ride in the atmosphere will not even slow it down. It will punch through it like it isn't even there. Worse is it will punch through the crust to the mantel causing shockwaves all around the planet.

    We wont' use one nuke. We will blowup the big one then we will blow up the smaller ones into smaller pieces. We will do this until the chunks are small enough that the atmosphere will handle. With smaller chunks there is more surface area for the atmosphere to work on. Most importantly the smaller chunks will not "crack the crust" as one fat ass one would.

    • by smoker2 (750216)
      You forgot :
      "And no, I am not an astro physicist !"
    • by 1729 (581437) <slashdot1729@NOsPam.gmail.com> on Monday August 04, 2008 @02:01PM (#24470007)

      Gravity Tractor? You know I love these sky high fantasy ideas to deflect asteroids as much as anyone else but shouldn't we be concentrating on what is real? If an asteroid does threaten Earth in the next few years we will use nuclear demolitions on it. We will not use a gravity tractor, laser beams, or giant snow balls. Nor will we attach plasma engines or mass drivers to it. We will use nuclear demolitions because that is, simply, all we have.

      [...]

      We wont' use one nuke. We will blowup the big one then we will blow up the smaller ones into smaller pieces. We will do this until the chunks are small enough that the atmosphere will handle. With smaller chunks there is more surface area for the atmosphere to work on. Most importantly the smaller chunks will not "crack the crust" as one fat ass one would.

      Blowing up an asteroid isn't necessary, and with only a couple of years' notice, it isn't very effective, either. For details, see:

      https://e-reports-ext.llnl.gov/pdf/343984.pdf [llnl.gov]

      Nuclear explosives are a good tool for this job, just not in the way that you think they are.

    • by cnettel (836611)

      You realize that no human has been beyond LEO (more or less) for over 30 years? We DO have the technology to whip up something similar to Deep Impact, or Rosetta, or Deep Space 1, rather quickly. And, frankly, that's all it takes. A reprogrammed ICBM might also makes sense, but a manned mission does not.

      We have lots of ICBMs to expend if the first one fails, and the biggest concern with that approach (ignoring the "many small, but still huge" chunks one, which is quite important) is still the risk for failu

    • Re: (Score:3, Insightful)

      by Tim C (15259)

      Gravity Tractor? You know I love these sky high fantasy ideas to deflect asteroids as much as anyone else but shouldn't we be concentrating on what is real?

      And how exactly do we turn those sky high fantasy ideas into reality, other than by concentrating on them?

      We already have teams looking at doing as you suggest, what do we lose by having other teams look at other ideas? More people working on the same thing won't necessarily make it happen any faster or better. People here of all places should understand

    • by jdigriz (676802) on Monday August 04, 2008 @03:20PM (#24471119)
      A gravity tractor is not a sky-high fantasy idea. It's simply giving a name to the fact that all objects attract each other at least a little. They're hovering a little spacecraft near the asteroid and then moving it away under low-power. The asteroid follows slowly due to the laws of physics, not because the ship carries a star-trek tractor beam. Yes, in the future we will send men to explore non-threatening asteroids. But currently we don't even have manned ships built that can get beyond low earth orbit. So no, the odds are we won't be sending men to do it, except perhaps as a backup. A small robot "tug" like this one is the only option other than nuking it with ICBMs until we have longer-range craft built.
    • by AlecC (512609) <aleccawley@gmail.com> on Monday August 04, 2008 @03:29PM (#24471309)

      I don't think you know what a "Gravity Tractor" is. It is about 20 tons (min) of rock. We have that. It is put close to the asteroid in the direction we want to pull it, and good ol' Newtonian gravity is allowed exert traction (mMG/d^2). Then thrusters on the lump of rock thrust to get rid of the "equal and opposite" reaction pulling the lump onto the asteroid. So, like the classic donkey with a carrot on a string, the asteroid is gently lead away from the collision with earth.

      This is *much* safer than nukes, and *much* lower technology. We can do. No. For mere money.

      • Re: (Score:3, Insightful)

        by Lord Apathy (584315)

        I know exactly what a gravity tug is. I suggested that we use one a few posts back to move a moon from Jupiter to Venus. So, yeah I know what is and how it works. I also know this, we don't have the experience to make it work. And we don't have time to learn.

        You say this is "much safer" like nukes are a bad thing? Actually the nuclear option is much safer because with out present level of understanding it is the best thing that will work. Hell, it's the only thing. Are you opposed to using nukes bec

  • by moteyalpha (1228680) * on Monday August 04, 2008 @02:25PM (#24470343) Homepage Journal
    It seems that if you wanted to change it's course by a continuous amount, that simply increasing it's mass by pushing other material into it's field would make the sun do the work. Just a random thought.
  • Suckers! (Score:3, Funny)

    by g0dsp33d (849253) on Monday August 04, 2008 @02:45PM (#24470653)
    I just got a patent on giant space traffic signs to divert and manage traffic flow. More effective and it does not get stuck in space mud. Plus I can't get sued if my product fails! hahaha I'll be rich.
  • Why is this useful? (Score:3, Informative)

    by david.given (6740) <dg@cowl a r k .com> on Monday August 04, 2008 @03:20PM (#24471109) Homepage Journal

    If you place your massy spacecraft near an asteroid and let go, the two will mutually attract each other and eventually collide. The centre of gravity of the system won't change.

    So, in order for this to work at all, you need a manouevering system on the spacecraft in order to maintain its separation from the asteroid.

    The thing is, though, that from a pure orbital mechanics point of view, this is absolutely equivalent to simply mounting the spacecraft's thrusters on the asteroid itself. In fact, using the gravitational tractor is probably going to be rather less efficient, because the geometry of the system is such that you have to fire your thrusters towards the asteroid --- and a certain amount of your thruster exhaust is going to bounce off the asteroid's surface, imparting momentum in the wrong direction to the asteroid.

    The only things I can think of that the gravity tractor does for you that direct acceleration doesn't is:

    • With the gravity tractor, you don't have to manipulate the asteroid in any way; this may make the engineering considerations easier, as you don't have to worry about the rock collapsing under your rocket's thrust, etc;
    • The gravity tractor operates on the entire mass of the asteroid, rather than applying thrust to one point and relying on the asteroid's integrity to distribute the thrust. This avoids embarassing situations where the asteroid simply disintegrates rather than accelerating, or leaving stuff behind.

    But, given the type of accelerations we're talking about --- which will be the same regardless what technique you're using --- I wouldn't imagine that either of these would be a problem in practice. So, what makes the gravity tractor so much better than just using rockets? Indeed, what makes it better than alternative approaches like spraying the asteroid with aluminium powder (which raises the albedo, causing increased photon pressure, which alters the orbit over time)?

  • by tinkerton (199273) on Tuesday August 05, 2008 @03:48AM (#24477183)

    In one of his books Dyson warns against the idea of blowing up approaching asteroids and proposes mass drivers that push the asteroid into a new orbit over extended periods, possibly years. Factors I can think of from the top of my head:

    - a mass driver needs mass, which it gets from the asteroid. So you don't have to sling huge masses towards the asteroids.
    - anchoring the mass drivers can be a problem if the asteroid is brittle. Bigger asteroids may be stronger but you also have to push harder.
    - you can have many mass drivers
    - the more time you have the better.

    So the special thing about a gravity tractor is that it's a reliable way to attach a mass driver to any type of asteroid. The tractor still needs mass drivers and it needs a lot of mass to build up the gravity. If you see the asteroid as a "soft waterball" that is hard to push, you could consider the tractor as an empty box with . It flies over, fills itself with parts of the asteroid and starts pulling.

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