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Gravity Tractor Could Deflect Asteroids
Posted by
ScuttleMonkey
on Mon Aug 04, 2008 12:55 PM
from the massive-undertakings dept.
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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British Company Takes Lead To Stop Asteroids 198 comments
An anonymous reader writes to tell us that following the news of NASA's budget cuts impacting their ability to do things like watch the sky for asteroids, a British company has decided to create a "gravity tractor" ship that could divert asteroids away from Earth if the need should arise. Of course, a gravity tractor certainly isn't a new idea. "Dr. Cordey said the company had worked with a number of space authorities on other methods of protecting the Earth from asteroids, but this one would be able to target a wider range. He said: 'We have done quite a lot of design work on this with the European Space Agency and we believe this would work just as well on a big solid iron asteroid as well as other types.' But the high cost implications mean that before the device could be made, it would have to be commissioned by a government or a group of governments working together."
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If I buy a gravity tractor... (Score:5, Funny)
Armageddon 2 (Score:5, Funny)
Re:Armageddon 2 (Score:5, Funny)
Parent
Preliminary testing. (Score:5, Funny)
Will be done by holding monthly Gravity Tractor Pulls at the local fairgrounds, with free beer.
Re:Preliminary testing. (Score:5, Funny)
But it'll take a while until all testing is completed because they'd only do testing on Sunday, Sunday, Sunday!
Parent
Coaxing vs Pushing (Score:4, Insightful)
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?
Re:Coaxing vs Pushing (Score:5, Insightful)
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Re:Coaxing vs Pushing (Score:4, Interesting)
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.
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Action and reaction, man (Score:5, Insightful)
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.
Parent
Um, dumb question time (Score:4, Insightful)
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.
Re:Um, dumb question time (Score:5, Insightful)
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.
Parent
Re:Um, dumb question time (Score:4, Insightful)
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.
Parent
OT: Orbit@Home is now NASA-funded (Score:5, Interesting)
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.
Dupe (Score:4, Informative)
Screw the asteroids tracking systems (Score:4, Funny)
We should be watching to see if the dolphins leave the Earth by their own means.
Nice, but lets keep it real. (Score:5, Insightful)
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.
Re:If they ever do this... (Score:5, Informative)
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).
Parent
Re:If they ever do this... (Score:5, Insightful)
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.
Parent
Compensated summation (Score:5, Informative)
(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)
Parent
Re:If they ever do this... (Score:4, Interesting)
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.
Parent
Re:If they ever do this... (Score:5, Funny)
I agree. They should use furlongs and fathoms.
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Re:If they ever do this... (Score:4, Funny)
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Re:How about we move this rock instead? (Score:5, Insightful)
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?
Parent
Re:How about we move this rock instead? (Score:5, Funny)
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Re:I hate to accuse JPL of forgetting something... (Score:5, Informative)
Who modded the parent "insightful". The answer is pretty simple, and is even illustrated in the article. The picture shows a craft with three thrusters all angled away from the asteroid. The resulting thrust is a vector normal to the target. Sure, it sacrifices efficiency, but it works.
Parent