Gravity Tractor Could Deflect Asteroids

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.'"

Comments

If they ever do this...

[DoofusOfDeath]

I hope their simulations use doubles, not floats!

Re:If they ever do this...

[Z_A_Commando]

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!

Re:If they ever do this...

[HairyCanary]

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.

Coaxing vs Pushing

[flaming error]

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

[Dr. Eggman]

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.

Re:Coaxing vs Pushing

[MyLongNickName]

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 fast acceleration.

Ramming the ship into the asteroid is more efficient, but has the problem of potentially busting up chunks of the asteroid, imprecise measurements, and the inability to monitor and make adjustments as the asteroid is moved.

Re:Coaxing vs Pushing

[caywen]

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 the logistics. Perhaps landing safely on the thing is much harder than maintaining a fixed distance. If that's the case, I would get it. Sorry, I didn't do any real math, this is just a from the hip question.

Re:Coaxing vs Pushing

[Bemopolis]

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.

Re:Coaxing vs Pushing

[ceoyoyo]

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.

Re:Coaxing vs Pushing

[AlecC]

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 be extremely unpredictable. By contrast, the gravity tractor is pretty precise to start with, and the longer it runs the better we know the asteroids mass and the prciser it becomes.

This is not new news - I first heard it about a year ago. But the idea is getting critical oversight, and standing up to it, which is promising. But basically, when I heard this idea, I stopped panicing about asteroids. If we get three years warning, we can fix it. It may take Apollo-scale expenditure, but it can be done. Actually, my estimate would be lower - range $10-40 billion. Which has to be a hell of a sight better cost/benefit ration than the Iraq war.

Re:How about we move this rock instead?

[4D6963]

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?

Um, dumb question time

[smooth wombat]

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

[Colonel Korn]

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.

Re:Um, dumb question time

[the_other_chewey]

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.

Opportunity to Weaponize

[garnkelflax]

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.

Nice, but lets keep it real.

[Lord Apathy]

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:Nice, but lets keep it real.

[Tim C]

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 that.

Re:Nice, but lets keep it real.

[AlecC]

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:Nice, but lets keep it real.

[Lord Apathy]

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 because they have that word nuclear in them?

We would not just start lobbing nuclear bombs at a target and hoping one of them works. We would send a highly trained team out there to ether demolish it into manageable chunks or change is course with nuclear demolitions, with a carefully thought up plan.

You know why all this solar sail and gravity tug plan came up? or why the nuclear option has been pretty much shunned? Because some hippie scientist didn't like the idea of using nukes in space, if forget his name, not even to save the planet.

Action and reaction, man

[Moraelin]

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.

Re:Action and reaction, man

[Mattsson]

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.

"Long enough time"

[Anonymous Coward]

If you do the math, the shift is about 1.3 km during the first year. In ten years, the shift is 130 km, and in a hundred, 13000 km (~ one earth diameter). So we need to detect the rock a century before the impact, make a very precise calculation of its trajectory and park the rocket next to the asteroid for the whole duration possibly replacing or refueling it many times over.

Freeman Dyson's proposal

[tinkerton]

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.