Gravity Tractor Could Deflect Asteroids 372
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.'"
If I buy a gravity tractor... (Score:5, Funny)
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If they ever do this... (Score:2, Insightful)
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Re:If they ever do this... (Score:5, Funny)
I agree. They should use furlongs and fathoms.
Re:If they ever do this... (Score:4, Funny)
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).
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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.
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It's not always insignificant. The problem mainly comes up when you're trying to accumulate something over a large number of iterations. For example, suppose your speed is the 1.0e0 term and the 1.0e-18 is your acceleration. At first it's insignificant, but over a few thousand (or million) cycles the error in the result becomes noticeable.
You can get around the issue with approaches like the one that this AC [slashdot.org] mentioned, but that requires additional consideration up front. You have to consider the limitations
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.
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)
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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
How about we move this rock instead? (Score:2, Interesting)
In a sense, you could apply the same approach, except try to modify earth's orbit, which might actually be easier...
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?
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Says you. You'll be proved wrong when an asteroid the size of Venus crosses our path. Then who'll be so smug? (Me!)
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
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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.
Re:How about we move this rock instead? (Score:5, Funny)
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I'd like to move it a little bit further away. I'm tired of all this ranting about global warming. Let the polar bears have their little ice floes, and I can get back to reading about something else!
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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.
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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.
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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*
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Oh sure. What could possibly go wrong?
Armageddon 2 (Score:5, Funny)
Re:Armageddon 2 (Score:5, Funny)
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You THINK he died, but he actually hid in a refrigerator. He got the tip from his friend Harrison Ford.
He is Hard to Kill!
Oh wait, that's Steven Seagal. Well anyway, it escapes me now but I know there is something hard about killing Bruce Willis.
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He didn't get destroyed, he got mutated and is now Super Gravity Willis.
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He didn't get destroyed, he got mutated and is now Super Gravity Willis.
Is he constantly orbited by Gary Coleman?
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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!
Sounds overly complex (Score:2)
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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?
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You are presuming that landing is possible. The object in question might be a loosly-conglomerated gravitationally bound pile of rocks and dust.
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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.
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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
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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
Keyhole - Is that a standard term? (Score:2, Interesting)
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.
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Darn (Score:3, Funny)
... and I thought it would be more like this [wikia.com].
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)
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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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.
Re:Coaxing vs Pushing (Score:4, Insightful)
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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
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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
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.
Re:Action and reaction, man (Score:4, Insightful)
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.
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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
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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.
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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.
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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!
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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
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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
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.
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.
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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...
Keyhole? (Score:2)
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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...
How to not get splattered (Score:3, Interesting)
Obligatory Hitchhiker's Guide to the Galaxy quote (Score:3, Funny)
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!
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.
pint sized (Score:2)
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)
Bad idea (Score:2)
Next thing you know, bugs from Klendathu will throw rocks at us too.
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From what I heard, the director hated the book so the movie turned out as a somewhat serious parody. That's why things are so exaggerated in the movie and is one of the main reason why so many people hate it.
Once you know that, IMHO it's a great sci-fi/action movie with over-the-top characters and situations.
Screw the asteroids tracking systems (Score:4, Funny)
We should be watching to see if the dolphins leave the Earth by their own means.
Opportunity to Weaponize (Score:2, Insightful)
Microns... (Score:2)
A micron is a unit of length, not velocity. Do they mean microns per second?
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.
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"And no, I am not an astro physicist !"
Re:Nice, but lets keep it real. (Score:4, Informative)
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.
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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
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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
Re:Nice, but lets keep it real. (Score:4, Informative)
Re:Nice, but lets keep it real. (Score:4, Insightful)
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.
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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
Different approach (Score:3, Interesting)
Suckers! (Score:3, Funny)
Why is this useful? (Score:3, Informative)
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:
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)?
Freeman Dyson's proposal (Score:3, Insightful)
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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Or 1/3rd US population ?
Yes take 1/3rd of the US population and put them on early warning duties. It sounds as this has some pretty hefty notification requirements to be an effective scheme.
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I agree. As a backup plan, we need to be ready to ram a nuke in the keyhole.
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Nukes are better suited for the primary plan:
https://e-reports-ext.llnl.gov/pdf/343984.pdf [llnl.gov]
However, simply blowing up the asteroid at the last minute (Armageddon-style) won't help much.
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Would one or more nuclear explosions affect the orbit of a dinosaur-killer very much? Steering by explosion if you will.
Yup, that's the idea. If the asteroid is detected early enough, the orbit doesn't even need to be altered dramatically to avoid a collision. It might be sufficient to simply to use energy from the blast to heat one hemisphere of an asteroid, which will result in a change in velocity.
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"'m a space cowboy, bet you weren't ready for that! I'm a space cowboy, you know, you know where it's at, yeah yeah yeah yeah." -Steve Miller
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"Some people call me a space cowboy. Some call me the gangster of love. Some people call me Maurice. Cause I speak of the properties of love." - Also Steve Miller
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Cause I speak of the properties of love.
Pompatus [wikipedia.org].
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.
An oversight... (Score:2)
Even so, I am not fully convinced by the illustration as presented. While it may not be a 100% accurate depiction, this is still throwing mass toward the "towed" object. Keep in mind that outside the nozzle, gas tends to expand outwards. It seems to me (as I mentioned) that to negate this effect, you would have to aim the reaction mass even more to the side, which would make the whole thing even more inefficient.
If they
Exhaust is not a pencil beam. (Score:2)
And depending on the distance (though further away poses problems for the gravity-part) the object can subtend a small portion of the exhaust "pattern."
Further, directly above is not the only option. Halo orbits of a single body require constant thrust to maintain, which is actually quite ideal for the situation at hand: allowing the gravity tug to get closer to the center of mass (cos(phi)/r^2 is better than 1/r^2 when you can have a smaller r) and keep the body out of the way of the preferred thrust di
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The scenario you're describing is so mind-numbingly trivial to avoid that even I could do the math involved.
I challenge that. It is anything but trivial. (Score:2)
In addition to that, you have to keep from pushing reaction mass AT the object you are towing. As another poster mentioned, at a minimum this will decrease the efficiency of the operation by a great deal, which makes the balance even more precar
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