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Author Topic: How does a gravity tractor work to pull colliding objects out of Earth's path?  (Read 10187 times)

Offline adamg

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http://www.newscientist.com/article.ns?id=dn8291&feedId=online-news_rss20 [nofollow]

So If a 20 ton ship can deflect a 200 meter asteroid in 20 years at a distance of 1.5 radii, does this mean that moving the gravity tractor a mere 37.5 meters away would allow you to deflect it in 1/16th the time.

I know that the force of gravity is dependent on the inverse square of the distance between bodies, so why do these scientists want to be so far away from the asteroid?

With the asteroid weighing 3 million times more than the tractor wouldn't you want to be as close as possible.

I imagine that it would require more fuel to move the two, but wouldn't this be a means of deflecting NEO that are only 1-2 years away from impact with the earth?

On that note, what does everyone think of the concept of gravity tractors?

They aren't affected by the spin of an object, or its composition, and without contacting the surface they face less risk from damage.

But, it seems like this is only a solution for small NEOs.

Anything larger than 1 kilometer across would be so massive that a 20 ton craft may not be able to move it. Assuming it could, it would require all of its fuel and decades to do it. It may simply run out of fuel before work is accomplished.

Of course humans could always build an enourmous craft of 1000 tons, which carries 350 tons of Uranium as fuel. I imagine that such a craft, if positioned .3 radii from an NEO, (permiting it is not a cylindrical shape that spins rapidly) could, perhaps in 10 years deflect an object as large as, perhaps 3 kilometers.

This leaves us with the obvious problem of dealing with some marge NEOs.

The dinosaurs were done in by a 10 kilometer sized object, which was the size and possible mass of Mount Everest.

If faced with such a threat a tractor would be useless, as the size of the damned thing would require a craft far to big for us to build and that probably couldn't carry enough fuel to stay the necessary time.

In that case there is the option of useing a space mirror to focus sunlight onto a singular point on the object, thus vaporising some material and creating a small jet effect that will slowly push the object away. Of course this would require the mirror to be syncronized to the spin of the object, and the jet effect would be marginalized by the lack of sunlight at certain points as well as the constant spinning altering the deflection path.

Tugs would face the same challenge, but would also face the danger of being damaged.

Nukes are a favorite option of miltary cowboys and hollywood, but unless the object is solid, as indicated by a faster spin, (conglomorations of rubble would fly apart if spinning to quickly.

A ruble pile has been tested in lab conditions and shown to absord the blast of a nuke.

On top of that, nuclear weapons in space are extremely inefficient. With no atmosphere to create a shock wave, the only thing a hydrogen bomb can do is unleash gamma rays and heat. Due to the cold nature of space and the laws of thermo dynamics, the fireball of a nuclear blast dissipates quickly and even if in contact with an object, will not fully vaporise it unless it is massivly engulfing the thing.

Thus, some in the miltary have proposed building a bomb so big that the amount of energy released would vaporize any and all objects as large as 10 kilometers.

They claim that a 1000 megaton bomb, atop a 150 meter long rocket would be sufficient to save us from a large object.

In this sense they are correct. The trouble is that such a device would be incredibly expensive, and if accidentally detonated on earth, would generate a fireball large enough to melt the earths crust, as well as generate a nuclear winter that would most likely kill us all.

Another issue is this. What if we discover a 5 kilometer wide asteroid that is only 1-2 days or even hours from impact with earth?

We may have to detonate the bomb 2-3,000 kilometers above the earth. This would generate an EMP that would likely destroy an entire hemisphere's worth of electronics.

Any thoughts on this most interesting and pressing of matters? All replies are greatly appreciated.

Adam Andrew Galas
« Last Edit: 24/12/2006 09:22:45 by chris »


 

another_someone

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I suspect the reason for the great distance is because of the time required, and because of tidal forces.

The following are purely speculation on my part as for the possible reasons not to get too close to the NEO.

Firstly, as much as the spacecraft will pull the NEO from its trajectory, so too would the NEO pull on the spacecraft, and so the spacecraft will have to constantly fire its motors to hold its position.  Ofcourse, it would want to be far enough away from the NEO in order to ensure that its rocket exhausts do not impact upon the NEO, and thus have the exact opposite effect from that desired.  Also, the closer the spacecraft is to the surface of the NEO, the greater the force of the NEO on the spacecraft, and thus the greater the amount of thrust the spacecraft's engines will have to deliver in order to hold position.  Not only would this greater amount of thrust have greater risk of impinging directly on the NEO, but any rocket motor that is expected to deliver power efficiently over many years will not be expected to be able to deliver a great deal of peak thrust.

Secondly, the closer the two objects are, the greater the differential in gravitational force (i.e. if the spacecraft were very close to the NEO, the effect of the spacecraft's gravity on the nearby surface of the NEO would be very much greater than the effect of the gravity on the surface on the far side of the NEO, and it will thus have the effect of potentially pulling the NEO apart.  By stationing the spacecraft further away from the NEO, the different in gravitational force affecting the near and far side of the NEO would be less, and thus the likelihood of tearing the NEO apart is less.  In a similar vein, if the NEO is spinning, if the spacecraft if too close to the surface of the NEO, it could cause a drag on that spin.



George
 

Offline Atomic-S

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Well, this is a difficult subject, but we likely have a few hundred thousand years to figure it out.
 

another_someone

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quote:
Originally posted by Atomic-S

Well, this is a difficult subject, but we likely have a few hundred thousand years to figure it out.



The problem is that we do not know how long we have it may be within the next decade (or, if something has slipped under our radar, even tomorrow), or it may not be for another million years or more.



George
 

Offline daveshorts

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If it is very close you are in trouble whatever happens, even if you blow the thing up, you will probably get a nuclear winter.

A Gigatonne bomb sounds a little unsubtle to me, I would have thought that a stream of smaller bombs, each that give it a small push would be the best way to go. And would still be far cheaper than building some kind of nuclear powered rocket that needs hundreds of tonnes of nuclear fuel, and then launching it. Using lots of small bombs also has the advantage that if one bomb is a dud you will still move it most of the way.
 

Offline daveshorts

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If it is very close you are in trouble whatever happens, even if you blow the thing up, you will probably get a nuclear winter.

A Gigatonne bomb sounds a little unsubtle to me, I would have thought that a stream of smaller bombs, each that give it a small push would be the best way to go. And would still be far cheaper than building some kind of nuclear powered rocket that needs hundreds of tonnes of nuclear fuel, and then launching it. Using lots of small bombs also has the advantage that if one bomb is a dud you will still move it most of the way.
 

another_someone

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quote:
Originally posted by daveshorts

If it is very close you are in trouble whatever happens, even if you blow the thing up, you will probably get a nuclear winter.

A Gigatonne bomb sounds a little unsubtle to me, I would have thought that a stream of smaller bombs, each that give it a small push would be the best way to go. And would still be far cheaper than building some kind of nuclear powered rocket that needs hundreds of tonnes of nuclear fuel, and then launching it. Using lots of small bombs also has the advantage that if one bomb is a dud you will still move it most of the way.



If one is going for an explosive solution, then I would tend towards the multiple small explosion option, but not sequential explosions, rather a shaped explosion created by an arc of explosive charges in order to try and contain the shrapnel from the shattered NEO.

The problem with whatever explosive solution you take is how you manage the spin of the object if it has any?  If the object shatters, then the spin could cause shrapnel to go flying of at tangents; and even if it holds together, the spin could still deflect the force of the explosion.



George
 

Offline adamg

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I believe the rational given for the gigaton bomb was that it is so large the fireball it generates will engulf any small to medium sized NEO and vaporize it, no matter its spin, velocity, density, composition, ect. The idea was that a big enough bomb will simply overwhelm all variables and blow them all to hell, leaving no debris to worry about.

Many smaller bombs would face the problem of a ruble pile is that the pile may disintegrate and the subsequent bombs would no longer have 1 target but dozens or hundreds. The incoming missiles would continue to detonate which would only scatter the rubble worse.

The idea however, was in the end dubbed "insane" as it would have been a larger threat to the earth than any likely NEO.

Of course, that leaves us with the question of saving the world when we are threatened by large space rocks. I somehow doubt that a gravity tractor could be built in time. After all, who is going to fund a gravity tractor? It will cost tens of billions, and if the Americans are not directly threatened they will not pay for it, at least not the current administration.

I fear that if Apophis does threaten us in 2036 that each government will simply ask the question, "will it hit us or cause a Tsunamai that will?" If the answer is no, they will find excuses not to pay the costs.

Perhaps we can convince Richard Branson, if he is still alive, to build the tractor. If he can't afford it, then maybe the new $61 billion Bill and Melinda Gates Foundation could pony up the necessary funds.

Adam Andrew Galas
 

another_someone

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The problem is that it is not enough simply to build a device when we need it.  We need to develop and test the technologies long before we actually need to use it in anger, knowing full well that many of the early attempts will fail.  It is thus all the more important that we develop technologies that we are able to test with safety, and don't find that one of the tests that go wrong (as some will) end up putting us in almost as much peril as the actual situation we are trying to protect ourselves from.  For this reason, it is also necessary to have long term funding, not merely a single large cash injection.



George
 

Offline daveshorts

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I wasn't intending on usin the little bombs to destroy the thing, just to push it away, you set them off nearby the radiation causes the surface to vaporise giving the object a hefty shove. I have a feeling you could probably design the bomb in a clever way so most of the radiation goes in the direction you want it to. The rubble pile exploding would be a pain, but i would have thought that with a couple of year's warning you could move most of the mass out of the way of the earth, even if you got a fairly impressive fireworks display from the odds and ends. This way you may have a couple of bad winters from the dust in the atmosphere, but you avoid the tsunamis, much worse nuclear winter (as the earth's surface will be vaporised too), etc etc.

In fact even if you vaporised the thing you would still have several cubic km of dust hitting the atmosphere anyway.
 

another_someone

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quote:
Originally posted by Atomic-S

Well, this is a difficult subject, but we likely have a few hundred thousand years to figure it out.



The problem is that we do not know how long we have it may be within the next decade (or, if something has slipped under our radar, even tomorrow), or it may not be for another million years or more.



George
 

Offline daveshorts

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If it is very close you are in trouble whatever happens, even if you blow the thing up, you will probably get a nuclear winter.

A Gigatonne bomb sounds a little unsubtle to me, I would have thought that a stream of smaller bombs, each that give it a small push would be the best way to go. And would still be far cheaper than building some kind of nuclear powered rocket that needs hundreds of tonnes of nuclear fuel, and then launching it. Using lots of small bombs also has the advantage that if one bomb is a dud you will still move it most of the way.
 

Offline daveshorts

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If it is very close you are in trouble whatever happens, even if you blow the thing up, you will probably get a nuclear winter.

A Gigatonne bomb sounds a little unsubtle to me, I would have thought that a stream of smaller bombs, each that give it a small push would be the best way to go. And would still be far cheaper than building some kind of nuclear powered rocket that needs hundreds of tonnes of nuclear fuel, and then launching it. Using lots of small bombs also has the advantage that if one bomb is a dud you will still move it most of the way.
 

another_someone

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quote:
Originally posted by daveshorts

If it is very close you are in trouble whatever happens, even if you blow the thing up, you will probably get a nuclear winter.

A Gigatonne bomb sounds a little unsubtle to me, I would have thought that a stream of smaller bombs, each that give it a small push would be the best way to go. And would still be far cheaper than building some kind of nuclear powered rocket that needs hundreds of tonnes of nuclear fuel, and then launching it. Using lots of small bombs also has the advantage that if one bomb is a dud you will still move it most of the way.



If one is going for an explosive solution, then I would tend towards the multiple small explosion option, but not sequential explosions, rather a shaped explosion created by an arc of explosive charges in order to try and contain the shrapnel from the shattered NEO.

The problem with whatever explosive solution you take is how you manage the spin of the object if it has any?  If the object shatters, then the spin could cause shrapnel to go flying of at tangents; and even if it holds together, the spin could still deflect the force of the explosion.



George
 

Offline adamg

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I believe the rational given for the gigaton bomb was that it is so large the fireball it generates will engulf any small to medium sized NEO and vaporize it, no matter its spin, velocity, density, composition, ect. The idea was that a big enough bomb will simply overwhelm all variables and blow them all to hell, leaving no debris to worry about.

Many smaller bombs would face the problem of a ruble pile is that the pile may disintegrate and the subsequent bombs would no longer have 1 target but dozens or hundreds. The incoming missiles would continue to detonate which would only scatter the rubble worse.

The idea however, was in the end dubbed "insane" as it would have been a larger threat to the earth than any likely NEO.

Of course, that leaves us with the question of saving the world when we are threatened by large space rocks. I somehow doubt that a gravity tractor could be built in time. After all, who is going to fund a gravity tractor? It will cost tens of billions, and if the Americans are not directly threatened they will not pay for it, at least not the current administration.

I fear that if Apophis does threaten us in 2036 that each government will simply ask the question, "will it hit us or cause a Tsunamai that will?" If the answer is no, they will find excuses not to pay the costs.

Perhaps we can convince Richard Branson, if he is still alive, to build the tractor. If he can't afford it, then maybe the new $61 billion Bill and Melinda Gates Foundation could pony up the necessary funds.

Adam Andrew Galas
 

another_someone

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The problem is that it is not enough simply to build a device when we need it.  We need to develop and test the technologies long before we actually need to use it in anger, knowing full well that many of the early attempts will fail.  It is thus all the more important that we develop technologies that we are able to test with safety, and don't find that one of the tests that go wrong (as some will) end up putting us in almost as much peril as the actual situation we are trying to protect ourselves from.  For this reason, it is also necessary to have long term funding, not merely a single large cash injection.



George
 

Offline daveshorts

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I wasn't intending on usin the little bombs to destroy the thing, just to push it away, you set them off nearby the radiation causes the surface to vaporise giving the object a hefty shove. I have a feeling you could probably design the bomb in a clever way so most of the radiation goes in the direction you want it to. The rubble pile exploding would be a pain, but i would have thought that with a couple of year's warning you could move most of the mass out of the way of the earth, even if you got a fairly impressive fireworks display from the odds and ends. This way you may have a couple of bad winters from the dust in the atmosphere, but you avoid the tsunamis, much worse nuclear winter (as the earth's surface will be vaporised too), etc etc.

In fact even if you vaporised the thing you would still have several cubic km of dust hitting the atmosphere anyway.
 

Offline bigtim

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quote:
Originally posted by adamg

...force of gravity is dependent on the inverse square of the distance between bodies...



This is a common missconception. Gravity is not a force. The force is the product of mass and the strength of the gravitational filed, ie. the quotient of the ratio force/mass.

Big Tim
 

Offline lightarrow

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quote:
Originally posted by bigtim


This is a common missconception. Gravity is not a force. The force is the product of mass and the strength of the gravitational filed, ie. the quotient of the ratio force/mass.
I don't think it's really necessary to be so nitpicking in this case.

About the distance between the spacecraft tractor and the asteroid, in addition to what another_someone said, maybe it's also because gas exaust from the spacecraft's engine shouldn't hit the asteroid (or the effect would partially vanish).
« Last Edit: 23/09/2006 07:36:12 by lightarrow »
 

Offline chris

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Here's a link to a transcript of my interview with NASA astronaut Stan Love, who published the Asteroid Tractor article on which this thread is based, in late 2005.

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx
« Last Edit: 24/12/2006 09:21:50 by chris »
 

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