How Can We Effectively Destroy An Asteroid Headed for Earth?

[Line]

They say if we needed to destroy and asteroid headed for Earth the fastest easiest way is witha nuclear bomb. If we used it it would destroy it but it would create thousands of pieces falling to Earth which would be much worse.

In a special it was said that it would take a bomb on the order of 1,000 megatons to completely destroy an asteroid. It would not only be the biggest bomb ever built but it would need to be put on the biggest rocket ever built. My soloution could be simple. INstead of building one big bomb build about 10 smaller ones. launch 10 smaller nuclear warheads and attach them all to the asteroid. THen at the precise moment they all go off.

SInce there would be 10 each one would be around 100 megatons. And since it not one concentrated into one place they would have a better chance of destroying all the asteroid. I think 100 megatons is well within our bombmaking capabilitites. If we make them light enough we could put 2 on each rocket only requiring 5 launches. This sounds easy and simple. Why hasn't anybody ever thought of this?

 

[DaveC426913]

This sounds easy and simple. Why hasn't anybody ever thought of this?

They have. Because it isn't.

There is a wealth of info to read on the shortcomings of any variation of the concept of blowing up an asteroid with a missile.

 

[Astronuc]

Why hasn't anybody ever thought of this?

How does one know that some have not already addressed this matter?

Those who would contemplate such technology do not talk about it openly.

It is more likely that one would divert an asteroid rather than blowing it up. Blowing it up would put a lot of debris near earth, and debris in orbit is something to be avoided. It has the potential to lots of damage to satellites, spacecraft , and space stations.

 

[Line]

How does one know that some have not already addressed this matter?

Those who would contemplate such technology do not talk about it openly.

It is more likely that one would divert an asteroid rather than blowing it up. Blowing it up would put a lot of debris near earth, and debris in orbit is something to be avoided. It has the potential to lots of damage to satellites, spacecraft , and space stations.

Yaaay let's blow up an asteroid, that can destroy us,mankind,our history,and all our fututre. But wait, the debris will hit our sattelites...damn. Oh well.


And the moral is, sattelites are mroe important than mankinds survival.

 

[Astronuc]

Yaaay let's blow up an asteroid, that can destroy us,mankind,our history,and all our fututre. But wait, the debris will hit our sattelites...damn. Oh well.

And the moral is, sattelites are mroe important than mankinds survival.

No quite. Find a way to deflect the asteroid so that it misses Earth and avoid damage to satellites, which provide for monitoring of weather and for communication. Perhaps the asteroid could be blown up after is passes earth.

 

[Line]

If it's going to pass Eartht here's no need to worry.

Anyhow, it would take some state of the art stuff to deflect it. Unless of course you want to use nuclear weapons. That of course would be if we have time. If we find an asteroid tommorow and finfout it's goign to collide with Earth 2 weeks from now there'd be little we could do but destroy it.

Deflectin take lots of time,planning, and cordination. My favorite is we attach huge rockets to it, slow it down, and mine the thing for minerals.
There's enough raw material in the average asteroid to wipe out the national debt.

 

[LURCH]

Setting aside the risk to the satellites and space stations, etc., deflecting an incoming threat is far more desirable than blowing it to bits. A single impact at a single location on Earth's surface would be devastating, and if you would survive, but breaking the abject into thousands of impacts over the entire surface would ensure that no one survives. But it just so happens that nuclear weapons are the best tools for deflecting a potential impactor.

I think this is another excellent reason why we need a lunar launch facility. Storing thousands of nuclear weapons mounted on launch vehicles on Earth has always been a bad thing; it's inherently dangerous. The same weapons sitting on the surface of the moon (or in orbit around the moon) pose no threat to those of us here on Earth. If the orbit decays, or there is an accidental launch or detonation, the event will go virtually unnoticed. If someone launches a weapon "in anger", there will be almost a week during which to intercept it.

A lunar launch facility for nukes also puts them in a position to be launched with greater velocity, and therefore intercept the object sooner.

 

[DaveC426913]

The same weapons sitting on the surface of the moon (or in orbit around the moon) pose no threat to those of us here on Earth. If the orbit decays, or there is an accidental launch or detonation, the event will go virtually unnoticed.

Are you completely ignoring the lessons learned from Space: 1999?

 

[theCandyman]

I thought that scientists and astronomers had a pretty good idea of anything threatening that was headed toward earth?

 

[DaveC426913]

I thought that scientists and astronomers had a pretty good idea of anything threatening that was headed toward earth?

No way. It is well-known that this is impossible with today's technology and manpower.

 

[Line]

Once again ou can destroy an asteroid with nuclear power. YOu could breakit up into piece with a nuclear bomb. Or you can completley destroy it with a 1,000 megaton bomb.

 

[DaveC426913]

Once again ou can destroy an asteroid with nuclear power. YOu could breakit up into piece with a nuclear bomb. Or you can completley destroy it with a 1,000 megaton bomb.

No. You can't.

- we don't have the sky watch system to see it in time
- we don't have the rocket power to get the payload there at all, let alone in any decent time frame
- we don't have guidance systems that could put it in the right place at the right time (it will be moving quite fast)
- nuclear bombs simply will NOT destroy an asteroid
..- we have no way of controlling what the detonation(s) do in terms of destruction, best we can hope for is to crack it into a few very large pieces, all of which are now on a collision course with Earth
..- even the most ambitious collection of nuclear warheads simply does not vapourize of cubic kilometers of solid rock

(A moderate asteroid is between a thousand and a million Gigatons. Does that put it in perspective?)

I mean, the idea sounds good at first blush, but when you get down to brass tacks, it simply doesn't work on many, many levels.

 

[Line]

We can send bombs, if we do smaller ones. If we can fit space telescopes on rockets and send probes to Neptune I'm sure we can get a bomb to just outside Earth orbit.

Guidance should be no problem since we've landed things on asteroids and flown into comets. Once again it would take 1,000 megatons to completely destroy it. If ou think destroying it takes time guess how long deflecting it takes.

And I don't see how a bomb could deflect it. Nukes just let off lots nergy like heat.If heat just travels to the asteroid it should just melt it. The reason nukes blow things up on Earth is because the air expands, moves and knocks things down. IN space there is no air and hence nothing can be blown off course.

 

[DaveC426913]

Some of this stuff is pretty basic physics.

We can send bombs, if we do smaller ones. If we can fit space telescopes on rockets and send probes to Neptune I'm sure we can get a bomb to just outside Earth orbit.

There's timing logistics. If we were to blow it up near Earth, we'd be pelted with an asteroid's worth of debris, causing even more damage than a single rock.

Do you know how long those trips take? Years. Decades.

Guidance should be no problem since we've landed things on asteroids and flown into comets.

Well, we haven't landed on asteroids, no. We've flown one spaceship into one comet, yes.

Once again it would take 1,000 megatons to completely destroy it.

I reeally don't know why you keep repeating this, like it's some sort of mantra. Other than a wild guess, do you have any evidence to support this claim?

And I don't see how a bomb could deflect it. Nukes just let off lots nergy like heat.

Yes, kind of a lot like a rocket engine. You do know that rockets work in space without benefit of air, right?

 

[Line]

Deflecting it would take more time and there was a special on TV about doomsday. It has been calculated that about 1,000 Megs is needed.

ANd yes rockets need no wind to work. And we have ladned on an asteroid. It wasn't all hi-tech withl legs but I probe was landed on an asteroid.

 

[LURCH]

Yes, the NEAR probe landed on Eros.
http://liftoff.msfc.nasa.gov/news/2001/news-NEAR.asp

Unfortunately, early detection is still a very large problem. Not too many years ago, a rather large asteroid (the kind that could do damage on a global scale) was detected after it had made a close pass by the earth. I remember hearing that Earth had occupied that exact spot only six hours earlier. IIRC, the highly elliptical orbit of the object brought it very near to the sun, and it passed the Earth on its way back out to space. This caused it to come at us "out of the sun", like that tactic used by fighter pilots, with its dark side towards us. As a result, we did not see the object until after it had passed, and sunlight was seen reflecting off its rear as if to say, " bite my shiny metal...".

This object is thought to be about a half-mile in diameter, and would have released an energy of about 2000 megatons, leaving a crater anywhere from 5 to 10 miles in diameter. The first warning anyone on Earth would have had would be the sprouting of a huge mushroom cloud.

I'm wondering if nukes could be used in a combination of "breaking up" the object and deflection?

 

[DaveC426913]

I'm wondering if nukes could be used in a combination of "breaking up" the object and deflection?

These two methods are mutually exclusive and counterproductive. If you break it up, you now have a zillion unpredictable pieces to deflect. If you're going to deflect it, you want it to be in one piece.

 

[LURCH]

That's what I used to think, but now I begin to question whether that is necessarily true. I have been thinking of a strategy in which several missiles are sent to intercept an object at various points along its collision trajectory. This could add redundancy to the system in case one or more missiles failed to guide, or to detonate. Sadly, early detection would still be the key. And early detection is exactly where we are weak.

The idea occurred to me when I was thinking about the fact that, in order to be on a collision course with Earth, an object must follow a very precise path through both space and time. Disrupting such an object with a large explosion would cause it to break into hundreds, or possibly thousands of pieces. But, by the very nature of the event, these pieces are no longer on the same path. Neither are they on parallel paths. The fact that they are moving away from one another means that they cannot all be headed toward the same destination. So, if a particular fragment is still on a collision course with Earth, then any other fragment moving away from that fragment cannot be on that same collision course. In fact, only a very small number of those fragments could still be on a collision course, and these will all be in fairly close proximity to one another. This small group of fragments will be on or clustered around the original path of the original object (the "collision trajectory").

Objects on that trajectory will be present at the detonation of several subsequent nuclear devices. The ionizing radiation that makes a nuke useful for deflecting one large object will work even better on several smaller objects. These subsequent detonations will hollow out a "hole" in the debris field, centered around the "collision trajectory". It is this hollow area that will collide with Earth. The remaining fragments in the debris field will be on new trajectory so deviant from the original that they should no longer pose a threat.

 

[DaveC426913]

I see a few problems with your approach:

"...intercept an object at various points along its collision trajectory..."

How is this better than "all at once?" I grant that redundancy is a good thing, but spreading the impacts out over time gains nothing while costing a certain amount of predictability.


"...these pieces are no longer on the same path. Neither are they on parallel paths..."

The relative speed of expanding objects compared to the relative speed of the approach to Earth would make them effectively parallel. The question becomes "If we blow off large chunks, how much speed - if any - did we impart upon these small chunks to deflect them from Earth?"

Also, don't forget that, as individual free-floating bodies, these large chunks are now free to be pulled toward Earth on their own individual courses as they near. Just because they were given a shove doesn't mean they won't still converge on the Earth.

 

[LURCH]

The relative speed of expanding objects compared to the relative speed of the approach to Earth would make them effectively parallel. The question becomes "If we blow off large chunks, how much speed - if any - did we impart upon these small chunks to deflect them from Earth?"

Yeah, that's what I was sayin' about the importance of early detection. The original idea - sending a device to detonate outside the threat object and counting on radiation pressure to give a tiny nudge - was always dependant on getting to the object early enough so that such a tiny nudge would be sufficient.

The two advantages I think we could gain are:

1) that the initial blast would be contained inside an enclosed space, so far more of the device's energy would be imparted to the change of course for the resulting fragments. Whatever acceleration this imparts to the fragments would be much greater than that which would be imparted to the whole object by a nearby, exterior blast.

2) The majority of the object's mass having been deflected, the use of radiation pressure to alter the course of those fragments which still pose a threat would be far more effective, because it would be using the same amount of energy to move a far smaller mass.

Each blast will accelerate these small fragments a little more, with those pieces that are closest to the collision trajectory getting the most acceleration. This will create an ever-expanding hole at or near the center of the debris field. This hole will continue to expand at whatever speed was imparted to the fragments by the final blast. If all of this takes place many years before the proposed collision date, then by the time that date arrives, no fragment will be anywhere near the Earth.

At least, that's the theory.

 

[Morbius]

How does one know that some have not already addressed this matter?

Those who would contemplate such technology do not talk about it openly.

It is more likely that one would divert an asteroid rather than blowing it up.

Astronuc,

The issue of Planetary Defense against asteroids and other NEOs was the subject of
a workshop held at Lawrence Livermore National Laboratory a little over a decade ago:

http://www.llnl.gov/planetary/

Astronuc is quite correct here - the focus is on deflecting rather than destroying the
asteroid.

Dr. Gregory Greenman
Physicist

 

[Chronos]

Deflection is definitely the preferred alternative for reasons already well explained. Defelection, however, is more complicated than it appears at first glance. Nuclear blasts in space would not generate shock waves like they do on Earth - there is no atmosphere. You need to apply kinetic energy to the trajectory. A nuke would would not be effective unless detonated very near the surface of the intruder.

 

[POSITION_VECTOR]

diverting an asteroid is more plausible...and it does not need NUKE YOO LAR bombs. have you ever heard of gravitational force? you place a big enough mass with rockets near the asteroid and its gravitational force can divert the asteroid.

maybe you ought to work for jerry bruckheimer or some other hollywood movie studio focused on entertaining the ignorant and the ADD public.

 

[LURCH]

As mentioned earlier, nukes are the only method we have. Some other approaches that would be very nice have been proposed, but all involve equipment that does not exist.

 

[Morbius]

Deflection is definitely the preferred alternative for reasons already well explained. Defelection, however, is more complicated than it appears at first glance. Nuclear blasts in space would not generate shock waves like they do on Earth - there is no atmosphere.

Chronos,

I'm afraid your are in ERROR.

A nuclear blast in space in the vicinity of the asteroid DOES generate a shock wave in
the asteroid, and it doesn't require an atmosphere. You just don't understand the
mechanism.

When the nuclear device is detonated in the vicinity of the asteroid; it will intensely
radiate the surface of the asteroid. It is the radiative heating and blow-off of asteroid
material that WILL launch as shock wave into the asteroid.

That's how inertial confinement fusion, or laser fusion works also. The intense
radiation by the laser, and the blow-off or ablation of the fusion pellet surface, will
launch a shock wave into the fusion pellet.

Dr. Gregory Greenman
Physicist

 

[Morbius]

diverting an asteroid is more plausible...and it does not need NUKE YOO LAR bombs. have you ever heard of gravitational force? you place a big enough mass with rockets near the asteroid and its gravitational force can divert the asteroid.

maybe you ought to work for jerry bruckheimer or some other hollywood movie studio focused on entertaining the ignorant and the ADD public.

POSITION_VECTOR,

I'm afraid YOU are the one that should be working for Hollywood.

Have you calculated how large a mass one needs to divert an asteroid?
Where are you going to get this large mass?

Yes - we've heard of gravitational force; and we also know that the gravitational
force is the WEAKEST of all the primordial forces.

Your suggestion is ridiculous. If you had rockets that were powerful enough to boost
enough mass to deflect the asteroid, then you could just use those rockets,
themselves to push the asteroid.

Any energy you put into deflecting the asteroid via gravity, essentially
ultimately came from energy of the rockets that put your large mass in
the vicinity of the asteroid. So why not use those rockets directly?
That way you won't have wasted the residual gravitational potential energy
that will be in the mass after it deflected the asteroid.

Additionally, we don't have rockets that are that powerful.

In order to divert a large asteroid; we need to alter its course; and that is going to
take a good deal of energy. The most compact way of transporting energy to the
asteroid is to transport it as nuclear energy!

Pound for pound, nuclear energy is about 1 Million times more "energy dense" than
any type of chemical energy.

This was all very thouroughly discussed / studied over 10 years ago at the
Planetary Defense Workshop that I refer to above.

Dr. Gregory Greenman
Physicist

 

[DaveC426913]

diverting an asteroid is more plausible...and it does not need NUKE YOO LAR bombs. have you ever heard of gravitational force? you place a big enough mass with rockets near the asteroid and its gravitational force can divert the asteroid.

maybe you ought to work for jerry bruckheimer or some other hollywood movie studio focused on entertaining the ignorant and the ADD public.

omg this must be a joke.

 

[Morbius]

omg this must be a joke.

Dave,

That's what I would expect; but he forgot to tag the end with a

Dr. Gregory Greenman
Physicist

 

[POSITION_VECTOR]

one only needs to find the asteroid before it gets anywhere close to Earth. within a time frame of about 5-10 years, do you not think that we can divert an asteroid with a satellite that can be trasported near it? It's more plausible than blowing it up with nuclear missles. or transporting a rocket there. A satellite like object near the asteroid can offset it's course by a very small amount, but if done at a large distance away from earth...it can significantly deflect the asteroid's path. use of missles are just plain ridiculous and may be considered if we detect this thing too late...but then again...it's too late.

 

[LURCH]

omg this must be a joke.

This is NO JOKE, of that I am quite certain. I have seen this method discussed before, and it is being considered quite seriously. It was proposed by a couple of astronauts at NASA. However, the OP seemed to be suggesting the transportation of an object large enough to have a gravitational pull on the threat-object sufficient to use for towing. This may not have been the poster's intent, it just reads that way. Morbius' response was quite correct; if you could move something that big, move the meteor.

The proposal I saw was to send a rocket out to the meteor, and let the meteor's own gravity grab the rocket. Then, the rocket could thrust away from the meteor with some force less than the force it would need to escape, and the meteor gets towed by its own gravitational field. This would forstall many of the problems of planting a thruster on the surface, since the technique will work whether the object is spinning or not, and regardless of composition.

DS1 proved that the required navigational precission exists. But this technique would also require a thrustre with extreme longevity. The only known candidate would be an ion propulsion system. This is another technology used by DS1, but the job at hand would require a truly enourmous ion drive. At present, no such thruster exists.

Also, this approach would be entirely dependant on the object being large enough (massive enough) to have a sufficient gravitational field to keep it gravitationaly attached to the rocket while the rocket supplies enough thrust to generate the necessary change in course. Very few NEO's have that much mass, yet many have more than enough mass to be serious threats.

 

[DaveC426913]

The proposal I saw was to send a rocket out to the meteor, and let the meteor's own gravity grab the rocket. Then, the rocket could thrust away from the meteor with some force less than the force it would need to escape,

Okay, well this is functionally identical to flying a rocket out and attaching it the asteroid and letting the rocket thrust do the work. The fact that you are attaching the rocket to the asteroid by gravity is a logistical detail.

 

[DaveC426913]

Dave,

That's what I would expect; but he forgot to tag the end with a

Dr. Gregory Greenman
Physicist

And no, he doesn't seem to be joking.

 

[LURCH]

Okay, well this is functionally identical to flying a rocket out and attaching it the asteroid and letting the rocket thrust do the work. The fact that you are attaching the rocket to the asteroid by gravity is a logistical detail.

Exactly. It's just a way of elliminating the problems of attaching a thruster to the asteroid if it's spinning, or if it's composition is a loose conglomorite rather than a solid rock. (Or if these properties are unknown)

BTW, I think it should be stated that this is a rather enginious solution to those problems, but with the limitations mentioned earlier, it just doesn't seem practicle.

 

[DaveC426913]

Exactly. It's just a way of elliminating the problems of attaching a thruster to the asteroid if it's spinning, or if it's composition is a loose conglomorite rather than a solid rock. (Or if these properties are unknown)

BTW, I think it should be stated that this is a rather enginious solution to those problems, but with the limitations mentioned earlier, it just doesn't seem practicle.

Right. OK, this all stemmed from P_V's suggestion of using gravity of a moveable object to deflect the asteroid.

These two solutions have nothing whatever to do with each other.

 

[Chronos]

Chronos,

I'm afraid your are in ERROR.

A nuclear blast in space in the vicinity of the asteroid DOES generate a shock wave in
the asteroid, and it doesn't require an atmosphere. You just don't understand the
mechanism.

When the nuclear device is detonated in the vicinity of the asteroid; it will intensely
radiate the surface of the asteroid. It is the radiative heating and blow-off of asteroid
material that WILL launch as shock wave into the asteroid.

That's how inertial confinement fusion, or laser fusion works also. The intense
radiation by the laser, and the blow-off or ablation of the fusion pellet surface, will
launch a shock wave into the fusion pellet.

Dr. Gregory Greenman
Physicist

Perhaps you missed my point. Nuclear blasts in space do not produce shock waves like they do in air, water, etc, since there is no medium available to propogate a shock wave. So, a nuke would have to be detonated near enough to the surface of the asteroid to vaporize some of its mass in order to impart any momentum. Explosions in a vacuum generate very high energy projectiles. Explosions in a medium generate high energy shock waves, which is a far more effective way to impart kinetic energy to a mass. Detonating a nuke near enough to the surface of an asteroid to be effective risks fragmenting it.

 

[Astronuc]

Nuclear blasts in space do not produce shock waves like they do in air, water, etc, since there is no medium available to propogate a shock wave. So, a nuke would have to be detonated near enough to the surface of the asteroid to vaporize some of its mass in order to impart any momentum. Explosions in a vacuum generate very high energy projectiles. Explosions in a medium generate high energy shock waves, which is a far more effective way to impart kinetic energy to a mass. Detonating a nuke near enough to the surface of an asteroid to be effective risks fragmenting it.

Morbius certainly understands the physics of nuclear explosions in a vacuum.

The point of detonating a nuclear warhead is exactly to use the 'radiation/thermal' energy to vaporize some mass of the asteroid which will 'deflect' it. Most of the nuclear warhead will be vaporized, so high energy projectiles are not a significant problem. A nuclear blast will likely cause some fragmentation of the asteroid, but those fragments would likely be volatized.

The radiation and thermal energy from a nuclear detonation will reach the asteroid surface before any blast wave, so the blast wave will interact with vapor and liquid, rather than solid, so fragmentation should not be a problem. I would imagine that someone is doing simulations to predict the thermo-mechanical response of a nuclear blast on an asteroid.

 

[Morbius]

Perhaps you missed my point. Nuclear blasts in space do not produce shock waves like they do in air, water, etc, since there is no medium available to propogate a shock wave. So, a nuke would have to be detonated near enough to the surface of the asteroid to vaporize some of its mass in order to impart any momentum. Explosions in a vacuum generate very high energy projectiles. Explosions in a medium generate high energy shock waves, which is a far more effective way to impart kinetic energy to a mass. Detonating a nuke near enough to the surface of an asteroid to be effective risks fragmenting it.

Chronos,

How is the nuke going to fragment the asteroid? You're thinking like the nuke was
still in the atmosphere! When you blow up a nuke; you expect it to "fragment" or
blow apart the stuff that's near it - but that is due to the blast wave from the nuke.

However, as you correctly point out - there is no blast wave in the vacuum of space!

Astronuc is quite correct - the material that constituted the nuke will be vaporized -
[actually it's beyond being vaporized - it's a plasma] so you don't have projectiles.

No - the major effect of the detonation of a nuke in space on material nearby is the
radiation. Place the nuke at the proper distance from the asteroid that you wish to
deflect - and the radiation will vaporize some of the material on the surface of the
asteroid, and the resultant blow-off will yield a reaction force that will deflect the
asteroid.

The effect of the radiation heating of the surface of the asteroid DOES PRODUCE
a shock wave IN THE ASTEROID! There'e no conventional nuclear blast wave
because the detonation is in a vacuum. However, the radiation can cause shock
waves to be generated in the material the radiation hits.

What one does is to detonate the nuke at the correct distance from the asteroid, so
that the radiation effects generate the proper deflection force. This was well covered
in the papers at the Planetary Defense Workshop that I cited above.

Dr. Gregory Greenman
Physicist

 

[Morbius]

one only needs to find the asteroid before it gets anywhere close to Earth. within a time frame of about 5-10 years, do you not think that we can divert an asteroid with a satellite that can be trasported near it? It's more plausible than blowing it up with nuclear missles. or transporting a rocket there. A satellite like object near the asteroid can offset it's course by a very small amount, but if done at a large distance away from earth...it can significantly deflect the asteroid's path. use of missles are just plain ridiculous and may be considered if we detect this thing too late...but then again...it's too late.

POSITION_VECTOR,

If we are going to deflect an asteroid, which can be miles in diameter; we are going to
have to deflect it YEARS ahead of time. But even then, you have to apply some
very large forces to the asteroid. The gravitational force from some once Earth-bound
satellite isn't going to do it.

Besides, any deflection forces you get from the gravity of the satellite will have
come from the rockets used to put the satellite there.

Suppose I have a ball of magnetic material sitting on the table. I hold in my hand a
magnet, and I swipe the magnet close to the ball and deflect it. The ball rolls off.
Where did the energy to roll the ball come from? It came from ME. The magnetic
field merely conveyed the energy via the magnetic force. But the energy came
ultimately from me. Using the magnet actually made the process less efficient -
I could have done better by hitting the ball directly with my hand.

The same is true with your satellite. Ultimately, the energy to deflect the asteroid
came from the rockets. By using the gravity of the satellite; you merely made the
process LESS EFFICIENT. It would be better to use the force of the rockets
directly. Deflecting an asteroid is going to take a LOT of force, and we don't have
any to spare; so we really won't be able to accommodate the inefficiencies of using
gravity.

As I stated before, the most efficient way to transport energy to some distant
place is via nuclear energy, hence it will be useful to have some nuclear weapons
on hand for the purpose. Perhaps that factored into President Clinton's decision
on August 11, 1995 to alter US policy to one that will retain nuclear weapons in
the US stockpile, for the indefinite future.

Dr. Gregory Greenman
Physicist

 

[Morbius]

Exactly. It's just a way of elliminating the problems of attaching a thruster to the asteroid if it's spinning, or if it's composition is a loose conglomorite rather than a solid rock. (Or if these properties are unknown)

LURCH,

Using gravity doesn't help you plant the rocket on the asteroid, even if it's spinning or
a loose consistency.

Gravity is a "central force". The force vector will be along the vector from the center
of mass of the asteroid to the center of mass of the rocket. Therefore, since the
force vector and radius vector are co-linear; the gravity can't induce a torque on the
rocket. Therefore, if the asteroid is spinning, the rocket WON'T spin up to match it.
You will still have the problem of a rocket attaching itself to a spinning object in any
case.

No - gravity is pretty useless here. Anything you can do with gravity, can be done
better by doing it directly.

If your asteroid is spinning, or even if it has a surface of loose consistency; if you
accellerate your rocket in space and slam it into the asteroid in a collision; the
asteroid is going to pick up the momentum. Where else would the momentum go?

Dr. Gregory Greenman
Physicist

 

[Morbius]

The radiation and thermal energy from a nuclear detonation will reach the asteroid surface before any blast wave, so the blast wave will interact with vapor and liquid, rather than solid, so fragmentation should not be a problem. I would imagine that someone is doing simulations to predict the thermo-mechanical response of a nuclear blast on an asteroid.

Astronuc,

EXACTLY CORRECT!

Yes - the interactions and responses of materials to the effects of nuclear explosions
indeed have been, and are; being simulated. Those are very complex calculations;
certainly not "back of the envelope" or "hand-waving" types. However, it's the type
of thing that been done for year for other reasons.

Dr. Gregory Greenman
Physicist

 

[LURCH]

LURCH,

Using gravity doesn't help you plant the rocket on the asteroid, even if it's spinning or
a loose consistency.

Gravity is a "central force". The force vector will be along the vector from the center
of mass of the asteroid to the center of mass of the rocket. Therefore, since the
force vector and radius vector are co-linear; the gravity can't induce a torque on the
rocket. Therefore, if the asteroid is spinning, the rocket WON'T spin up to match it.
You will still have the problem of a rocket attaching itself to a spinning object in any
case.

No - gravity is pretty useless here. Anything you can do with gravity, can be done
better by doing it directly.

If your asteroid is spinning, or even if it has a surface of loose consistency; if you
accellerate your rocket in space and slam it into the asteroid in a collision; the
asteroid is going to pick up the momentum. Where else would the momentum go?

Dr. Gregory Greenman
Physicist

You missunderstand the method being proposed. The idea is that the thruster be attached to the object by gravity alone; it never comes into physical, "material" contact with the object. This way, the rocket never has to sit on the surface. I believe the term used was "gravitational tethering".

It is precisely because gravity is a central force that the problems of spinning or loosely conglomerated targets are elliminated. If the object is spinning, the rocket, which is a short distance away, does not spin, and can point its nose (and its thrust) in one constant direction. Because the force and radius vectors are co-linear, no angular momentum is exchanged (well, not enough to mention, anyway). If the object is of loose composition, the tug of gravity will work on the whole group of objects as a single mass. Where a thruster on the surface or an impactor might just blow a hole right through the object, a thruster at a distance would pull on the center of mass for the group.

 

[theCandyman]

For gravity, F = G mM/r^2. [G = 6.7 * 10^-11 N m^2/ kg^2]

Now use this equation and calculate how much force using just gravity will affect the asteroid. Now only that, but if the asteroid is of a threating size, it might be impossible to make a rocket massive enough to affect it.

 

[DaveC426913]

For gravity, F = G mM/r^2. [G = 6.7 * 10^-11 N m^2/ kg^2]

Now use this equation and calculate how much force using just gravity will affect the asteroid. Now only that, but if the asteroid is of a threating size, it might be impossible to make a rocket massive enough to affect it.

The rocket is not supposed to be massive; the rocket acts by applying thrust.

And how much it affects the asteroid is determined only by how much time it has to do so. Even a small rocket will have an effect; it's only question of 'is it enough to do the trick?'

 

[DaveC426913]

You missunderstand the method being proposed. The idea is that the thruster be attached to the object by gravity alone; it never comes into physical, "material" contact with the object. This way, the rocket never has to sit on the surface. I believe the term used was "gravitational tethering".

So, if I understand correctly, the rocket's acceleration is constrained by the escape velocity of the asteroid. No matter how powerful the rocket has the potential to thrust, it must stay below escape velocity or it will break free.

 

[Morbius]

So, if I understand correctly, the rocket's acceleration is constrained by the escape velocity of the asteroid. No matter how powerful the rocket has the potential to thrust, it must stay below escape velocity or it will break free.

Dave,

You are correct. The amount of force the rocket can exert on the asteroid is limited
by the mutual gravitational attraction between the rocket and asteroid, and the
requirement that the rocket stay below escape velocity.

This SEVERELY restrincts the amount of force that can be applied. Even if we
deflect the asteroid YEARS in advance; it is going to take some VERY LARGE forces
to deflect asteroids of a size that would be a threat to the Earth.

Dr. Gregory Greenman
Physicist

 

[LURCH]

Yes, in this method detection would have to be even earlier than the other techniques mentioned. And, as we all were reminded last week,

https://www.physicsforums.com/showthread.php?t=130554

We don't have very adequate early detection.

This SEVERELY restrincts the amount of force that can be applied. Even if we
deflect the asteroid YEARS in advance; it is going to take some VERY LARGE forces
to deflect asteroids of a size that would be a threat to the Earth.

Also very true. But on the plus side, this method would put a direct correlation between the amount of energy available (in practicale terms) and the amount needed. Larger objects would require more thrust, but they would also have a stronger gravitational tether, and be able to handle more thrust. Nevertheless, the amount of thrust needed compared to the amount that could be applied per unite of time is such a huge ratio, we'd have to start with much more advanced warning than we're likely to get.

And we'd have to know the strength of the gravitational field around the object before we could begin thrusting, so we don't just escape. This is notoriusly difficult to calculate for an object traveling through open space. I suppose we could find that out by firing a preliminary projectile near the object (kind-of a shot across its bow), and measure how the path of that projectile is altered by the close encounter. But the margin of error would be pretty big. I think we'd have to send an engine with veriable thrust, so we could dial it back if it started to escape, or crank it up if it started to get closer to the object. Not impossible, but pretty complicated.

And of course, as has been stated, this method reduces the amount of effect we could have on the object. Current scenarios are dependant upon detection years in advance, so that the small effect we could have on the cousre of the object would be sufficient when multiplied by time to impact. Gravitational tethering makes the effect we could have so small that the time to impact would have to be many decades; [probably about a century. This raises two big questions,
1)Can we accurately predict a collision event 100 yrs in advance?
I'm doubtfull, we have a cattelog of a few objects that might hit in the next twenty years, and we can't even speek with much certainty about them. They "probably won't".
2)Projecting the kind of tech we are likely to have 100 yrs from now, why bother?
If we have a hundred years of warning, then we can be pretty sure that we'll have a much more reliable method in place in plenty of time.

I still say the best solution is to blast 'em!

 

[Morbius]

I think we'd have to send an engine with veriable thrust, so we could dial it back if it started to escape, or crank it up if it started to get closer to the object. Not impossible, but pretty complicated.

LURCH,

The main engines on the space shuttle are variable thrust. During the launch of a
space shuttle, the engines throttle back to reduce aerodynamic stress on the vehicle,
and then as the air gets thinnner with altitude, the engines throttle up again.

Dr. Gregory Greenman
Physicist

 

[LURCH]

Ah yes, of course. I was still thinking of ion propulsion. If this method were to be viable at all, I think that the low specific impulse and long duration of Ion Drive would be a necessity.

VASIMR's (ion drives with veriable specific impulse) exist, but last I heard there were only prototypes in testing. Have any been flown? I thought I remembered eharing that the ESU was going to use that propulsion system for its next Lunar orbiter, but I don't know if that's still in the future.

Aslo, here's an interesting article on this very topic;

http://flux.aps.org/meetings/YR04/DPP04/baps/abs/S1905.html

NASA's Prometheus program seeks to develop new generations of spacecraft nuclear-power and ion propulsion systems for applications to future planetary missions...

...Another potential Prometheus mission of high science interest would be an extended tour of primitive bodies in the solar system, including asteroids, Jupiter family comets, Centaurs, and Kuiper Belt Objects (KBO). The final landed phase of this mission might include an active keplerian experiment for detectable (via downlink radio doppler shift) acceleration of a small kilometer-size Centaur or KBO object, likely the satellite of a larger object observable from Earth. This would have obvious application to testing of mitigation techniques for Earth impact hazards.

 

[Astronuc]

VASIMR's (ion drives with veriable specific impulse) exist, but last I heard there were only prototypes in testing. Have any been flown?

VASIMR have yet to be perfected - they are still problematic and have little thrust. None have been flown.

 

[Morbius]

Ah yes, of course. I was still thinking of ion propulsion.

LURCH,

"It beats me. But isn't she a beauty? Interesting design.
I've never seen anything like her. And ion propulsion at that.
They could teach us a thing or two."

--LCDR Montongomery Scott, from the Star Trek episode "Spock's Brain"

Dr. Gregory Greenman
Physicist