[Fortran (OP)]

I am a little puzzled by the idea of destroying comets using nuclear war-heads. As far as I can tell nuclear explosions occur on earth by merely producing an intense burst of energy which superheats the atmosphere, and thus causes rapid expansion of the air, after a few seconds cooling begins and the atomic 'wind' reverses in direction. In addition to this there is the intense heat/light radiation across a wide spectrum.  apart from possibly melting the surface of a comet/asteroid can anybody explain why so many people think a nuclear explosion would disintegrate such an object?

Conventional explosives (black powder derivatives excluded) detonate ie turn from a compact solid into gas and thus produce a real blast wave, using conventional explosives would be far more effective.  Of course I might have missed something in my understanding,   discuss?

[lyner]

The effect of an explosion at the side of a comet would be that the momentum of the bits of the bomb going towards the comet would share their momentum with it - pushing it to one side. The idea bomb, for this would, presumably, have some considerable mass.

[Bored chemist]

A nuclear explosion releases a lot of energy If there's no air nearby it will still heat whatever is in the vicinity. The energy released by a nuke near a commet would vapourise part of the commet and the vapour moving one way would move the remainder of the rock the other way.
If you do this while the commet is far enough away you can move it enough that it won't hit the earth.

[Fortran (OP)]

Bored chemist,

Please explain how the energy radiated by a nuke would move 'vapour away' ?

It would heat it from one side only. As for the first answer, the momentum of the bits of the bomb is due to the conventional trigger and not the nuclear 'blast' -

Well I'm not sure whether mythbusters are watching but if they are....

Nuclear weapons splitting comets into millions of bits - BUSTED!

[Edster]

If a conventional fission ( or fission/fusion)device is detonated in the proximity of a "gasteroid" or comet ( mainly frozen water and co2 plus  some rocky component) then the effect on its trajectory and integrity is mainly  dependent on proximity and orientation, and of course the make up of the body.

If the device is landed on the asteroid in contact then at that point there is concentrated energy and a shock wave will be generated by vapourisation and expansion of the solid material from the heat as the device radiates its converted mass. I will neglect the  relatively insignificant contribution from the vapourised structure of the device. (You don`t get shrapnel from a nuke casing!)

 This heated surface will also outgas or produce ejecta in the direction the device was, like a fountain ( without gravity to turn it back). This will produce a reaction thrust moving the body  in the opposite direction.
 The shock wave might actually shatter the body. ( sorry it is not impossible) but the broken remnants wouldn`t necessarily spread very far unless internal areas of gas or liquid were present to expand and impart some relative velocity

A close encounter will simply be a high energy irradiation which depending on  a close enough separation  may vapourise any frozen material and still possibly the rock and produce a reaction jet as above  which will alter its trajectory by some amount.

IMHO .. []

[Bored chemist]

Bored chemist,

Please explain how the energy radiated by a nuke would move 'vapour away' ?

It would heat it from one side only. As for the first answer, the momentum of the bits of the bomb is due to the conventional trigger and not the nuclear 'blast' -

Well I'm not sure whether mythbusters are watching but if they are....

Nuclear weapons splitting comets into millions of bits - BUSTED!

A gas will expand in a vacuum like space. It can't move one way, because there's a rock there (on which it will push); but it will move the other way.

[lyner]

It's true to say that, if you can penetrate the side of the comet (the correct side, that is), you can use the material of the comet as the (Newton 3) reaction mass. That would be best value because you could use a vehicle with the lightest mass. If you could actually land on the object, you could even tunnel into it prior to detonation.
Either way, it's a momentum transferring exercise.
As with a terrestrial explosion, the stronger the casing, the higher the pressure build-up before the explosion, so the more useful energy you can get out of it.

[Bored chemist]

In the face of a nuclear explosion, nothing has any strength worth thinking about. Most materials tensile strength falls dramatically with temperature and a nuke will boil the shell anyway.
What can (and does) contain a nuclear explosion for a short while is inertia. The bomb casing neesd to be massive, rather than strong.

[lyner]

Yes, you're right. The so called Tamper, surrounding a nuclear device relies on its mass, rather than its strength, to delay the initial expansion.
I think it's still true to say that you'd get a more useful energy transfer by implanting the bomb an appropriate distance under the surface, though. The longer the force acts then the more momentum is transferred (Impulse = Force X time) so a few more milliseconds of containment time would mean more momentum / deflection speed.

I know that Nuclear devices are very concentrated sources of energy but an impulse is hardly the most subtle way of moving an object in space. If you can get to the target in time, then the course modification need not be very great to avoid an Earth impact - a minor nudge could be enough (a miss is as good as a mile). A more controlled and extended way of applyng the impulse could be better value. A nuclear reactor (rather than a bomb) could produce a (very dirty) directed jet plume which could utilise an equivalent amount of fuel in a more efficient way - if you could bolt it to the target. It would be a fantastic firework display.

[Fortran (OP)]

Here's another thought, a small diversion at an early enough phase would be enough to cause a miss, another way of cuasing a near miss would be a head-on high speed collision of a simple mass, slowing the target by even the tiniest amount would also effectively produce a miss. If the target was broken up early enough the debris field would probably widen to many times the earth's diameter and thus only a proportion (if any) would get through.

[lyner]

The best way to affect the orbit depends where the comet happens to be. I have played for hours with a simple orbit program I wrote and there are many available for Web use. Speeding up or slowing down when the comet is in the outer parts of the orbit can make a huge difference to how near or far it will pass the Sun.
The main message is that you do what you have to do as soon as possible. I have often thought that the ideal defence would  be a couple of weapons placed in orbit, all ready to zoom off to a rendezvous as soon as a likely object shows up.
It might prove to be an expensive exercise, though, because we would almost certainly fire it off at 'possible' candidates for a crash, rather than 'probable' candidates - just to be on the safe side.
If it could be shown that there had been even a partial result then the people responsible would be bathed in glory. Morgan Freeman, eat yer heart out. (It was him in the film, wasn't it?)

[Bored chemist]

Drilling down into the surface a bit would certainly help but it would be tricky, not least because you don't know what you are drilling untill you get there.
The best bet ( I think) would be to send a few really big bombs so you had several shots (in case one failed). If the bomb is bigger than it needs to be then you just move the rock further than you need to- a waste of energy; but not life threatening. If the bomb's not big enough then you have lost a lot of time and it's much harder to get a successful shot second time round.

[lyner]

I was thinking of some sort of penetrating charge - along the same lines as armour piercing techniques - rather than a mining operation - to get a charge under the surface.

[Madidus_Scientia]

Yeah, what about a series of bunker buster style missles ahead of the nuclear charge, it would take precision to make them all hit the same crater though

[Raghavendra]

If the disaster happens, lots of energy is released...

[Yomi]

And that would affect the normal motion or path of celestial bodies in space

[Chemistry4me]

Depends on where you put them in space. After all, it is 'space'

[Yomi]

Look position really doesent matter where ever you explode it would have an impact on the path of celestial bodies directly or indirectly.......

[Raghavendra]

In earth the effect of energy is more because of gravitation, where as in space the energy live for a long period of time,in the form of other.... Untill and unless other object is near it, to feel

[lyner]

Yeah, what about a series of bunker buster style missles ahead of the nuclear charge, it would take precision to make them all hit the same crater though

If we're supposing multiple projectiles then they would have to impact almost simultaneously. Would there be any point in using several - just an added complication. One craft would need to carry the whole caboodle. It could split up just before impact, I suppose, if you needed to protect the nuke from damage initially but your 'bunker buster' would be on the same path as the nuke so they should both find their way to the same bit of the target.