# Why can't a nuclear explosion QUITE A DISTANCE FROM asteroid work?

Okay, so I just skimmed an article about an asteroid that may smack the Earth a few decades from now. A question has been vexing me. Why is it so impossible to detonate a big-ass atomic bomb quite a distance away, thus using the energy of the blast to throw the object off course, instead of trying to blow it up?

I’m not asking about trying to blow the ogdamn thing up. I’m asking why we cannot use the speeding particles of the blast, which occures at a distance from the object, to change its course.

I would guess that to be effective the explosion would have to be within a mile of the asteroid (the effect of a bomb would scale down as the distance to the power 3). Now if you can get your bomb within a mile you are probably good enough to hit it anyway.

The energy from a nuclear blast in a vacuum is primarily radiation. Unless it was very close, all it would do was make the asteroid more radioactive. So instead of a hunk of rock zeroing in on Earth, you have a radioactive hunk of rock heading for earth. :eek:

Remember: there are no shockwaves in space. To actually cause damage, the primary method would be the energy radiated from the explosion. Close to the asteroid, it would vaporize it, but that effect would drop off quite rapidly.

Even on Earth, where the atmosphere creates a shockwave, the damage from an atomic bomb only extends a few miles from ground zero. In space, missing by 20 miles is pretty damn close, but it’s too far from the target to do any good.

Blowing up an asteroid is either very difficult if it’s solid iron or possible dangerous if it’s a loose aggregate of smaller (yet big) boulders, which it might fragment. By moving the blast off the surface you wind up trying to vaporize part of the asteroid and use that jet of material to nudge the rock off course. Again you wind up with the problem of not knowing exactly how it’ll react if it not a solid hunk of material.

A neat idea is to use a mass to gravitationally tug the asteroid into a slightly different orbit.

Please do note that they are talking about detonating the nuke from “several hundred yards” away, though, and NOT quite a distance away, as you proposed in your post.

Because the necessary amount of energy can’t be delivered over a sustained period, and asteroids can be surprsingly fragile.

It would be quite simple to knock an asteroid off course like that if you could harness all the energy of a largish nuke and manage to direct it at the asteroid over a period of several days or probably even several hours. But we can’t do that. A nuke detonates in seconds, not hours.

So all that energy has to be applied in seocnds. And if the energy is sufficient to cause a large enough direction change within seconds it is also going to be large enough to blow a fragile object into peices.

It’s like throwing something at your TV screen to move the set. If the object has enough energy to move the weight of the set then it must also have enough energy to smash the glass. And if it is thrown softly enough not to smash the glass there is no way it has enough energy to move the weight. To move without breaking you need either a sustained gentle force over along period or a gradually increasing force, and bombs can’t manage that.

You could use a series of timed detonations to gradually build up momentum in the desired direction, but that seems unnecessarily complicated.

From SPACE.com:

I’m not quite sure I follow this.

The reason I finally asked is that we already have plenty of nuclear bombs hanging around. I figured we could spare a few.

Why not the inverse of the square of the distance?

Thanks all! You’re the first people who’ve ever listened to the actual question rather than assuming I was asking about blowing the damn thing up.

The main issue is really just how big an explosion you need to give it enough of a nudge to make it not dangerous. Shoemaker-Levy 9, the comet that was broken apart and then slammed into Jupiter was drawn into its collision course by the planet’s gravity on prior passes. (It broke up after passing Jupiter within the Roche limit, and IIRC, impacted the gas giant on its next orbit.) Therefore, the asteroid has to miss Earth by enough of a margin not to become a threat on subsequent orbits. Obviously, this is a significantly smaller amount of energy than would be required to blow the asteroid up (which, as the Bad Astronomer amongst others tells us, is a Bad Idea[sup]tm[/sup]). Intuitively, it seems to me that for something the size of, say, Ida (roughly 35 km diameter with a mass of about 4.2E16 kg), you’re talking of a nuclear device significantly more powerful than 50 megatons (the most powerful weapon yet detonated on Earth). And, as others have noted, giving the asteroid a single kick big enough to move it significantly is likely to vapourize part of it.

A more likely scenario is a form of rocket engine mounted at the proper vector on the asteroid with enough fuel to burn for enough time to remove the asteroid from the danger zone.

All it’s saying is that a nuke detonated at close range will turn the surface of the asteroid into gas. Nothing surprising there, a nuke detonated on Earth turns the ground under it into gas.

All that gas has to go somewhere. It can’t go through a solid asteroid for fairly obvious reasons. So it has to shoot off into space, ie in the direction that the explosion started from.

Every action has an equal reaction. If several tonnes of gas are flying off into space in the direction of the explosion that will apply several tonnes of pressure in the oposite direction, away form the explosion. That will cause the asteroid to move away from the explosion.

So even without any atmospehre to carry a shockwave a nuke can still create a hell of a push by effectively turning the asteroid itself into a rocket.