You dont blow it up. People who think you do blow it up got that information from others who are wrong, or from a film.

It is called mass ejection redirection. You detonate close to the surface so that you blow off just enough material to move the impactor slightly. The key here is you have enough warning time.

It would take a huge amount of power to destroy a serious impactor, or deflect it once close. At that point, you might as well focus on digging tunnels.

If time is short, use nukes. If the asteroid breaks up into chunks, use more nukes. Eventually you'll end up with a really epic meteor shower and a little radioactivity. Better than a giant smoking hole in the ground.

One question: If we see a comet coming at us from the great void, as in Lucifer's Hammer, will we have time to deflect it with nukes?

I double checked. m should indeed be the mass of the asteroid, as I originally contended, not the mass of the bomb (the E term comes from the energy emitted by the bomb). Sending a nuclear device this powerful on an accurately aimed rocket could reach an asteroid and detonate, changing course, in a matter of months.

Other solutions, like the ion drive + painting one side (assuming stabilization against the rotation which would result) would take decades for a comparable effect. I'm not saying that your solution isn't safer or just as effective given sufficient lead time, I'm saying that it's a long way down the pipeline.

Ethan

And explosions! Don't forget the explosions.

I like explosions.

Low density carbon style asteroid? 70% or so of the asteroids out there… What would a nuclear blast do to it? Fuse it into a solid lump or somehow impart enough energy to overcome the gravity that formed it? Gravity tractor sounds more feasable to me

Medium density silica/iron style asteroid? 10% or so population wise. 50/50shot grav tractor vs hydrogen device nuclear weapon?

2 mile wide chunk of nearly pure iron/nickel? 5ish % population, would probably be the best candidate for deflection goal nuclear detonation

Ice/Comet who knows what effect a nuke would have…

Ethan mentioned a no-fallout fusion device but the problem there is getting one of those monsters out of earth orbit. Any other would be a plutonium – deturium / tritium weapon.

As far as space detonations go, both US and Soviets did experiments as far out as 300 or so miles, most of the american tests resulted in pretty lights in the sky and knocking out the power grid of Hawaii

As for this:

"BUT, since I don't know that we actually have any alternatives ready, I'd keep a bunch of nukes ready just in case. Like war or anything else, the nukes are the last chance card. You play it when there is not other choice."

Umm, alternatives? We don't have asteroid-killing nukes ready yet either. It's ALL theoretical at this point, because we've never moved an asteroid. So the nuclear option is just as experimental as the gravity nudge.

Oh and…

Geokstr: Good one!

In terms of effectiveness, if you could use a rocket motor and some other technology to move an asteroid to a new orbit or divert it more efficiently, then I'd be for that. BUT, since I don't know that we actually have any alternatives ready, I'd keep a bunch of nukes ready just in case. Like war or anything else, the nukes are the last chance card. You play it when there is not other choice.

Avoiding use of nukes in space is foolishness. We have nukes now, hitting a habitat with one would be no enormous challenge. In addition, a spacecraft with a rocket motor and moderate sized fuel tank is at least as dangerous a weapon as a nuke. Nukes have many peaceful uses in space, and refusing to use them in those ways simply means that they'll only be used for violent means…except that the availability of rocket engines makes kinetic weapons far more likely.

We need to use the energy to lift a large mass off the surface of the asteroid at a low velocity to get the most momentum for a given energy. The key is to use part of the asteroid as the reaction mass. The only way to achieve Ethan Siegel's limit is to use the bomb to split the asteroid into two halves. This would require landing and drilling as in the worst sci-fi movie ever, Armageddon.

With lots of lead time, one could just drop lime on the asteroid to paint it white, which would change the Yarkovsky force and deflect it.

A 20 tonne spacecraft with an ion drive could be used as a gravity tractor with enough lead time.

With less lead time one could land a solar powered "gravel pit" on the asteroid and use a conveyer belt as a low speed mass driver to send gravel off the asteroid in the direction opposite to the desired deflection.

With minimal lead time one could try standoff nuclear explosions designed to lift a layer of rock about a meter thick off the surface of one side of the asteroid with a speed of 100 m/sec or so. Fragmenting the asteroid has to be avoided, so this would be a pretty dangerous operation. But one could try a large standoff distance first and then gradually increase the energy density delivered to the surface by moving the explosion points inward. So many bombs would have to be delivered toward the incoming asteroid with enough time in between to judge the effects of the previous shots. An exciting time for all concerned.

So we need a long lead time! Find all the dangerous objects. Then we only have to worry about comets which can't be predicted so far in advance even if found early.

In his famous book "High Frontier" Gerard O'Neill has suggested a method for maneuvering moderately sized asteroids (the size that nukes would have a prayer of moving) and it may be worth considering for something like this. He was a great fan of the mass driver and suggests that mass drivers could be installed on a hunk of rock and it could be turned into a controlled object; the rock itself providing the reaction mass and energy from the sun powering the driver. If it works for changing a random space nugget into a spaceship, why not change an asteroid with its eye on earth into something similar. I presume that we would have to find it in time to get to it and apply the small but continuous thrust that would be needed to change its trajectory enough to matter, but other than that one admittedly large difficulty, everything else is known technology and simple physics.

UP here near the Arctic Circle, we found it much more efficient to defeat all the Bears at once by using a Favre instead.

I think the calculation needs to be a lot more careful. What is the momentum of half the photons and debris from the bomb? That tells you how much the asteroid will move. From that you can work out the kinetic energy imparted to the asteroid. The difference between that and the (half-)energy of the bomb presumably goes into heating the asteroid up.

I think, ultimately, this is about the second law of thermodynamics: just because you've got a lot of energy stored somewhere, doesn't mean you can automatically use it for anything you want. Efficiency, as Ian said, comes into it.

On the other hand, breaking one large lump into a lot of smaller ones might still be a good thing. The small pieces will have a greater surface area in aggregate, and so may be more effectively stopped or slowed by the atmosphere. You'd still end up with the same amount of energy delivered to the earth, but a series of huge airbursts may still be better than one really ginormous groundburst (or splash in the sea).

Either way the answer involves actual intellectual hard work and study, i.e. the opposite of what internet discussions are about.

You guys are watching too much movies if you believe that we could attach a engine to asteroid and it would skip hitting Earth.

1.) You should find the asteroid early enough to haul the massive engine and the massive load of fuel.

2.) You have to assemble that complex at the asteroid.

3.) You could try smaller option, but then you would have to find the asteroid even earlier.

Same applies on other SCIFI options. And yes, the composition of the asteroid plays huge role when you are planning to plant something on a asteroid.

Massive laser is much better option and imho mankind should develope massive lasers capable of heating and by that breaking or just changing the characterists of the asteroid. Three of those in the Earth orbit could cover up all directions towards Earth and they could be used together too.

But before that, we don't have any other option than nuclear warhead. We don't have even missiled to do that right now. And I don't believe that we have big enough nukes.