Energy.
A large enough meteor impacting the Earth at speeds high enough to cause an extinction-level event has a lot of kinetic energy.
Intuitively it seems like the troublesome part is the meteor hitting the Earth and causing earthquakes, tsunamis, and clouds of ash and debris as a result. And those things are really bad. But they’re a consequence of that mountain of kinetic energy.
So you break the asteroid up into a cloud of dust. Fantastic! No more crater, no more big boom. But you still have a giant ball of kinetic energy headed right at you, and that energy is just going to dump into the atmosphere. And boil it.
Of course that’s even assuming we totally vaporize the thing. More likely is instead of one multi-mile asteroid we have a bunch of giant chunks that will spread death and destruction all across the side of the planet unlucky enough to be facing it… before boiling the atmosphere anyway.
P.S. I you fragmented an asteriod - would at least _some_ of the pieces tend to change heading/speed enough so they miss earth?
That's how you turn a fission bomb into a fusion bomb. If you just put LiD in a fission bomb, it doesn't squeeze hard enough. If you focus the x-rays from a fission bomb onto an ablation drive, it does squeeze hard enough and you 1000x+ the yield with fusion. Which is why fusion bombs look like metal peanuts.
Conveniently, this means we are also really good at designing x-ray reflectors and lenses that focus the primary output of a nuclear bomb for the purposes of building ablation drives. Depending on just how good, we could probably persuade an asteroid pusher to be far more unidirectional than one typically pictures from a "bomb," if we set it off some distance away and focused the x-rays in the direction of the asteroid.
Far away in time doesn’t even need to mean far away in space, since many of the objects of concern are orbiting the Sun along with us. They may even have multiple close passes before being perturbed enough to become a threat.
Also, the masses involved don’t have to be huge. Two rocks intersecting in space are likely to be doing so with very different velocities. Kinetic energy is mass times velocity squared, so velocity rapidly overtakes mass as the dominant factor.
Suppose over one second you can be hit the earth with with either 1 30 gigaton weapon, 3 10 gigaton nuclear bombs. 3,000 100MT weapons, or 3 million 10 KT weapons etc. All of those suck in ways that are hard to comprehend.
Hmm - I guess not! According to [1] it would take less then a ton of TNT to accelerate 1 million tonnes (eg like asteroid 12932-zephyr) to 0.1 m/s...So its probably a good thing I'm not responsible for these things!
Thanks to everyone for their help!
Presumably that depends a lot on the composition of the asteroid, e.g. whether it is mostly metallic or a pile of loose rubble.
Normally when a single bit of sand hits the atmosphere it dissipates energy in every direction. So you a tiny fast moving glowing dot radiating energy in every direction. However you can’t dissipate energy when you’re right next to another particle at the same temperature. Thus energy from each bit of sad can basically only go in 2 directions down or up and you’re not transferring energy through the entire thing before a crust of plasma followed by ~10km wide ball of sand hits the surface.
Hell it could be a ball of water that size and the same basic effect kicks in.
Spread a 10km wide ball of material into a 1,000 km wide diffuse cloud and you are basically slapping the earth with 1,000 km wide disk with an average width of ~17 cm (maximum width ~1m) going 10’s of thousands of km/h. Again things don’t go well, you may be shifting how many minutes various people survive but it’s not actually addressing the issue.