Comment by stouset | Hacker News Reader

Comment by stouset | Hacker News Reader

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> I don't understand the whole 'we don't want to break up a loose conglomeration of rock'. I'd actually think breaking an asteroid up into multiple pieces that can be 'eaten' by the atmosphere would be easier/safer?

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.

That seems like it would require an absolutely huge amount of matter, given what's happened after old impacts or large volcanic eruptions. Would we realistically have enough nukes/energy on hand to divert something that massive? It just seems like an incredibly difficult way to do it?

P.S. I you fragmented an asteriod - would at least _some_ of the pieces tend to change heading/speed enough so they miss earth?

The idea is that you catch them far enough away in time so that a small nudge pushes them enough to miss.

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.

P.P.S. Hmm - actually, if the object is that big...would we even have enough nukes/energy to break it up. Seems like either way (breakup/push) we're going to be limited by the amount of energy we can deliver compared to the mass of the asteroid? Oh! also, would a nuke actually keep stuff out gassing long enough before it cools back down? how long would you need to have that tiny lil push going on before it made any practical difference? thats a lot of mass to try and push ??

These aren't numbers, but a quick intuition reminder: nuclear-powered ablation drives are really good and we are really good at building them.

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.

Thanks! It actually appears it requires a lot LESS energy then I expected. According to [1] less then a tonne of TNT could accelerate 1 million tonnes to 0.1 m/s !! So it appears I was WAY off base (by at least 10^6 :-P). I swear I'm a better programmer then physicist!! :-D

1] https://www.omnicalculator.com/physics/kinetic-energy

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