Death from the Skies! Read Online Free PDF Page A

explode, breaking apart completely. But if it’s hit just softer than that critical speed, it won’t blow apart: the shock from the impact will shatter it in place, like a hammer tapping a crystal egg. The asteroid’s own gravity will still hold it together, but it’ll be riddled with crevices and cracks. In essence, it’s a floating rubble pile.
    What would happen if you tried to nuke something like that?
    Asteroid expert Dan Durda from the Southwest Research Institute in Boulder, Colorado, wanted to find out. He discovered that the scientific literature on asteroids didn’t have much information involving experiments on actual asteroidal material, so he set about to correct that. He obtained some meteorites known to have come from asteroids. One was dense, solid, and rocky, and the other was more porous, more like 253 Mathilde than a chunk of, say, quartz.
    He took his sample to NASA’s Ames Research Center in California, which boasts the ownership of an unusual gun: it uses compressed air to shoot projectiles at several kilometers per second.
    Durda set up his solid specimen in the gun’s sights, and slammed a BB into it at five kilometers per second. As expected, the meteorite exploded, disintegrating into hundreds of pieces.
    He then put a porous piece of rocky material in the crosshairs. When the projectile hit it, the meteorite absorbed the projectile and didn’t shatter.
    Durda asks, “What if an object like that were coming toward the Earth and you were trying to stop it? . . . How would it respond if we were to throw some sort of a projectile into it at a very high speed to try to break it up? What if you were to try to put a small nuclear device next to it to try to break it up? Would it actually respond in the way you normally think of a solid hunk of rock?
    “You put a brick next to you, and take a hammer, and you slam the brick, and it goes flying into pieces . . . that’s what you think of when you talk about breaking up an asteroid.
    “But you take a sandbag, and you whack it with that same hammer, and nothing happens. It just kind of goes thud, and that’s the end of it.”
    That’s bad news for us. A rubble pile is pretty good at absorbing damage, and so a nuke won’t destroy it. If we see one headed our way, we can bomb the heck out of it, and it’ll laugh all the way down to impact.
    Actually, many scientists are rethinking the idea of nuking an asteroid. A huge disadvantage of blowing up an incoming threat, even if we could, is that it would create thousands of small potential impactors out of one big one. That may sound better than the alternative (a giant one hitting intact), but even a rock a hundred yards across could easily take out a city. A dozen of those impacting at the same time would be disastrous no matter where they hit. While the explosions would be smaller, they’d be more spread out, scattering damage across the globe instead of confining it to one place.
    Durda adds another danger of blowing up small asteroids. “If you take the cosmic composition of a typical asteroid of that size . . . there’s enough chlorine and bromine in that object to destroy the ozone layer. So it doesn’t matter whether or not that object hits all at once, in one big piece, or if all of that small debris left over from breaking it up into a million pieces still comes raining through and vaporizing into our atmosphere. You’re still depositing all of that very harmful stuff into our fragile atmosphere.”
    It might be better, then, if there’s no way to stop it, to just let a smaller asteroid hit us.
    That’s unsatisfactory, of course, especially if you’re sitting in the bull’s-eye.
    But there may yet be another solution.
    One idea is to drop a bomb not on the asteroid, but near it. Blowing up a bomb next to an asteroid, say, a few hundred yards away, would generate a huge amount of heat and vaporize part of its surface. The solid rock or metal would turn into a gas and expand rapidly,