Death from the Skies! Read Online Free PDF

acting like a rocket. It would push a little bit on the asteroid, moving it.
    It wouldn’t be much, but in space you don’t need much: every little push adds up. If you blow up several bombs, you can actually generate enough thrust to move the rock significantly. If you move it enough, it’ll miss the Earth entirely.
    And a big advantage of this technique is that it would work on rubble piles too, though it’s not clear exactly how well.
    There are some disadvantages to this method. You need a lot of lead time, for one thing. The farther away an asteroid is from impact, the less you have to change its orbit to make it miss us. Most experts think ten years’ warning is enough, though they’d be happier with twenty. A century would be just fine. This technique would work best on smaller asteroids since they’re easier to move, but a smaller asteroid is also fainter, and thus harder to find. Lead times would be shorter so there would be no room for mistakes. And getting one bomb to an asteroid is hard; getting twenty or more is a lot harder.
    Another problem is that it’s nearly impossible to know how an explosion would affect the orbit of the asteroid. It might be enough to have the rock miss us, or it might nudge it into an orbit that will hit us on the next pass around the Sun.
    For example, look at asteroid 99942 Apophis. It’s an Earth-crossing chunk of rock about 250 meters across, and is a potential impactor. At that size and mass, it would do considerable damage should it hit, exploding with the force of 900 megatons (more than a dozen times the yield of the largest nuclear weapon ever detonated). Apophis will pass by the Earth on April 13, 2029; there is no risk of impact at that time, but it will pass so close that it will actually be closer to the surface of the Earth than many weather and communication satellites.
    The asteroid will approach so close to us, in fact, that its orbit will be seriously affected by Earth’s gravity, and just how much its orbit is changed depends on just how close it gets to Earth in 2029. In fact, there is a region of space called the keyhole such that if Apophis passes through it, the orbit will be changed precisely enough that on its next return in 2036, Apophis will impact the Earth.
    This sweet spot is not terribly big, but our knowledge of Apophis’s exact trajectory isn’t good enough to completely preclude the asteroid’s passing through it. The odds are incredibly low, maybe less than 1 in 45,000, but it’s worth investigating.
    And what if it turns out that Apophis will glide right through the keyhole? We’ll have just seven years to move it enough to miss us. A better idea is to prevent it from passing through the keyhole in the first place. If we get to Apophis before 2029, then we hardly have to nudge it at all; calculations show that changing its velocity by even a few thousandths of an inch per hour would work. So you might think that a well-placed nuclear weapon would do the trick.
    Unfortunately, it won’t. That keyhole isn’t alone: there are dozens of keyholes, thousands. That first keyhole is just for a return of Apophis in seven years, but other keyholes will bring it back in ten years, twelve, twenty . . . instead of saving us, a detonation just buys a little bit of time, and there’s no guarantee that we can move it away from some other keyhole—or knock a chunk or ten of it into another keyhole.
    Controlling the resulting orbit is a key issue, and blowing up a nuclear weapon is not exactly subtle. 4 We need more fine-tuning on asteroid steering.

RAMMING SPEED
    By now it may have occurred to you that maybe we don’t need a bomb. The impact of an asteroid on the Earth releases energy like a bomb, so why not try impacting the asteroid itself? If we hit it hard enough with some sort of impactor, we won’t need a nuke.
    There is a very big advantage of this method: we’ve done it before. On, appropriately enough, July 4, 2005, NASA’s Deep Impact