Death from the Skies! Read Online Free Page B

telescopes—robots, in a sense—that use pre-programmed patterns to scan the sky. The vast amounts of data generated are then analyzed by computer to look for moving objects. It’s actually relatively rare these days for a human to find an asteroid.
    While the majority of all known asteroids orbit the Sun in the main belt, not all of them do. Various processes, gravitational and otherwise, can change the shapes of the orbits of some main-belt asteroids over time. Their orbits can become more elliptical, dipping them closer in and farther out than the other asteroids in the main belt. Some cross Mars’s orbit, and some cross the Earth’s.
    It’s the latter we need to be concerned about.
    The search for these Earth-crossers (called Near Earth Objects, and dangerous ones tagged as Potentially Hazardous Objects) is a multinational effort, but it’s still somewhat small—fewer than two dozen astronomers work on it full-time, with the majority of the work being done in the United States. Even if we had more people looking, using better and bigger and more equipment, the smallish rocks a hundred or so yards across are still a threat: it’s very difficult to spot them with any reasonable lead time. Many this size are discovered just after they pass the Earth, in fact. It’s quite possible that the first warning we may get of a small Tunguska-level impact is the flash of light as it streaks across the sky.
    So astronomers keep searching, and hoping they’ll catch the next impactor with plenty of time to do something about it. The goal is to find 90 percent of all Earth-approaching asteroids bigger than about a thousand yards across by the end of 2008. There are thousands upon thousands of such objects, so the astronomers have plenty of work to do. While the formal 2008 goal was not officially met (it will be eventually), the important thing to note is that, statistically speaking, a large number of asteroids with initially uncertain impact probabilities have been relegated to the “harmless” category.
    We’ve known about asteroids for two hundred years, and it’s taken us this long to recognize their danger. The dinosaurs never had a chance.

ARMAGEDDON NOW
    Of course, the big difference between us and the dinosaurs is that they didn’t have a space program.
    You’ve seen the movie a hundred times: an asteroid miles across is discovered and its orbit puts it on a direct collision course with Earth. If we don’t do something, it’ll wipe us out. Enter the team of brave hero astronauts/oil riggers/military men. Heroically they launch into space, heroically face down the monstrous rock, and heroically blow it to smithereens, which then rain down harmlessly as gawkers look on.
    That sounds, well, heroic. There’s only one problem: it won’t work.
    Actually, there are lots of problems with this scenario. For one, there’s no guarantee that nuking an asteroid will destroy it. A lot of asteroids are almost solid iron, so throwing a nuclear bomb at one might only warm it up a little.
    Even if an asteroid is made of rock, there’s no guarantee a nuke will disintegrate it. First, if it’s really big, a nuclear weapon may not do all that much damage to it. But it also depends on the asteroid’s consistency.
    Some asteroids have been found to have very low density, which was initially puzzling. Rock has a density of about two to three grams per cubic centimeter (roughly an ounce per cubic inch, or two to three times the density of water). But some asteroids have lower density than that. An asteroid called 253 Mathilde, for example, which orbits the Sun between Mars and Jupiter, has a density of about 1.3 grams per cc. It must have a texture like Styrofoam. How can that be?
    When asteroids were finally observed up close by space probes, they were seen to be heavily cratered. Obviously, asteroids eat their own: they hit each other, leaving giant pits across their surfaces. If an asteroid gets hit hard enough, it’ll