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Conveniently, a natural test of the theory arose when the Shoemakers and Levy discovered that Comet Shoemaker-Levy 9, which they soon realised was headed for Jupiter. For the first time, humans would be able to witness a cosmic collision - and witness it very well thanks to the new Hubble space telescope. Most astronomers, according to Curtis Peebles, expected little, particularly as the comet was not a coherent sphere, but a string of twenty-one fragments. "My sense," wrote one, "is that Jupiter will swallow these comets up without so much as a bump." One week before the impact, Nature ran an article, "The Big Fizzle is Coming," predicting that the impact would constitute nothing more than a meteor shower.

The impacts began on July 16, 1994, went on for a week, and were bigger by far than anyone - with the possible exception of Gene Shoemaker - expected. One fragment, known as Nucleus G, struck with the force of about six thousand gigatons - seventy-five times more than all the nuclear weaponry in existence. Nucleus G was only about the size of a small mountain, but it created wounds in the Jovian surface the size of Earth...

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I asked [Anderson and Witzke] how much warning we would receive if a similar hunk of rock was coming toward us today.
"Oh, probably none," said Anderson breezily. "It wouldn't be visible to the naked eye until it warmed up, and that wouldn't happen until it hit the atmosphere, which would be about one second before it hit Earth. You're talking about something that is moving many tens of times faster than the fastest bullet. Unless it had been seen by someone with a telescope, and that's by no means a certainty, it would take us completely by surprise."

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An asteroid or comet travelling at cosmic velocities would enter the Earth's atmosphere at such a speed that the air beneath it couldn't get out of the way and would be compressed, as in a bicycle pump. As anyone who has used such a pump knows, compressed air grows swiftly hot, and the temperature below it would rise to some 60,000 Kelvin, or ten times the surface temperature of the sun. In this instant of its arrival in our atmosphere, everything in the meteors path-people, houses, factories, cars-would crinkle and vanish like cellophane in a flame.

One second after entering the atmosphere, the meteorite would slam into Earth's surface, where the people of Manson had a moment before been going about their business. The meteorite itself would vaporize instantly, but the blast would blow out a thousand cubic kilometers of rock, earth, and superheated gases. Every living thing within 150 miles that hadn't been killed in the heat of entry would now be killed by the blast. Radiating outward at almost the speed of light would be the initial shockwave, sweeping everything before it.

For those outside the zone of immediate devastation, the first inkling of catastrophe would be a flash of blinding light-the brightest ever seen by human eyes-followed an instant to a minute or two later by and apocalyptic sight of unimaginable grandeur: a roiling wall of darkness reaching high into the heavens, filling an entire field of view and traveling at thousands of miles an hour. Its approach would be eerily silent because it would be moving far beyond the speed of sound. Anyone in a tall building in Omaha or Des Moines, say, who chanced to look in the right direction would see a bewildering veil of turmoil followed by instantaneous oblivion.

Within minutes, over an area stretching from Denver to Detroit and encompassing what had once been Chicago, St. Louis, Kansas City, the Twin Cities-the whole of the Midwest, in short-nearly every standing thing would be flattened or on fire, and nearly every living thing would be dead. People up to a thousand miles away would be knocked off their feet and sliced or clobbered by a blizzard of flying projectiles. Beyond a thousand miles, the devastation from the blast would gradually diminish.

But that's just the initial shockwave. No one can do more than guess what the associated damage would be, other than it would be brisk and global. The impact would almost certainly set off a chain of devastating earthquakes. Volcanoes across the globe would begin to rumble and spew. Tsunamis would rise up and head devastatingly for distant shores. Within an hour, a cloud of blackness would cover the planet, and burning rock and other debris would be pelting down everywhere, setting much of the planet ablaze. It has been estimated that at least a billion and a half people would be dead at the end of the first day. The massive disturbances to the ionosphere would knock out communications systems everywhere, so survivors would have no idea what was happening elsewhere or where to turn.

... etc, the climate would be cooled and poor growing seasons would result for up to 10,000 years...

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We can't send people to it, we don't have any rockets powerful enough to send man as far as the moon. The last, Saturn 5 was retired years ago.

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Even if we did manage somehow to get a warhead to the asteroid and blasted it to pieces, the chances are we would simply turn it into a string of rocks that would slam into us one after the other in the manner of Comet Shoemaker-Levy 9 on Jupiter-but with the difference that the rocks would now be intensely radioactive. Tom Gehrels, an asteroid hunter at the University of Arizona, thinks that even a year's warning would be insufficient to take appropriate action. The greater likelihood, however, is that we wouldn't see any object-even a comet-until it was about six months away, which would be much too late. Shoemaker-Levy 9 had been orbiting Jupiter in a fairly conspicuous manner since 1929, but it took over half a century before anyone noticed.