The bursts probably result from the merging of two collapsed stars. These gamma-ray bursts, astrophysicists recently learned, originate in distant galaxies and are unfathomably powerful - as much as 10 quadrillion (a one followed by 16 zeros) times as energetic as the sun. Once a day or so, you would see a bright flash appear, briefly outshine everything else, then vanish.

If you could watch the sky with gamma-ray vision, you might think you were being stalked by cosmic paparazzi. If one of the big ones headed right for us, that would be it for pretty much all higher forms of life, even cockroaches. The Kuiper belt sends a steady rain of small comets earthward.

For a real chill, look to the Kuiper belt, a zone just beyond Neptune that contains roughly 100,000 ice-balls more than 50 miles in diameter. Humans would likely survive, but civilization might not.ДЄn asteroid five miles wide would cause major extinctions, like the one that may have marked the end of the age of dinosaurs. Objects more than a half-mile wide - which strike Earth every 250,000 years or so - would touch off firestorms followed by global cooling from dust kicked up by the impact. But with big asteroids, it doesn't matter much where they land. Many scientists question his interpretations: Impacts are most likely to occur over the ocean, and small ones that happen over land are most likely to affect unpopulated areas. As an example, he says one killed 10,000 people in the Chinese city of Chi'ing-yang in 1490. Astronomers estimate similar-sized events occur every one to three centuries.Д«enny Peiser, an anthropologist-cum-pessimist at Liverpool John Moores University in England, claims that impacts have repeatedly disrupted human civilization. In 1908 a 200-foot-wide comet fragment slammed into the atmosphere and exploded over the Tunguska region in Siberia, Russia, with nearly 1,000 times the energy of the atomic bomb dropped on Hiroshima.

But there is no question that a cosmic interloper will hit Earth, and we won't have to wait millions of years for it to happen. Once a disaster scenario gets the cheesy Hollywood treatment, it's hard to take it seriously. But the next statistic on the list could be us. At present, we may worry about snail darters and red squirrels in abstract terms. The current rate of extinctions is, by some estimates, 10,000 times the average in the fossil record. Human activity is severely disrupting almost all life on the planet, which surely doesn't help matters. The odds of being one of the people to witness doomsday are highest when there is the largest number of witnesses around - so now is not such an improbable time. Or turn the argument around: How likely is it that this generation will be the one unlucky one? Something like one fifth of all the people who have ever lived are alive today. By pure odds, it's unlikely that we would be among the very first hundredth of a percent of all those people. If humans were to survive a long time and spread through the galaxy, then the total number of people who will ever live might number in the trillions. In 1983, British cosmologist Brandon Carter framed the ' Doomsday argument ,' a statistical way to judge when we might join them. Yet 99 percent of all species that ever lived have gone extinct, including every one of our hominid ancestors. It's difficult to imagine it all coming to an end. In the 500,000 years Homo sapiens has roamed the land we've built cities, created complex languages, and sent robotic scouts to other planets.

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