Pallasite (stony iron meteorite) . Photo by Ron Hipschman. [ Click for a larger image.]		Nickel-iron meteorite. Photo by Ron Hipschman. [ Click for a larger image.]
	Nickel-iron meteorite. Photo by Ron Hipschman. [ Click for a larger image.]		Chondrite (stony meteorite) . Photo by Ron Hipschman. [ Click for a larger image.]
	Meteorite samples courtesy of the California Academy of Sciences .

hysicist Peter Fiske and his colleagues at Lawrence Livermore Labs in northern California are tracking down a meteorite impact in Southeast Asia, although they haven't found the crater yet. What they have found is a profusion of "tektites," glassy orbs created when soil melts from the intense pressure of an impact. From the abundance, characteristics, and location of these tektites, Fiske and his colleagues believe the impact of a mile-wide asteroid that formed them occurred relatively recently, about 770,000 years ago, the largest, most recent impact identified so far.

Because meteorite impacts have occurred so infrequently in recent history, there aren't many clues for scientists to work with. "Meteorite impact is the least well-understood geological process we know of," Fiske says. "Impacts have the potential to cause massive damage to the earth and life on it. But we really don't understand the details." He hopes to get at those details: How much hot debris is ejected from a large impact? How much forest is ignited? Do medium-sized impacts (smaller than the dino-killer) cause regional but not global extinctions?

What can be done to avert catastrophe once a large asteroid is identified as an imminent danger to earth? Eleanor Helin discusses our options if faced with this extremely remote possibility.

Sooner or later, history will repeat itself and a globe-threatening impactor will target our planet. There are programs in place to monitor the orbits of known asteroids and comets, and more near-earth objects are being discovered by amateur astronomers and the pros every year. In the best of circumstances, we could have decades, perhaps a century, of advance warning. But what would we do if a big one was on a collision course with earth? Doomsday movie scripts aside, scientists haven't yet figured out how to prevent global catastrophe. Launching a nuclear weapon could just break a meteoroid into pieces and spread out, not prevent, the damage. If we're lucky, we'll have enough time to come up with a solution.
  

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