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Close Encounters Of the Asteroid Kind

It's coming in 2028. Will it hit Earth or merely provide a spectacular flyby?

Don't go out and buy a hard hat yet. But an asteroid a mile wide is expected to rendezvous with the Earth in 2028.

The big question: Will it actually hit us? That's what astronomers are trying to answer following this week's Hollywood-like announcement that a chunk of material left over from the formation of the solar system may streak within 30,000 miles of Earth 30 years from now.

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Discovered Dec. 6 by University of Arizona astronomer Jim Scotti, the asteroid, 1997 XF11, is expected to pass closer to Earth than any other known asteroid during the foreseeable future.

The fly-by distance is uncertain. The mile-wide asteroid could come no closer than the moon, 240,000 miles distant, or it could pass closer than 30,000 miles.

"The chance of an actual collision [with Earth] is small," according to Brian Marsden, director of the International Astronomical Union's Central Bureau for Astronomical Telegrams in Cambridge, Mass. But a collision "is not entirely out of the question."

As a result, professional astronomers - not to mention amateur skywatchers everywhere - are likely to turn their telescopes on the asteroid with increasing frequency. They'll also rifle through archived material in hope of turning up images of 1997 X11.

Such efforts will help reduce uncertainty about the object's orbital path. In 2002, the asteroid is expected to pass within 6 million miles of Earth, close enough to track it with radar and pin down its 2028 pass to within 1,000 miles.

Search for falling objects

The discovery is perhaps the most significant to date for the Spacewatch program, established in 1981 in part to look for comets and asteroid that may be on orbits that will take them across Earth's path. Funded by the National Aeronautics and Space Administration and private donations, the Spacewatch project used a pair of telescopes atop Kitt Peak, outside of Tucson, Ariz., to scan the skies for objects that might strike Earth. So far, astronomers say they have identified 108 "potential hazardous objects" that could threaten Earth during the next several centuries.

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The good news about the asteroid's sighting is that it leaves lots of lead time for scientists to figure out where it's headed, Marsden says.

The discovery would be "scary" if the asteroid's encounter with the Earth-moon system were even three years away. But 30 years is far enough in the future to develop means of deflecting it and other asteroids, he says. Suggestions have ranged from using chemical explosives to nuclear warheads that would send an Earth-bound asteroid on a detour.

Telescope's eye view

The announcement of the asteroid's estimated path came late Wednesday, following observations earlier this month by University of Texas astronomer Peter Shelus using a 30-inch-diameter telescope at the MacDonald Observatory in western Texas. His tracking effort extended to 88 days the "arc" the asteroid was tracing over the night sky. Initially, a pair of amateur astronomers in Japan kept watch on 1997 XF11 for two weeks after its discovery. Their observations led to calculations showing that the minimum distance between the asteroid and Earth would be small. Additional observations spanning up to 60 days allowed astronomers to calculate an October 2028 encounter and a 500,000-mile "miss."

But when Dr. Shelus's data from March 3 and 4 were added to the existing information, calculations put the miss at 30,000 miles, with the closest encounter taking place at about 1:30 p.m. Eastern Daylight Time on Oct. 26, 2028. The asteroid should be visible to the naked eye after dark, with Europeans getting the best view.

Asteroids are routinely observed and plotted by astronomers around the world because of their potential for great destruction on Earth. An object 6 to 10 miles across is thought to have collided with Earth and wiped out the dinosaurs 65 million years ago.

Many scientists say the chances of 1997 XF11 hitting Earth are remote, but that hasn't stopped them from running the numbers. An asteroid the size of 1997 XF11 colliding with Earth at more than 17,000 miles per hour would explode with the energy of about 320,000 mega-tons of dynamite - the equivalent of 2 million Hiroshima-size atomic bombs. Such as asteroid hitting the ocean, they add, would create a tidal wave hundreds of feet high, causing flooding among thousands of miles of coastline. If it hit land, it would dig a crater as much as 20 miles across.