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ARTIST'S CONCEPTION of a catastrophic asteroid impact with Earth.
DON DAVIS

On June 15, 2002, an asteroid the size of a football field, dubbed 2002MN, came within 120,000 kilometers of hitting Earth. The rocky body was traveling at a speed of 36,800 kilometers per hour--and if it struck, it would have wreaked as much destruction as a nuclear weapon. It was one of the closest passes ever recorded for an object of that size. And astronomers didn't detect it till three days afterward.

"Years too late," is what former astronaut Rusty Schweickart would say. Schweickart is working with an informal group of scientists to try to prevent the crash of a giant asteroid--one even larger than 2002MN--that he says could destroy vast tracts of land, with disastrous, long-term effects on the climate. Schweickart says that future asteroids that threaten to strike must be pushed out of their orbits by advanced space technologies. He described his concerns recently at Forum 21, a private annual current-affairs meeting of Americans and Europeans, held this year in Divonne, France.

The lunar module pilot for the Apollo 9 space flight in 1969, Schweickart has worked on making several aspects of human space exploration safer. (For more, see his bio.) But the ex-astronaut is now focused more on dangers to Earth. He says he is worried about "the improbable but real threat to life posed by the existence of near-earth asteroids, which are being discovered today in quantities of hundreds."

From Shooting Stars to Smashing Siberia

Asteroids circle the sun on paths that may cross the orbit of Earth. From time to time through history, huge asteroids have collided with the planet. "The reality is we get hit 100,000 times every day; but they¿re so small, you call them shooting stars," Schweickart says. Earth's atmosphere protects us from bodies smaller than 50 meters. "But when they get to the size of 50 meters, they start coming though the atmosphere instead of burning up in it. The Tunguska thing didn¿t make it to the ground, but it blew up so close to the ground that it flattened a big hunk of Siberia." The famous 1908 explosion at Tunguska leveled 2,000 square kilometers of forest. It involved an asteroid some 100 meters in diameter. Impacts with bodies that size may occur once in a century on average, according to estimates. Though far less frequent, an asteroid of 10 to 15 kilometers in size could wreak the kind of destruction that wiped out the dinosaurs 65 million years ago, says Schweickart. Impacts of that size occur perhaps once in every 1,000,000 centuries, according to the JPL Near Earth Asteroid Tracking Team.

To learn more about such hazards, Congress in 1994 directed NASA to detect and track near-Earth objects greater than one kilometer in size, whose impact would create global catastrophe. Geochemistry shows that such collisions played a significant role in the creation and destruction of life on earth--and that this process is not over. (For more on the role of asteroids in life's development, see "Repeated Blows," by Luann Becker; Scientific American, March 2002, available for purchase on the Scientific American Archive.)

In 1998, NASA formally initiated the Spaceguard Survey--adopting the objective of finding 90 percent of all near-Earth objects larger than one kilometer in diameter. That year, NASA also set up a Near-earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, California; a similar facility exists in Pisa, Italy. Today, there are several teams tracking near-Earth objects. In addition to 2002MN, Spaceguard discovered another asteroid, 2002 EM7, that passed within 463,000 kilometers in March 2002; this asteroid also was not found until after its flyby of Earth.

Also, in April 2002, NASA announced that its Sentry automatic impact monitoring system had identified a potential close encounter. Asteroid 1950DA, which is two-thirds of a mile wide, is expected to come close to Earth in the year 2880. If it hits, it would destroy everything within a few hundred miles. Those far from ground zero would suffer from environmental effects similar to a nuclear winter, as the dust and debris blocked the sun for months or even years. NASA said that the odds were 300 to one that the asteroid would hit, and that there was plenty of time--eight centuries--to figure out what to do before then.