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 theyre 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 didnt 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.

Thousands of Threats

But Schweickart is still worried: "Don't relax; there's more where that one comes from." He says that one must assume that large asteroids exist in the thousands. Astronomers estimate that there are at least 1,000 near-earth objects of one kilometer or more and perhaps a million larger than 50 meters. "The estimate is that every month one that size [100 meters] not only passes between Earth and the moon, but has passed between Earth and the moon for billions of years. The difference is now we can see them because were looking."

Adds Schweickart: "Its like Las Vegas: the odds are the odds. We are going to get whacked at some point, and we should know it in advance. We probably will have decades of warning." Enough time, perhaps, to do something about the threat.

Toward that end, he says that work on the detection side needs to continue beyond the congressional mandate to identify 90 percent of near-earth objects over one kilometer. "Once we get those catalogued, there are many more in small sizes that are still very dangerous to life on Earth," he explains. "We need to go from the program we have now to a program that picks up things down to 300 meters in diameter. Even though they wont cause extinction, they could cause serious global disturbances--wipe out regions, let alone cities. And because there are a lot more [of the smaller-size asteroids], the probability is higher were going to find one thats going to hit us."

Stopping the Impact

Then the task will be to prevent the impact. Right now, says Schweickart, the only known technology for the purpose is "the wrong thing--a nuclear weapon that you send out there to blow it up." The result, he says: "You turn a rifle bullet into a shotgun blast."

Instead, "I'm interested in going beyond detection by using technology which is or will shortly become available to actively change the orbits of these potentially threatening asteroids in a controlled way, so that they no longer threaten life on Earth," he says. "You have to go out and meet it with a 'tugboat,' a space vehicle that would allow us to push it around for years at a time. You change the orbit by very small velocities, which causes it to miss, rather than hit, Earth."

The process would be based on developing rocket technology, but Schweickart declined to be more specific, explaining, "Were talking about things that have existed, but havent been put to use for this purpose."

He is discussing the problem in a small, private international group that includes astronomers, biologists, engineers and others. He says he will reveal more about the group in six months to a year, when it is ready to publish the results of its work. Some of the group members met in October 2001 at NASA Johnson Space Flight Center in a workshop organized by Piet Hut and Ed Lu. It was called Project B612, after the name of the small asteroid that was home to the Little Prince in the novel "Le Petit Prince," by Antoine de Saint-Exupery. Among the ideas discussed during the workshop was using a plasma-drive system such as nuclear-electric propulsion for controlled flight near an problem asteroid.

In the meantime, understanding of the issue is not at a point where the government could address it--that is, get a Congressional budget item and spend money, says Schweickart. "The problem, partly--as far as the general public is concerned--is that it would be a laugher," he says of the idea of "somebody talk[ing] about spending a few billion dollars to push the next asteroid out of the way." For the moment, he says, the matter should be studied and analyzed quietly. "Its one of those things which you want to keep peoples attention on and at the same time not panic. If we keep on with the detection program, we should know [about a problem asteroid] ahead of time, and that will give us, with developing technology, the opportunity to deflect it."

Lucy Komisar is New York journalist who writes on international issues.