Asteroid in the Neighborhood? The Public Wants to Know

Astronomers exhale, then redouble efforts after recent collision prediction is revised

One asteroid's brief brush with notoriety is serving as a wake-up call for improving the way the public is notified of potential hazards from space debris.

Last week's report that a recently discovered asteroid might swing to within 30,000 miles of Earth was quickly revised after astronomers spotted the object in an eight-year-old photo and revisited their orbit estimates.

"Presumably, people take these things quite seriously. We can't have cries of 'wolf!' coming out of the astronomy community," says Clark Chapman, a planetary scientist at the Southwest Research Institute's branch office in Boulder, Colo.

This is not the first time researchers have drastically revised a troubling outlook. In the early 1990s, comet Swift-Tuttle was projected to strike Earth 100 years hence - an announcement that later proved to be wrong, according to Dr. Chapman. And a mistranslated report in the New China News Agency in 1989 said the country faced an immanent asteroid impact, when nothing of the sort was going to happen.

The opportunity for false alarms is likely to grow, he continues, as observing programs uncover new near-Earth asteroids.

Earth sweeps through a messy part of the solar system. Within the past four years, Earth has had two close calls as asteroids sped by. The smallest material Earth encounters - dust and small meteoroids - burns up in the atmosphere in familiar meteor showers.

About once a year, Earth encounters an object 10 meters across, which is destroyed in a brilliant flash high in the sky. The explosions, roughly equivalent to detonating 20,000 tons of TNT, look very similar to air bursts with nuclear weapons, notes Arno Ledebuhr, a physicist at the Lawrence-Livermore National Laboratory in Livermore, Calif.

Astronomers are most concerned about objects from 1 to 2 kilometers across. An estimated 2,000 asteroids fall into this size range and have orbits that cross that of Earth. Only 10 percent, however, have been detected. These are not extinction-class asteroids, researchers say. But they add that a collision with one could have devastating effects on the planet.

Scientists put the probability of one of these asteroids striking Earth during the next century at one in a few thousand. In an effort to pin down the number of potentially hazardous asteroids and their orbits, a handful of professional astronomers are hunting for the objects.

The first dedicated efforts to hunt for near-Earth objects began in the early 1970s at the Mt. Palomar Observatory in California. Two teams - one headed by the late Eugene Shoemaker and his wife, Carolyn, the other led by Eleanor Helin of Caltech's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. - worked at the facility, looking at photographic plates for asteroids and comets.

Since 1995, Dr. Helin has used a telescope-mounted electronic camera operated by the US Air Force on Mt. Haleakala in Hawaii to search for asteroids. Her effort has detected more than 21,000 asteroids of all types; 60 percent of them are new finds. Of particular interest are a group of asteroids known as Atens, which she discovered in 1976. These asteroids never stray far from Earth's orbit, and some cross it as many as four times a year. Thus they are more likely to strike Earth than any other kinds of asteroids. So far, she says, her team has discovered 28 Atens.

In an attempt to get full-sky coverage, she has proposed putting similar cameras on a network of Air Force satellite-tracking telescopes worldwide. If the plan is approved, "within 10 years, we could document 95 percent or more of the estimated population of near-Earth asteroids," she says. Meanwhile, atop Arizona's Kitt Peak, a 14-year effort known as Spacewatch is adding a new telescope to its asteroid-hunting array.

The newest effort, however, is being mounted by astronomers at the Lowell Observatory in Flagstaff, Ariz. The Lowell Observatory Near-Earth Object Survey uses a 24-inch telescope at a site on nearby Anderson Mesa to search large areas of the night sky for asteroids.

"We're just getting cranked up this month," says Bruce Koehn, an astronomer at the observatory. Ultimately, he says, the system is designed to take pictures of a portion of the sky once every 30 seconds. By comparing images of the same patch of sky taken at different times, astronomers can isolate objects such as near-Earth asteroids as they shift against the background of stars.

Yet if recent experience is any guide, mining observatory archives for information on asteroids may be just as important for assessing the hazard they present as finding new ones. When the International Astronomical Union's office in Cambridge, Mass., reported last week that an asteroid known as 1997 XF11 might pass within 30,000 miles of Earth in the year 2028, calculations of its orbit were based on observations of its position only since last December.

The next day's estimate added information from a 1990 photo in which the asteroid appeared. Astronomers now had data that spanned eight years. Researchers at JPL ran a new set of calculations, and the miss distance emerged as 600,000 miles, with an uncertainty of 50,000 miles. Estimating the orbit with only 88 days of information "was like trying to figure out where to catch a fly ball after watching it travel only 3 inches," says Dr. Ledebuhr.

If an asteroid is discovered heading for Earth and with sufficient lead time, it will be critical to send a spacecraft to determine its size and composition, researchers say. That information would help them tailor a mission to divert the object - most likely with a nuclear warhead.

"If you have a lot of time, you only need to impart a small change in velocity to the asteroid," says Johndale Solem, a physicist at the Los Alamos National Laboratory in New Mexico. "We all pray that we never have to face this kind of situation," he says.

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