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Earth in a Cosmic Shooting Gallery

By Robert C. Cowen / December 30, 1992



COMET Swift-Tuttle has come and gone. So has the news-media hype about the "killer comet" that might return to clobber Earth in the 22nd century. But the overblown emphasis they gave to that highly uncertain prediction highlighted a serious reality.

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Earth does exist in a cosmic "shooting gallery" of asteroids and comets. More of those missiles are whizzing through our neighborhood than astronomers realized even a year ago.

In October, astronomers Tom Gehrels, David Rabinowitz, and James Scotti at the University of Arizona in Tucson estimated that every day as many as 50 "house-sized" asteroids probably pass Earth at a distance less than that to the moon. That's about 100 times the close-encounter rate previously suspected. Several such small asteroids may hit Earth every year. Generally, they burn up in the air, making brilliant fireball displays.

The Arizona astronomers are running a program called Spacewatch that keeps an eye out for such objects. They spotted the first of these house-sized asteroids when it passed by Earth at twice the distance to the moon in January 1991. They had seen six more by last October.

Old telescopic surveys using photographic plates couldn't spot these faint objects. But they show up readily in the sensitive detectors, called charge-coupled devices, that the Arizona team uses.

The small asteroids seen so far have diameters ranging from five to 50 meters. This overlaps the lower range of bodies big enough to do extensive damage. This, plus the astronomers' estimate that there are a large number of these previously undetected asteroids, strongly suggests that Earth's neighborhood is more dangerous than had been suspected.

Happily, humanity can minimize that danger by mounting a relatively inexpensive effort to learn what is out there. As Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., notes in the January issue of Sky & Telescope, "The real danger is from objects we have not detected." Objects that have been detected and whose orbits are adequately known do not seem to be a threat. We need as soon as possible to spot comets or asteroids that might eventually hit Earth.

Dr. Marsden notes that a diligent search, including careful orbit determinations, should pick out dangerous objects while they still have to make several orbits around the sun before hitting Earth. This should give ample time to prepare a spacecraft mission to deflect such objects, if warranted.

Comet Swift-Tuttle offers a case in point. Marsden's prediction of "a small but not negligible chance that Comet Swift-Tuttle will come scudding into Earth" in 2126 caught the news-media eye. Yet Comet Swift-Tuttle's orbit and the forces influencing that orbit are not known well enough to make such a prediction with certainty.

Marsden missed the comet's perihelion (the point in its orbit nearest the sun) this year by a day . He predicted Dec. 11. It occurred on Dec 12. If his prediction for 2126 is off by a day or more, there's no danger of collision. He explains that only if Swift-Tuttle's "perihelion were to occur during a three-minute interval on July 26, 2126, [would] Earth be hit 19 days later."

Marsden says that tracking Comet Swift-Tuttle now as it recedes from Earth could help clarify the situation. If large ground-based telescopes or the Hubble Space Telescope follow the comet for at least six years, he says he should be able to pinpoint its return.

That wouldn't cost much. Yet it could dismiss any concern about potential disaster or, if necessary, post a long lead-time warning for future generations.

Last year, a National Aeronautics and Space Administration study group recommended a program to monitor the skies for asteroids and comets. Called Safeguard, it would use several 2-meter or 3-meter telescopes. NASA has begun to fund such a search. But it has not yet committed itself to a full Safeguard program.

Such a program would be cheap insurance. It should be supplemented by a program to learn more about the nature of asteroids and comets using inexpensive spacecraft. There's little doubt that we live in a dangerous cosmic neighborhood. But most of the danger lies in not knowing what is out there.