It scares me," said Jack Hills, an astronomer at the Los Alamos National Laboratory. "It really does." He and the rest of the world had good reason to be worried. Astronomer Brian Marsden at the Harvard-Smithsonian Center for Astrophysics had just announced that a newly discovered asteroid a mile wide was headed for Earth and might pass as close as 30,000 miles in the year 2028. "The chance of an actual collision is small," Marsden reported, "but not entirely out of the question."
An actual collision? With a mile-wide asteroid? It sounded like the stuff of science fiction and grade-B movies. But front-page stories and TV newscasts around the world soon made clear that the possibility of a direct hit and a global catastrophe well within the lifetime of most people on Earth today was all too real.
Then suddenly, the danger was gone. Barely a day later, new data and new calculations showed that the asteroid, dubbed 1997 XF11, presented no threat at all. It would miss Earth by 600,000 miles--closer than any previously observed asteroid of that size but a comfortable distance. Still, the incident focused attention once and for all on the largely ignored danger that asteroids and comets pose to life on Earth. As Los Alamos senior scientist Greg Canavan put it, paraphrasing Dr. Samuel Johnson, "Nothing so clears the mind as the sight of the gallows."
XF11 was discovered last Dec. 6 by astronomer Jim Scotti , a member of the University of Arizona's Spacewatch group, which scans the skies for undiscovered comets and asteroids. Using a 77-year-old telescope equipped with an electronic camera, he had recorded three sets of images, 30 min. apart, of a small sector of the night sky. The digitized images, fed into a computer programmed to look for objects moving against the background of fixed stars, revealed an asteroid that Scotti, in an E-mail to Marsden, described as standing out "like a sore thumb."
Marsden promptly posted Scotti's data on the Harvard Center's Website, making them available to other astronomers. In early March, those data and newer observations by two Japanese amateur astronomers and a Texas scientist were fed into the Harvard Center's number-crunching orbit predictor, which spat out the 30,000-mile "miss distance" that led Marsden to make his dramatic announcement.
For a brief but exciting 24 hours, the big asteroid commanded everyone's attention. Astronomer Hills calculated that an asteroid the size of XF11 colliding with Earth at more than 38,000 m.p.h. would explode with the energy of 300,000 megatons--nearly 20 million times the force of the bomb that leveled Hiroshima. If it hit in the ocean, he predicted, it would cause a tsunami (commonly called a tidal wave) hundreds of feet high, flooding the coastlines of surrounding continents. "Where cities stood," he said, "there would be only mudflats." A land hit, he calculated, would blast out a crater at least 30 miles across and throw up a blanket of dust and vapor that would blot out the sun "for weeks, if not months."