Danger From Space
1997 XF 11
Asteroid 1997 XF11 was discovered by University of Arizona astronomer Jim Scotti at the Kitt Peak National Observatory. Scotti works with the Spacewatch program. The goal of this project, and NASA's Near Earth Asteroid Tracking program at the Jet Propulsion Laboratory in Pasadena, California, is to locate and characterize the thousands of asteroids that periodically cross Earth's orbit.
Scotti discovered XF 11 on Dec. 6 in a series of photographs. After subsequent observations by Japanese amateurs, it was added to the list of potentially hazardous objects, or PHAs, maintained by the International Astronomical Union.
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Objects on the PHA list are regularly monitored to refine knowledge about their orbits and their potential threat to Earth. The University of Washington has an animated GIF file posted on their Web site showing the asteroid's motion.
Astronomers quickly realized XF 11 would make a close pass by Earth in 2028. Observations have shown that the asteroid could pass within 600,000 miles of Earth's center at 1:30 p.m. EDT on Oct. 26, 2028.
But astronomers caution that more observations are needed to pin down XF 11's actual orbit.
Astronomers should be able to improve their forecasting in 2002, when it will pass within 6 million miles of Earth, making it an easy target for Earth-bound telescopes.
If it turns out XF 11 is a real threat, it can be diverted from its path by nearby nuclear or chemical explosions.
The Comet Threat
Of more concern to astronomers over the long haul are comets, which take longer than 200 years to complete one orbit. Because they are extremely difficult to detect until they are very close, there would be little time to prepare a defense.
"They're the stealth bombers" of the solar system, Eleanor Helin, director of NASA's NEAT program, said.
Some 2,000 objects a bit more than a half-mile across or larger are believed to be hurtling unseen along orbits that cross Earth's path around the sun. Many more thousands of smaller—but still potentially deadly—asteroids and comets also streak across Earth's orbit, astronomers believe, setting up a sort of cosmic shooting gallery in the inner solar system.
Luckily, the target is relatively small and the aim is rarely true. But until all the objects are located, the threat remains all too real.
"It's not necessary to set up an alarm, but you must recognize we are adding up a sizble list of these objects that should have our attention," Helin said.
No one really knows just how many such Earth-crossing asteroids and comets actually exist, much less if—or, more important, when—the planet and one of these rocky relics might end up at the same place at the same time. While the odds of a catastrophic collision are remote, only a few hundred such objects have been detected to date.
Large asteroid impacts are thought to have triggered mass extinctions in the past. Even small objects can cause catastrophic local damage.
"When you figure the small body that hit Meteor Crater in Arizona was on the order of [160 feet] or so, it doesn't take a [mile-size] object to really whack you," Helin said.
Additional background on this topic is maintained in an asteroid and comet FAQ at the NEAT Web site.
CBS News Space Consultant Bill Harwood ©1998, CBS Worldwide Inc., All Rights Reserved