Asteroid-Watchers Say Catastrophic Impact Is Unlikely
Rest easy, Earthlings.
Despite magazine cover stories and front-page articles warning that a catastrophic asteroid impact could at any moment condemn humanity to the fate of dinosaurs, Earth’s dense atmosphere is proving to be unexpectedly effective at postponing doomsday.
Ironically, this comforting conclusion grows out of a report today in the journal Nature that Earth shares its orbit with a swarm of asteroids, and its chances of encountering objects up to half a football field in length are 10 to 100 times greater than previously believed.
But, the researchers add, the conspicuous lack of asteroids flattening cities is no fluke.
Scientists from the University of Arizona and the National Aeronautics and Space Administration said that this surprising abundance of “Earth-approaching” boulders--a miniature version of the asteroid belt between Mars and Jupiter--went unnoticed for so long because most asteroids that fall toward Earth burn up or explode in the atmosphere.
“A lot of them are hitting the atmosphere all the time,” said David L. Rabinowitz of the University of Arizona Skywatch program at the Kitt Peak Observatory near Tucson, “but common sense says they’re not a problem because we’re not seeing a lot of impacts on the surface.”
This is not to say cataclysmic impacts never happen, scientists added. Several are known or believed to have occurred, such as the one 65 million years ago in Mexico that is suspected of triggering the extinction of the dinosaurs.
But since the atmosphere seems to be so efficient at chewing up intruders, the odds of meeting up with killer asteroids are slim indeed. Something comparable to the famous 1908 Tunguska event, in which a huge expanse of Russian forest was blown down by the midair explosion of a friction-heated asteroid, is expected only once every three centuries. Even then, it probably would occur over water, which covers nearly 75% of the Earth’s surface.
“Put it this way: We don’t need a fleet of rockets to blow these things off course,” said Christopher F. Chyba of NASA’s Goddard Space Flight Center in Greenbelt, Md. “This (collision of an asteroid) does not represent a major threat.”
Concern about a catastrophic celestial collision, long a cherished theme of science fiction writers, was renewed last year when the Harvard Smithsonian Center for Astrophysics calculated the eccentric orbit of a comet named Swift-Tuttle and decided that it could be a candidate for collision with Earth in 2126. The odds of a collision were estimated at roughly one in 10,000, but it was enough to spark the popular imagination.
NASA inadvertently encouraged speculation by sponsoring a conference on the topic last year and by commissioning a pair of researchers at the Jet Propulsion Laboratory and the California Institute of Technology in Pasadena to speculate about how to manage menacing meteoroids. They suggested rockets tipped with nuclear weapons or very heavy weights.
The space agency also sponsors a part-time asteroid watch at the Mt. Palomar Observatory in San Diego County. It is similar to the more comprehensive Skywatch program that produced these latest findings.
One matter, however, still puzzles scientists: If asteroids disintegrate in the atmosphere, why can’t we routinely see or hear the explosions? Some are as powerful as small atomic bombs, and several explosions occur in the upper atmosphere every year.
Although there is evidence of one such event--over the small town of Revelstoke in eastern British Columbia, Canada, in 1965--Chyba said sightings may be so rare because daylight masks half the flashes and three-fourths of the rest are likely to happen over oceans.
