If Worlds Collide : Southland Scientists Offer Ways to Avoid an Asteroid Accident

Peering up through their telescopes at the hundreds of asteroids and comets that routinely intersect Earth’s orbit, more than a few astronomers have asked themselves what has come to be known as the Doomsday Question:

What if some stray rock the twice the size of Gibraltar happens to arrive at the same spot in the boundless void of the universe at precisely the same moment as, say, Cleveland?

The likelihood of such a cataclysmic collision is remote--mile-wide objects are expected to smack into Earth only every 500,000 years or so--but it is not an entirely idle question. For example, on Aug. 14, 2126, Comet Swift-Tuttle is expected to come close enough to Earth to risk a collision.

It is generally accepted that just such a collision--Mexico’s Yucatan Peninsula has been suggested as one possible impact point--exterminated 90% of all species 65 million years ago. Among its suspected casualties were dinosaurs, then the undisputed dominant life form on the planet.

Fearing a similar disaster that may cost them that inherited status, some humans have been wondering what they should do about bothersome interplanetary flotsam.

Two Southern California scientists, tagged by the National Aeronautics and Space Administration to ponder the problem, suggest a couple of methods in an article in the current issue of the journal Nature: big rockets and small nuclear bombs.

Thomas J. Ahrens of Caltech and Alan W. Harris of NASA’s Jet Propulsion Laboratory calculate that rocket-launched masses of 220 to 2,200 pounds would be enough to deflect errant asteroids up to 110 yards in diameter--somewhat smaller than the Sports Arena.

For anything bigger, they figure, a relatively smallish atomic bomb or two would be needed to do the trick.

Deflection is the key, Ahrens and Harris said. Trying to blow an asteroid to smithereens would probably require burying a nuclear device in its rocky heart--no easy feat.

Besides, they add, sizable debris shards from such an explosion could still make it to Earth, and might cause more damage than a single large impact.

People have seen evidence of small collisions--in the form of meteorites--for a long time. Astronomers have known since the 1930s that catastrophic collisions are possible, that big asteroids and comets seemed to intersect Earth’s orbit. Hollywood has been thinking about the topic at least since 1951, when the movie “When Worlds Collide” was released.

But Ahrens said that scientists began to take the matter seriously only about a decade ago, when they began to realize how many potentially apocalyptic asteroids intersect Earth’s orbit. More than 2,000 objects at least half a mile wide have been sighted so far, he said.

That number sounds ominous at first, he conceded, but the real risk is still remote.

“It’s a hazard,” Ahrens said, “but it’s about as likely as an average person dying in a commercial airplane accident.”

In numerical terms, each person stands about one chance in 50 million of being killed by a giant asteroid in any given year. For this reason, Ahrens and Harris recommend against anyone taking steps to immediately implement any of their asteroid-bashing ideas.

Nonetheless, at a January seminar on the subject, NASA officials asked Ahrens and Harris to fulfill a congressional request to calculate what it would take to shove an onrushing asteroid out of Earth’s way or shatter it into so much cosmic gravel.

So the two men worked up a series of mathematical formulas that assumed “compressional wave velocities” and applied the “thermodynamic Grunesian ratio,” and drew their conclusions.

One suggestion, which they said would be particularly suitable for smaller objects, is to treat an errant asteroid like a hostile ballistic missile and attack it with a weapon made for the Strategic Defense Initiative. Their choice is the Lightweight Exoatmospheric Projectile by the Boeing Corp.

For bigger problems, Ahrens and Harris recommended dusting off some of the nation’s supply of neutron bombs. Detonated near enough to an oncoming hunk of rock or metal, these relatively small nuclear weapons would generate a pulse of subatomic particles strong enough to send most of the asteroid skittering harmlessly into space.

For the record, though, neither Ahrens nor Harris expects to see action on their suggestions any time soon.

“Although further study of the feasibility of diverting asteroids may be warranted,” they conclude their report, “we do not believe it is appropriate to conduct engineering designs of systems at this stage because of . . . the high cost in the face of a low-risk factor.”