Cosmic Collision Has Earthlings Looking Skyward
For astronomers and star warriors who already view the night sky with foreboding, the cataclysm that convulsed Jupiter last week was a possible preview of the fate of Earth, which one day may be caught off-guard by a stream of natural warheads--comets or asteroids on a collision course with the home planet.
The unexpectedly powerful impact of comet Shoemaker-Levy 9 on Jupiter--striking repeatedly with hundreds of times the energy contained in all Earth’s nuclear weapons--is lending new respectability to scientists who want to detect and ward off objects that may intersect Earth’s path.
As the comet fragments blossomed into plumes of superheated gas on Jupiter, the House Committee on Science, Space and Technology voted last week to require that NASA catalogue and track all major comets or asteroids that may cross Earth’s path. And several prominent scientists have quietly lobbied the U.S. Air Force Space Command to volunteer the use of its network of sky-watching satellites and telescopes for the same purpose.
Early next year, the Space Command is expected to make its early warning satellite system and at least one of its ground-based telescopes available to watch for incoming asteroids or comets, congressional sources said.
The programming of the satellite system, which normally monitors hostile rocket launches, will be modified so that it will also warn of objects approaching from deep space. The telescope, part of a system being upgraded, normally monitors foreign satellites.
Space is far from empty. Millions of asteroids and comets crisscross the solar system, but almost all of them are far from Earth, orbiting between Mars and Jupiter or circling in a vast cloud more than a light-year from the sun.
Astronomers have already identified about 200 objects large enough to wreak havoc on Earth. By tracking their orbits, they have determined that none of those would hit Earth. But most scientists suspect there are thousands more.
Geologists say Earth has been struck many times in its history. Scientists recently identified a 110-mile-wide impact crater on the Yucatan Peninsula. The 65-million-year-old crater, believed to be connected to the demise of the dinosaurs, is the largest-known impact structure on Earth.
Until recently, such scars were considered part of Earth’s past, not a preview of any future hazard.
Today, only a few aged telescopes are dedicated to searching for asteroids near Earth. If the House measure is adopted this fall, NASA will have a decade to catalogue every Earth-crossing object more than half a mile wide.
Physicist Edward Teller has suggested using hydrogen bombs to knock such intruders out of the sky, but scientists actively involved in planning such defenses say far more modest methods will suffice. No nuclear weapons or anti-comet ballistic defense systems, please, these scientists are quick to say.
They want just enough advance warning--say a century or so--to nudge any incoming asteroid out of the way by shoving it with a rocket tipped with a few tons of lead or to attach a solar sail that will waft it on a new course safely away from Earth.
Gregory H. Canavan, a senior scientific adviser at Los Alamos National Laboratory on Friday called such gentle precautionary maneuvers “cosmic croquet.” Canavan, who convened a group of scientists to evaluate the problem for NASA and Congress, estimated that an early warning system of small telescopes around the world might cost about $10 million a year and active defenses another $50 million a year.
“That is the insurance premium for things whizzing at you out of the cosmos,” he said. “With a hundred years warning, you would not need nuclear explosives. You would have lots of options.”
Many scientists are skeptical of the risk from comets or asteroids and are adamantly opposed to any effort to shoot them down.
“I would worry more about a car wreck, airplane crash or getting a bad can of Spam,” said University of Arizona astronomer Mark Sykes at the Steward Observatory.
David Morrison at NASA’s Ames Research Center in Mountain View, who helped prepare a report for Congress on ways of intercepting asteroids and comets, has made a study of the risk and determined that it is unlikely but large enough to warrant concern.
An impact that might kill 1 in 4 of the people on Earth can be expected about once every 500,000 years; everyone alive therefore faces about a 1 in 2 million chance of being killed in such a collision in any one year.
Over a lifetime that works out to about a 1 in 20,000 chance, which is comparable to the risk most people face of dying in an earthquake or an airplane crash, according to figures he presented to the American Assn. for the Advancement of Science.
“My (colleagues) think we are absolutely crazy to suggest government money should be spent to prevent a catastrophe that happens only once every several hundred millennia or so,” he told the symposium of scientists.
But, he said, “Unlike other natural hazards, impacts can kill billions of people and endanger civilization. Unlike any other natural hazard I can think of, you can avert an impact.”
Eugene Shoemaker, the astronomer who helped discover the comet that collided with Jupiter last week, supports the effort to develop an early warning system to watch for incoming celestial objects, but is skeptical about proposals to create missile defenses against them.
“Knocking a comet down is a tough thing to do,” he said Friday. Hidden by a dazzling cloud of dust and gas, a comet nucleus would be hard to target accurately. And any explosion powerful enough to destroy it might unintentionally knock it more directly into Earth’s path. Earth’s best protection against the hazards of cosmic hail is its relatively diminutive size.
Shoemaker said that Jupiter, which contains about 2.5 times the mass of all the other planets in the solar system combined--presents a much larger target than Earth, and so it is hit more frequently.
Although objects the size of the comet that rocked the planet last week might be expected to hit Jupiter every 1,000 years, he said, they might hit Earth about once every 100 million years.
But the mathematics of risk assessment take no account of disasters that may already be in progress.
“Of course if there is one coming,” said Lucy McFadden, a University of Maryland astronomer, “the statistics are irrelevant.”
