A top the granite outcroppings of 6,882-foot Kitt Peak in southern Arizona, the dome of the Steward Observatory sits like a gleaming white mushroom. And atop the dome Tom Gehrels sits cross-legged, enjoying the sunset. A red-tailed hawk soars nearby. Soon Gehrels descends into the dome and programs the 32-inch telescope to sweep the heavens. For all the placidity of the scene, the astronomer is looking for the stuff of sci-fi doomsday dramas: a huge asteroid on a collision course with Earth. Says Gehrels, “That damned thing may be out there right now.”
The chances, admittedly, are remote—once in one million years for asteroids one kilometer (.62 miles) wide, once in 100 million years for asteroids 10 kilometers wide. But the odds are good enough that the National Aeronautics and Space Administration, the state of Arizona and private donors thought it prudent to spend $3 million to create the Spacewatch Program in 1981 and appoint Gehrels, 59, its “principal investigator.”
One major asteroid striking Earth would be one too many. “The energy caused by the impact of a 10-kilometer asteroid would be incredible,” he says. “It would be the equivalent of one billion Hiroshimas—without the radiation. All weather would stop. There would be no wind, no rain, no sun. It would cause something like a nuclear winter, destroying all human life.”
Gehrels can speak confidently about such an Armageddon because many scientists believe that it has happened at least once already. According to a widely accepted theory advanced in 1979 by a group of researchers, including Nobel physicist Luis Alvarez, an asteroid was responsible for the extinction of the dinosaurs. Their theory: About 65 million years ago a large asteroid slammed into Earth, hurling so much dust into the air that sunlight was blotted out for at least six months. In the ensuing cold and darkness, more than 60 percent of all life on the planet perished, including the then-dominant giant reptiles.
Asteroids, which are believed to be chunks of planetary material left over from the solar system’s birth some four and a half billion years ago, travel in a belt running as close to the sun as Mars and as far away as Jupiter. There are millions of asteroids, ranging in size from dust particles to monsters 1,000 kilometers wide. (Only those 10 kilometers or smaller are susceptible to gravitational forces.) “Jupiter plays havoc with their orbits and can swing them into the path of Earth,” Gehrels explains.
Should a large asteroid appear headed our way, Earthlings will need to react quickly. But quickly in astronomical terms means everyone will still have to pay taxes next year. “We would need to observe it for 5 to 10 years to know when it would hit,” Gehrels says. Though not currently feasible, it’s considered likely that a manned spacecraft would be sent to the asteroid to attach to it rockets that would alter its course by speeding it up. If a dangerous asteroid “takes a hundred years to collide with Earth,” Gehrels says, “no problem. If it’s 30 years, no problem. But if it’s 10 years, with our present lack of technology, it would be a frightening situation. Of course, if it’s five years from now, we may as well kiss each other goodbye.”
Born in the Netherlands, Gehrels fought with the Dutch Resistance as a teenager in World War II. In 1951 he traveled to the U.S. and hitchhiked around the country before earning his Ph.D. in astronomy at the University of Chicago in 1956. In 1961 (the same year he joined the faculty at the University of Arizona, where he still teaches) Gehrels became a U.S. citizen. He and his wife, Liedeke, who teaches high school in Tucson, have three children.
Considered a pioneer in the measurement of light intensity, vibration and color, Gehrels headed the NASA team that built for Pioneer 10 and 11 the instruments that sent back pictures of Jupiter in 1973 and 1974; it also earned him a NASA Exceptional Scientific Achievement Medal.
A human-rights activist, Gehrels has named an asteroid he discovered after Soviet dissident Andrei Sakharov, and has campaigned for the physicist’s release. Thinking about the Cold War sometimes causes Gehrels to view the possibility of a renegade asteroid in a hopeful light. “It’s a well-known fact that when you have trouble within a country, the best way to divert attention from that is to find some outside enemy,” he observes. “Here it would have the terrific effect of getting nations together, particularly the Russians and the United States. I have a hunch this would be a turning point in human history.”