Each year Earth is pelted by space debris, from tiny grains to car-sized boulders, their impact equivalent to a kiloton of TNT. Every 1,000 years or so, a bigger rock smacks our planet with the force of a hydrogen bomb.
So why aren't we quaking in our boots? David Ropeik, director of risk communication at the Harvard Center for Risk Analysis, believes it's because asteroids are natural; we're far more terrified of man-made risks such as terrorism or bioengineered foods. Also, we've never watched an asteroid impact on TV, so we don't really believe it could happen.
Ropeik is intrigued by why people are disproportionately afraid of some things but can ignore others. The answer may lie in how our brains are wired, allowing us to respond to danger before we've even had time to think about it. At the 2003 meeting of the American Association for the Advancement of Science, he noted that if our ancestors hadn't been designed this way, they wouldn't have survived.
Experience and culture also teach us what to fear. We're born with some basic phobias, but we must learn that DDT and terrorists are dangerous. Ropeik says we're more afraid of catastrophic events such as airplane crashes than of everyday risks like cancer. It's partly a matter of media coverage that makes the danger appear greater than it is; and partly because the more grisly the prospect, the more it frightens us. The result is a certain degree of illogical behavior.
From a statistical perspective, says Clark Chapman, Ph.D., an asteroid researcher at the Southwest Research Institute, in San Antonio, Texas, it was "very strange" that the deaths of a few people from anthrax dominated the news in late 2001, while the risks of influenza, which killed 30,000, "were just buried." That distortion, adds Geoff Sommer, a graduate student at the think tank Rand Corporation, is the essence of terrorism. It works as a "weapon of mass distraction," siphoning attention from other arenas.
As for those not-so-scary asteroids: one did hit Siberia in 1908, leveling hundreds of square miles of forest. The National Aeronautics and Space Administration is now mapping the orbits of 2,000 large rocks likely to hit our planet in the future.