Mountains or Molehills: Sizing up the Impact Hazard

Mercury, November/December 2002 Table of Contents

Courtesy of Don Davis

by Ivan Semeniuk

Scientists agree that Earth-crossing asteroids put human life at risk. But how far should we go to stop them?

For Mike Belton the impact hazard wasn't a personal problem until he saw the numbers. As a former Kitt Peak planetary astronomer, Belton has long been aware of the infamous connection between the extinction of the dinosaurs 65 million years ago and a large impact. But he also knows the odds are heavily stacked against such an event occurring again for millions of years.

Then Belton read a magazine article that changed his perspective. The article included a graph showing how the likelihood of an impact increases as the size of the impactor decreases. What caught his eye were not the big dinosaur-killers at one end of the graph, but the far smaller and more numerous objects at the opposite end. These lesser rocks assault our planet at the rate of one every few millennia. Just one can deliver enough energy to destroy a large city. Given their frequency, Belton realized there was a good chance - maybe 1 in 10 - of one arriving within the next few generations of his own family. "That kind of shook me up," he recalls.

Belton has since become an active member of the Near-Earth Object (NEO) community and advocates paying more attention to potential impactors of intermediate size. Objects in that range have diameters between about 100 meters and 1 kilometer. The upper limit is the approximate threshold for a worldwide catastrophe (the dinosaurs were done in by a 10-kilometer body). The lower limit is about twice the size of the object that exploded over the Tunguska region of Siberia in 1908, flattening 2,000 square kilometers of forest and incinerating entire herds of animals. The next time an impact rattles Earth, it's almost certain to come in near the bottom end of that range - possibly before the end of this century. "Small impactors happen at rates which are of interest in human terms," says Belton. "I find that a compelling reason to learn more about them."

This past September, Belton co-chaired a NASA-sponsored workshop in Washington where he made his case for learning more and learning more quickly about the near-Earth objects (NEOs) that threaten us. Among the attendees were representatives from NASA, the Pentagon, the National Science Foundation, the aerospace industry, and most of the leading scientists involved in the NEO community. What emerged was a remarkably wide-ranging discussion - one that reveals the impact hazard to be a much more complicated and subtle issue than was apparent a decade ago.

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