The recent Leonid meteor shower gave skygazers a chance to observe pieces of space debris meeting their demise as they collided with the earth's atmosphere at high speeds. Most of the streaks on display during the Leonid shower are just specks of cometary dust and hence pose little risk to observers on the ground. But explosions of much larger asteroids high in the atmosphere can cause considerable damage to our planet. A study published in the current issue of the journal Nature calculates just how likely such collisions are. The good news? The findings suggest that 10-megaton explosions from objects about 50 meters wide will occur about one third as often as previously believed.

Peter Brown of the University of Western Ontario and his colleagues analyzed previously unreleased data collected continuously over the last eight and a half years by U.S. government satellites. While scanning the globe for evidence of nuclear explosions the satellites also detected nearly 300 optical flashes caused by planetary debris ranging from one to 10 meters across crashing into the earth's atmosphere. On average, the scientists report, our atmosphere is rocked by one explosion per year that is equivalent to five kilotons of TNT. The team's calculations show that larger bursts like the one that occurred in 1908 over Tunguska, Siberia, and flattened trees over hundreds of square kilometers, will occur once every 1,000 years. Although scientists are still a long way from cataloging all the interstellar objects tens of meters across, Robert Jedicke of the University of Arizona notes in an accompanying editorial that "we can all worry a little less about the risk of the next hazardous impact."