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."