Satellites tally small asteroid hits. (Bursting in Air).

Once a millennium, on average, a small asteroid slams into Earth's atmosphere and explodes with the energy of 1,000 Hiroshima-size blasts. That's less than one-third as often as scientists previously supposed.

The new estimate stems from observations of fireballs from extraterrestrial objects of a certain size that burned up in Earth's atmosphere between February 1994 and September 2002. During that period, U.S.-satellite sensors detected about 300 such fireballs, says Douglas O. ReVelle, an atmospheric scientist at Los Alamos (N.M.) National Laboratory.

The duration and brightness of each fireball's flash indicates the energy of the accompanying explosion, says Peter G. Brown, an astronomer at the University of Western Ontario in London, Ontario. The stony objects generating the fireballs probably ranged from 1 to 10 meters in diameter, says Brown. That's much larger than the dust grains that vaporize in the atmosphere to form most shooting stars, or meteors, but not large enough to crater Earth's crust.

In the Nov. 21 Nature, ReVelle, Brown, and their colleagues report the first comprehensive analysis of impacts of intermediate-size objects. Much of the study's data was gathered by Department of Defense and Department of Energy satellites, which are designed to look for nuclear explosions and other military activities. The most satisfying aspect of the new analysis is that it bridges the previous data gap between meteor-forming pebbles and objects large enough to be observed in space by telescopes, says Robert Jedicke, a planetary astronomer at the University of Arizona in Tucson.

From their analysis, the scientists say, chances are that once each year, a 4-meter-wide asteroid will burst in Earth's atmosphere and produce an explosion of about 5 kilotons of TNT--about one-third the size of the Hiroshima blast. Once a decade, on average, the impact of an object 9 m across produces the energy of a 50-kiloton bomb. Actual impact rates may be higher, say the researchers, because the current study spans only 8 years and may not be representative of longer periods.

The new analysis suggests that only once in a millennium is there an atmospheric blast that rivals the one that took place high over Siberia on June 30, 1908. That explosion, estimated to have equaled the energy released by detonating 10 million tons of TNT, leveled about 2,000 square kilometers of remote Russian forest. If it had occurred over a densely populated metropolitan area, the same blast could have killed millions.