The odds of a devastating asteroid or comet collision with the Earth may be significantly less than previously thought, according to a new study.

In a paper published in Thursday's issue of the journal Nature, a group of Canadian and American scientists used data from military spacecraft to estimate the number and size of objects that explode that collide with the Earth. They estimate that, on average, one asteroid collides with the Earth's atmosphere each year with the equivalent energy of five kilotons of TNT.

"We use Earth's atmosphere as a detector of small asteroids or comets by watching for the bright flashes produced as they impact the upper layers of the atmosphere," said Peter Brown of the University of Western Ontario, the scientist who led the Nature study. "This is an ideal way to see smaller objects (one to 10 meters) too small to be detected while still in space by ground-based telescopic surveys, but too large to be detected after they become bright fireballs by camera networks that watch the skies."

The study was based on data from military spacecraft in Earth orbit designed primarily to look for infrared flashes caused by manmade explosions or missile launches. The satellites, however, can also detect the flashes caused by the explosions of asteroids in the Earth's upper atmosphere. Over the past eight years the spacecraft recorded over 300 flashes associated with asteroid impacts.

Scientists used those statistics to extrapolate the odds of impacts by larger objects. An impact the size of the one seen by satellite in June over the Mediterranean Sea -- estimated to be approximately 25 kilotons -- takes place on average every four years. A one-megaton impact, by contrast, will occur only about once a century.

The most powerful asteroid collision in recent memory took place in 1908 in the atmosphere above Siberia. The Tunguska explosion, caused by an object 30 to 50 meters across and estimated to be the equivalent of 10 megatons of TNT, had previously been thought to take place on average once every 100 years. This new analysis, however, estimates that such impacts are far less frequent: once every 400 to 1000 years.

The results match other recent work that suggests that Tunguska-class impacts may be less frequent than previously thought. These results may have an effect on efforts to raise awareness of the threats posed by near-Earth asteroids, from new search programs to studies of ways to deflect or otherwise deal with objects that pose an impact threat.

Brown cautioned that the results of his study may represent only a lower limit on the risk of asteroid impacts. "It is important to realize the impact estimates we have measured are averages from the last eight and a half years," he said. "Based on past observations, it seems likely there is also a non-random component to the impact flux at these smaller sizes which would suggest our estimates are lower bounds to the true impact risk."