Environmental hazard: space junk

''On the other hand, we are not claiming the sky is falling," he said, ''We just need to understand what the risks are."

The most debris-crowded area is between 550 miles and 625 miles above Earth, Liou said, meaning the risk is less for manned spaceflight.

But the junk can pose a risk to commercial and research flights and other space activities.

Much of the debris results from explosions of satellites, especially old upper stages left in orbit with leftover fuel and high pressure fluids.

A NASA report issued in 2004 identified Russia as the source of the largest number of debris items, closely followed by the United States. Other sources were France, China, India, Japan, and the European Space Agency.

Even without any launches adding to the junk, the creation of new debris from collisions of material already there will exceed the amount of material removed as orbits decay and items fall back to Earth, the researchers estimated.

Only removal of existing large objects from orbit ''can prevent future problems for research in and commercialization of space," they wrote. ''As of now there is no viable solution, technically and economically, to remove objects from space," Liou said. He said he hopes the report will encourage researchers to think about better ways to do this.