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Junk created by the collision of two satellites last month is poised to fall into Earth's atmosphere this week.
When the U.S. Iridium 33 communications satellite and the defunct Russian military communications satellite Cosmos 2251 collided and were destroyed, the smashup left a fresh sea of fragments orbiting at various angles. The collision occurred about 490 miles (790 km) over Siberia.
Engineers have been watching the debris, which has raised the risk of other collisions in space.
Now the U.S. Strategic Command says a fragment catalogued as 1993-036PX will re-enter the atmosphere Thursday, according to Spaceweather.com. Other bits are expected to take the plunge March 28 and March 30.
The fragments are small, however, perhaps just a centimeter or so. They should burn up on the way in and pose no threat to anyone on the ground.
"As of March 7, there were 355 catalogued fragments of Cosmos 2251 and 159 fragments of Iridium 33," said Daniel Deak, who prepared a map of the debris for Spaceweather.com. "The Cosmos fragments are not only more numerous, but also more widely scattered, ranging in altitude from 198 km to 1,689 km [123 to 1,049 miles]. For comparison, Iridium fragments are confined to altitudes between 582 km and 1,262 km."
Researchers don't know exactly why the debris spread out so differently. They suspect Cosmos 2251 might have ruptured (it was pressurized) and blown apart, the web site reports.
The risk of the Space Shuttle Discovery being struck by debris rose 6 percent due to the new debris, NASA has said.
Discovery is due to launch Wednesday night. It will fly up to 220 miles (354 km) above Earth, where it will dock with the International Space Station.
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