NASA on Friday warned there was still a small chance that remnants of a falling research satellite could end up in parts of the U.S., though scientists weren't able to predict the precise trajectory or exactly when the space junk would reach the ground.
The difficulty of pinpointing where the parts will land highlights broader international concerns about tracking more than 20,000 pieces of orbiting space debris, some no larger than a football. Experts say the debris poses a potential threat to commercial and military satellites, as well as to the international space station.

The anticipated breakup of the defunct, 13,000-pound NASA climate satellite, set to tumble uncontrolled through the atmosphere after 20 years of operation, could result in dozens of pieces hitting the Earth by early Saturday. Most pieces are expected to burn up as they streak through the atmosphere, though experts at the National Aeronautics and Space Administration have indicated that some could survive re-entry and possibly weigh hundreds of pounds.
After days of being unable to say precisely where the shower of parts is likely to land, NASA's website said Friday there was "a low probability" of pieces ending up in North America. Partly because solar flares have caused disruptions in the atmosphere, the agency said it couldn't predict the satellite's exact path or rule out the possibility of parts hitting the U.S. Such a scenario, according to NASA, "cannot be discounted."

The risks posed by orbiting debris, a topic that increasingly preoccupies some military and aerospace experts, are significantly greater. The debris is essentially the unavoidable residue of hundreds of rocket launches, exploration missions and other man-made objects left circling the Earth.

With an estimated 750 or more satellites currently in orbit—and many more nations now seeking to launch satellites than ever before—overall collision hazards are expected to increase. Experts worry the threats are particularly significant around some widely used orbital locations. Astronauts aboard the international space station periodically are forced to take emergency steps to deal with threats.

Two years ago, a drifting and powerless Russian satellite smashed into and destroyed a commercial satellite operated by Iridium Communications Inc., a provider of phone and data services based in McLean, Va.

The collision occurred because Pentagon radar sites on the ground and U.S. government assets in space weren't closely tracking the merging courses of the two satellites. At the time, top Air Force officials said the U.S. had the capability of closely tracking and issuing pre-collision warnings for only a couple of hundred pieces of orbiting debris.

Since then, both military and commercial satellite-operators have put greater emphasis and resources into tracking space debris.

Surveillance efforts have been stepped up; military and corporate experts have shared information about the condition and orbits of satellites; some manufacturers have recommended installing additional sensors on satellites to warn of potential threats; and there has been enhanced international cooperation to try to avoid in-orbit surprises

The Cosmos News Astronomy&Space Videos