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Falling NASA satellite could hit Earth by Friday, landing location unknown

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Back when UARS, the Upper Atmosphere Research Satellite, was launched to study the ozone layer in 1991, NASA didn't always pay attention to the "what goes up must come down" rule. Nowadays, satellites must be designed either to burn up on re-entering the atmosphere or to have enough fuel to be steered into a watery grave or up into a higher, long-term orbit.

NASA is unsure exactly where the falling satellite will land. (Photo: Associated Press)

The International Space Station — the largest manmade structure ever to orbit the planet — is no exception. NASA has a plan to bring it down safely sometime after 2020.

Russia's old Mir station came down over the Pacific, in a controlled re-entry, in 2001. But one of its predecessors, Salyut 7, fell uncontrolled through the atmosphere in 1991. The most recent uncontrolled return of a large NASA satellite was in 2002.

The most sensational case of all was Skylab, the early U.S. space station whose impending demise three decades ago alarmed people around the world and touched off a guessing game as to where it might land. It plummeted harmlessly into the Indian Ocean and onto remote parts of Australia in July 1979.

The $740 million UARS was decommissioned in 2005, after NASA lowered its orbit with the little remaining fuel on board. NASA didn't want to keep it up longer than necessary, for fear of a collision or an exploding fuel tank, either of which would have left a lot of space litter.

Predicting where the satellite will strike is a little like predicting the weather several days out, says NASA orbital debris scientist Mark Matney.

Experts expect to have a good idea by Thursday of when and where UARS might fall, Matney says. They won't be able to pinpoint the exact time, but they should be able to narrow it to a few hours.

Given the spacecraft's orbital speed of 17,500 mph (28,160 kph), or 5 miles (8 kilometers) per second, a prediction that is off by just a few minutes could mean a 1,000-mile (1,610-kilometer) error. It probably won't be clear where it fell until afterward, Matney says.

If it happens in darkness, it should be visible.

"If someone is lucky enough to be near the re-entry at nighttime, they'll get quite a show," says Matney, who works at Johnson Space Center in Houston, also in the potential strike zone.

Space junk in general is on the rise, much of it destroyed or broken satellites and chunks of used rockets. More than 20,000 manmade objects at least 4 inches (10.16 centimeters) in diameter are being tracked in orbit.

It's mostly a threat to astronauts in space, rather than people on Earth. In June, the six residents of the International Space Station took shelter in their docked Soyuz lifeboats because of passing debris. The unidentified object came within 1,100 feet (335 meters) of the complex, the closest call yet.

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