Satellite likely fell in Pacific; 'we may never know'

— -- Where in the world is NASA's crashed satellite?

On the ocean floor most likely, what's left of it, but nobody seems to know for sure. The best bet is that the fiery breakup of NASA's Upper Atmosphere Research Satellite (UARS), designed to study atmospheric chemistry, scattered debris across a 500-mile swath of the Pacific Ocean.

Coming decades after the nuclear era led to U.S. missile-warning radars being placed around the globe, the question remains: Why it wasn't possible to pinpoint the exact areas where pieces of the NASA satellite fell to Earth?

"This is a completely different situation than an incoming ballistic missile," says orbital debris expert Nicholas Johnson of NASA's Johnson Space Center in Houston. "We may never know" exactly where the 6.2-ton spacecraft broke apart, he says.

The Defense Department's Joint Space Operations Center at Vandenberg Air Force Base in California requires reports from three radar-tracking stations to officially declare a satellite downed. Since the stations are located so far apart, the satellite could cover about 30,000 miles before the last station could confirm it was no longer flying, Johnson says.

That is why NASA put the window for re-entry from 11:23 p.m. ET Friday to 1:09 a.m. ET Saturday. Best estimates place the re-entry at 12:16 a.m. ET, plus or minus 20 minutes, on Saturday at a point roughly 1,000 miles north of Honolulu.

"That suggests most of the debris fell into the Pacific," Johnson says.

As of Monday, the space agency had no information on landing sites of any of the spacecraft's 26 parts, such as its fuel tank or stainless steel gyroscope pieces, that were expected to survive the heat of re-entry. Observers looking for the meteor trail of the satellite saw no signs of the re-entry in Canada.

Other factors complicating a more precise re-entry time, Johnson says, are the uncertainty about the density of the upper atmosphere as the satellite fell, its exact tumbling pattern and questions about its exact location in the sky.

"Certainly there are differences between an (inter-continental ballistic missile) and a satellite," says tracking radar expert George Lewis of Cornell University in New York. "Most of our radars are looking the other way," toward the North Pole trajectory that nuclear missiles would follow from Russia or China, he says. "We have only two radars that might reasonably track something coming from the direction of Australia."

Ballistic missiles are designed to fly smoothly arcing trajectories, "their point being accuracy," Lewis says. A satellite falling to Earth, though, tumbles through the atmosphere like a stone skipping over water, which is harder to track, he says. "The primary concern in all of this is not to mistake a satellite for a missile. Beyond that, the re-entry of a satellite is not a high priority," he says.

Anyone who enjoyed the drama of the UARS re-entry saga can look forward to the middle of November, plus or minus five weeks, when the German space agency says its defunct ROSAT X-ray telescope will fall to Earth. It carries four heat-resistant mirrors, the largest 32 inches across and is likely to survive re-entry intact, according to the German Aerospace Center.

"We followed your satellite coming down in Germany," says German Aerospace Center spokesman Andreas Schütz. "Now you can follow our satellite."