'We May Never Know' Whereabouts of Satellite Debris. NASA:

NASA scientists are still not sure exactly where pieces of a huge, defunct satellite landed after re-entering Earth's atmosphere this morning, but early evidence suggests that the debris landed somewhere in the Pacific Ocean, which could complicate recovery efforts.

During a Saturday afternoon conference call with reporters, Nicholas Johnson, chief orbital debris scientist for NASA, acknowledged that "we may never know" the whereabouts of the Upper Atmosphere Research Satellite (UARS).

Data from the Department of Defense's Joint Space

Operations Center (JSOC) indicate that UARS fell to the Earth sometime between 11:23pm Eastern Friday night and 1:09am Saturday morning. At that point, the satellite passed over Canada, the African continent, and the Pacific, Atlantic, and Indian Oceans. The mid-point of that path, and likely point of entry, according to those calculations, is off the West Coast of the U.S., as indicated by the green circle on a map published by NASA (click below for larger image).

"There were several folks along the western coast of North America, the U.S. Northwest, and Canadian Southwest who were looking to observe UARS as it came over," Johnson said. "Every one of those attempts came up negative, [so] that would suggest that re-entry did happen before it reached the North American coast [and] that most of this debris fell into the Pacific Ocean."

This data is consistent with predictions made by the Inter-Agency Space Debris Coordination Committee (IADC), a group made up of 12 space agencies from around the world. Every year, they conduct a re-entry exercise, and UARS was this year's target.

"We're now waiting to see if we can verify that," Johnson said. "If it came down over one of the oceans, of course, that's going to be problematic."

How did NASA know that UARS was back? To find out if a satellite is no longer in orbit, you send out sensors. If you don't get a response back, that's a good indication that it has re-entered the Earth's atmosphere, Johnson said. Typically, scientists will try sending out these sensors at least three times just to make sure the data is accurate. That can take between two and three hours, which is why NASA did not immediately know if UARS had successful re-entered the atmosphere.

The debris recovery process, meanwhile, will rely mainly on eyewitness accounts.

"Most of the time, we do rely heavily on people on the ground," Johnson said. "UARS, whether it came in during local day or night, would've been clearly visible, so if we continue to have a lack of reports, particularly in Canada or Africa … that would typically give further credence to the fact that it's probably over the water."

Crew members on naval vessels might have seen something, but it takes longer for those reports to trickle in, so NASA will collect reports over the next few days and make an assessment. "If we have sufficient evidence … we will certainly make that available," Johnson said.

The UARS satellite was launched in 1991 by the Space Shuttle Discovery. At launch, it was 35-feet long, 15 feet in diameter, and weighed 13,000 pounds. It was used to measure ozone and chemical compounds found in the ozone layer, which affect ozone chemistry and processes, as well as winds and temperatures in the stratosphere and the energy input from the Sun. "Together, these help define the role of the upper atmosphere in climate and climate variability," according to NASA.

UARS was officially decommissioned on December 14, 2005 and has since been hanging out in space. While pieces of satellites typically burn up upon re-entry into the Earth's atmosphere, other pieces survive the process and crash land on Earth. Early on, NASA said the chances of UARS pieces hitting someone here on Earth were remote, but really, officials had no idea where UARS would hit, so all eyes have been on the sky.

A recent NASA-sponsored report from the National Research Council found that the space agency needs a better plan for dealing with this space junk, or orbital debris, before it gets out of hand and damages valuable spacecraft.

According to NASA, most orbital debris is within 1,250 miles of the Earth's surface; the greatest concentrations of debris is found between 500 and 530 miles. Most of this debris circles the Earth at speeds between 4-5 miles per second.