UARS is scheduled to de-orbit Friday, plus or minus a day, according to NASA scientists' calculations.
NASA scientist Mark Matney says UARS is most likely to de-orbit Friday morning. "Right now, UARS is actually re-entering a little faster than anticipated because solar activity has gone up, which speeds re-entry," he said.
This is the largest NASA satellite to come back to Earth uncontrolled since Skylab in 1979. UARS weighs about 6 tons and is the size of a school bus, but most of it will burn up on re-entry. It's what will survive re-entry that worries NASA.
Nicholas Johnson, NASA's chief orbital debris scientist, says their analysis shows that 26 parts of UARS will survive. "These 26 components, which we anticipate will survive all the way down, will be going at a moderate velocity of tens to hundreds of miles an hour," he said. "All these 26 have been identified as potentially causing damage if they hit a structure or a person but the odds of that are very, very, low."
Johnson said any one person's chances of getting hit by debris are tiny -- something like 1 in 21 trillion. The chances that of the 7 billion people on Earth, one of them, somewhere, could be hit -- those odds are more like 1 in 3,200.
About 150 tons of meteorites, in pieces large and small, land on the planet every day. Some are visible, such as the meteor spotted over California last week, but most of the debris is so small it's never found.
What makes UARS so remarkable is its size and the advance warning from NASA officials that it's coming. Because of the varying density of the upper atmosphere, they won't be able to tell where it is going to land until two hours before it plunges back to Earth.
Johnson says the potential target area is wide. "It's a pretty wide area, 57 degrees north to 57 degrees south," he said.
That "pretty much encompasses the entire populated world. I mean, there is a small percent that live above and below those latitudes, but the vast majority of the 7 billion people on the planet live within those latitudes," he said.
That means pretty much anyone south of the Arctic and north of Antarctica. Johnson says to blame the sun for mystery. "The sun is constantly changing the output of energy, which affects the atmosphere, and that can accelerate of delay the re-entry of the satellite," he said.
UARS (Upper Atmosphere Research Satellite) was launched by the space shuttle Discovery in 1991. UARS was NASA's first multi-instrumented satellite to study the chemistry of Earth's atmosphere and measure the amount of light that comes from the sun at ultraviolet and visible wavelengths. UARS stopped being productive in 2005.
When UARS launched, it orbited the planet at an altitude of 350 miles. It is now down to 140 miles, and will continue to lose altitude until there is enough air around it to slow it from orbital speeds.
U.S. Space Command at Vandenberg AFB will be tracking UARS as it returns and NASA will post regular updates.