NASA officials know exactly what to expect when a defunct science satellite falls to Earth on Friday, except for the most important information — precisely where and when its scattered pieces will land.
Predicting the exact trajectory for a hunk of space junk that will crash into the planet is an inexact science, even for the experts at America's space agency, said Geza Gyuk, director of astronomy at Adler Planetarium.
"There are a lot of different things that go into this," Gyuk said. "It's a fairly complicated thing because it's not a controlled entry (into the Earth's atmosphere)."
NASA's 6.5-ton Upper Atmosphere Research Satellite collected measurements of ozone and other chemicals from 1991 to 2005, when it was decommissioned and positioned in a lower orbit where gravity would pull it back to Earth more quickly, according to NASA.
The satellite's orbit passes over most of the planet, from northern Canada to the southern part of South America. NASA said Wednesday that it expects UARS to re-enter the atmosphere sometime Friday afternoonbut that the satellite will not be passing over North America at that time.
NASA has been forced to rely on estimates because of the wide range of variables that affect the satellite as it nears its plunge to Earth, Gyuk said.
The satellite encounters increasing friction as it comes closer to Earth, which can cause it to begin tumbling, Gyuk said. The drag on the satellite changes as it tumbles, making it difficult to predict how the satellite will respond, he said.
The satellite will eventually begin to break apart, and many chunks will burn up as they fall to Earth. But NASA expects that up to 26 pieces of the satellite will survive re-entry and land in a 500-mile stretch somewhere on Earth.
Despite the uncertainty, NASA officials said Earth's residents shouldn't fear being squished by space junk.
NASA said there is a 1-in-3,200 chance that one of those objects will strike a person somewhere on the planet. The odds that it will strike you in particular are far less — about 1-in-22 trillion.
An average of one piece of space debris tracked by space agencies has fallen to Earth each day over the last 50 years, but there are no confirmed reports of injuries or property damage caused by falling space junk, according to NASA.
The largest NASA spacecraft to experience uncontrolled re-entry was Skylab, the 83-ton abandoned space station that fell from orbit in 1979 and scattered debris into the Indian Ocean and in a remote section of western Australia.
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