LOS ANGELES (CBS) — One of NASA’s six-ton satellites is expected to fall to earth later this week.

NASA’s Upper Atmosphere Research Satellite, or UARS, was launched from a space shuttle in 1991.

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The satellite is expected to re-enter earth’s atmosphere sometime Friday afternoon, but scientists are having trouble determining where it’s headed.

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“At this point, it’s really too hard to tell,” Michael Duncan of the Vandenburg Air Force Base told CBS2. “When we create these orbits, they’re only accurate to a certain period of time, and when you’re talking about being off by 15 minutes, it can relate to a 5,000- to 7,000 -mile-footprint difference.”

UARS will likely come down into an ocean. NASA says the chance of it hitting a person is 1 in 3,200, far better than winning a lottery, but much more painful.

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While the satellite will fall apart upon re-entering earth’s atmosphere, some pieces are expected to be several hundred pounds.