Nasa is trying to determine where its six-tonne UARS satellite fell back to Earth early on Saturday.
The spacecraft was expected to re-enter the atmosphere by about 0500 GMT - officials say its debris might have landed in Canada but are not sure.
The Upper Atmosphere Research Satellite (UARS) is the largest American space agency satellite to return uncontrolled into the atmosphere in about 30 years.
Officials said the risk to public safety was remote.
Unconfirmed reports on Twitter suggested some debris may have fallen over Okotoks, a town south of Calgary in western Canada.
"If debris fell on land (and that's still a BIG if), Canada is most likely area," a Nasa official had tweeted earlier. A later statement on the UARS website read: "The satellite was passing eastward over Canada and Africa as well as vast portions of the Pacific, Atlantic and Indian oceans during that period. The precise re-entry time and location are not yet known with certainty."
Most of the decommissioned spacecraft should simply have burnt up, but modelling work suggested perhaps 500kg could have survived to the surface.
Pieces of debris were expected to be scattered over a 800km path. With more than 70% of the Earth's surface covered by water, many experts said the pieces were most likely to end up in the ocean.
UARS was deployed in 1991 from the space shuttle Discovery on a mission to study the Earth's upper atmosphere.
It contributed important new understanding on subjects such as the chemistry of the protective ozone layer and the cooling effect volcanoes can exert on the global climate.
9/24/11
Nasa's UARS satellite plunges to Earth - but where?Nasa is trying to determine where its six-tonne UARS satellite fell back to Earth early on Saturday
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