Because two-thirds of the Earth is ocean, space debris usually hits water.
The 6-ton Upper Atmosphere Research Satellite ran out of fuel in 2005 and will fall uncontrolled out of orbit. Only about 1,200 pounds of metal should survive, NASA said.
It could land anywhere from south of Juneau, Alaska, to the northern tip of South America.
NASA scientists estimate a 1-in-3,200 chance a satellite part could hit someone. Most of it will burn up after entering Earth's atmosphere.
The agency says there wasn't enough fuel left in the 20-year-old satellite to move it to a higher "graveyard orbit" or steer it down safely.
Space debris bigger than 5 tons doesn't often fall to Earth. But this will be the third time this year for something that big to reach Earth, according to Jonathan McDowell, a Harvard University astrophysicist who tracks objects in orbit.
NASA orbital debris chief Gene Stansbery said there probably is no hazardous material left in the falling pieces, but people should not touch any fallen satellite parts just in case.
NASA will be tracking the satellite on a weekly and later daily basis until it falls.
The Associated Press contributed to this report.