No injuries or damage have been reported on land, which NASA officials said was a good indication the satellite went into the ocean.
That doesn't necessarily mean it all fell into the sea. Some debris could have fallen over areas such as Portland, Ore.; Seattle; Calgary, Alberta; and Saskatoon, Saskatchewan, said Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics.
"Pieces are falling off of this flaming fire ball, and some of it has enough momentum to go hundreds of miles," he said.
Speculation was rampant on sites such as Twitter. There were no credible reports of debris on the ground, said Nick Johnson, NASA's chief scientist for orbital debris. But if the satellite fell even five minutes later than estimated, some of it could have hit land, he said.
"We don't know where the re-entry point exactly was. We don't exactly know where the debris field is," Johnson said.
NASA's earlier calculations had predicted that the former climate research satellite would fall over a 500-mile swath and could include land. Officials said the 35-foot satellite fell sometime between 11:23 p.m. EDT Friday and 1:09 a.m. EDT Saturday.
Much of the speculation focused on unconfirmed reports and even video of debris from the Upper Atmosphere Research Satellite over Alberta, Canada.
NASA spokesman Steve Cole said that was possible because the last track for the satellite included Canada, starting north of Seattle and then in a large arc north then south. From there, the track continued through the Atlantic south toward Africa, but it was unlikely the satellite got that far if it started falling over the Pacific.
Some 26 pieces of the satellite representing 1,200 pounds of heavy metal had been expected to rain down somewhere. The biggest surviving chunk should be no more than 300 pounds.
NASA urges anyone who thinks they've found satellite debris to call police. It's government property and illegal to keep it or try to sell it. The debris has no toxic contamination, but there could be sharp edges, NASA officials have said.
UARS is the biggest NASA spacecraft to crash back to Earth, uncontrolled, since the post-Apollo 75-ton Skylab space station and the more than 10-ton Pegasus 2 satellite, both in 1979.
Russia's 135-ton Mir space station slammed through the atmosphere in 2001, but it was a controlled dive into the Pacific.
Before UARS fell, no one had ever been hit by falling space junk and NASA expected that not to change.
NASA put the chances that somebody somewhere on Earth would get hurt at 1-in-3,200. But any one person's odds of being struck were estimated at 1-in-22 trillion, given there are 7 billion people on the planet.
The satellite ran out of fuel and died in 2005. UARS was built and launched before NASA and other nations started new programs that prevent this type of uncontrolled crashes of satellite.
Kennedy reported from Miami.
Online: NASA: http://www.nasa.gov/mission-pages/uars/index.html