NASA is waiting for confirmation that satellite pieces are down
About 26 pieces, some weighing hundreds of pounds, are expected to survive reentry
It is not clear exactly where the pieces might have landed
Miami (CNN) -- Pieces of a defunct satellite plummeting toward Earth may have come to rest, NASA said Saturday morning
NASA says "it's possible" that the Upper Atmosphere Research Satellite "is down by now," according to the agency's Twitter page early Saturday. But the agency said it is seeking official confirmation with the United States Strategic Command.
About two dozen pieces of the satellite were expected to survive the crash through the Earth's atmosphere.
Late Friday night, NASA predicted satellite parts would pass "over Canada and Africa, as well as vast areas of the Pacific, Atlantic and Indian oceans."
U.S. in falling satellite's strike zone
FAA: Pilots watch for falling satellite
"The risk to public safety is very remote," the space agency added.
It was not immediately clear where fallen pieces may have ended up.
About 26 pieces were expected to survive the descent. Those pieces, made of stainless steel, titanium and beryllium that won't burn, will range from about 10 pounds to hundreds of pounds, according to NASA.
Earlier, NASA said "there is a low probability" surviving debris will land in the United States, but on Saturday morning the space agency tweeted, "The U.S. is very safe from (the satellite) ... It's final orbit did not cross the United States."
Mark Matney of NASA's Orbital Debris team in Houston said there's no way to know exactly where the pieces will come down.
"Keep in mind, they won't be traveling at those high orbital velocities. As they hit the air, they tend to slow down. ... They're still traveling fast, a few tens to hundreds of miles per hour, but no longer those tremendous orbital velocities," he explained.
"Part of the problem is, the spacecraft is tumbling in unpredictable ways, and it is very difficult to very precisely pinpoint where it's coming down even right before the re-entry," Matney said.
Because water covers 70% of the Earth's surface, NASA has said that most -- if not all -- of the surviving debris will land in water. Even if pieces strike dry land, there's very little risk any of it will hit people.
However, in an abundance of caution, the Federal Aviation Administration released an advisory Thursday warning pilots about the falling satellite, calling it a potential hazard.
"It is critical that all pilots/flight crew members report any observed falling space debris to the appropriate (air traffic control) facility and include position, altitude, time and direction of debris observed," the FAA statement said.
The FAA said warnings of this sort typically are sent out to pilots concerning specific hazards they may encounter during flights such as air shows, rocket launches, kites and inoperable radio navigational aids.
NASA said space debris the size of the satellite's components re-enters the atmosphere about once a year. Harvard University astrophysicist Jonathan McDowell noted that the satellite is far from being the biggest space junk to come back.
"This is nothing like the old Skylab scare of the '70s, when you had a 70-ton space station crashing out of the sky. So, I agree with the folks in Houston. It's nothing to be worried about," McDowell said.
Pieces of Skylab came down in western Australia in 1979.
The only wild card McDowell sees is if somehow a chunk hits a populated area.
"If the thing happens to come down in a city, that would be bad. The chances of it causing extensive damage or injuring someone are much higher."