Scientists go back on earlier prediction and say falling satellite could hit the U.S. tonight or early Saturday.

This story was updated at 11:36 a.m. ET

A huge, dead satellite tumbling to Earth is falling slower than expected, and may now plummet down somewhere over the United States tonight or early Saturday, despite forecasts that it would miss North America entirely, NASA officials now say.

The 6 1/2-ton Upper Atmosphere Research Satellite (UARS) was expected to fall to Earth sometime this afternoon (Sept. 23), but changes in the school bus-size satellite's motion may push it to early Saturday, according to NASA's latest observations of the spacecraft.

"The satellite's orientation or configuration apparently has changed, and that is now slowing its descent," NASA officials wrote in a morning status update today. "There is a low probability any debris that survives re-entry will land in the United States, but the possibility cannot be discounted because of this changing rate of descent." [Complete coverage of NASA's falling satellite]

NASA expects about 26 large pieces of the UARS spacecraft to survive re-entry through Earth's atmosphere and reach the planet's surface. The biggest piece should weigh about 300 pounds. The spacecraft is the largest NASA satellite to fall from space uncontrolled since 1979. [6 Biggest Spacecraft to Fall Uncontrolled From Space]

NASA officials have said the the chances that a piece of UARS debris hits and injures one of the nearly 7 billion people on the planet are about 1 in 3,200. However, the personal odds of you being struck by UARS satellite debris are actually about 1 in several trillion, NASA officials have said.

As of 10:30 a.m. EDT (1430 GMT) today, the UARS satellite was flying in an orbit of about 100 miles by 105 miles (160 kilometers by 170 km), and dropping.  NASA launched the UARS satellite in 1991 to study Earth's ozone layer and upper atmosphere. The satellite was decommissioned in 2005.

"Re-entry is expected late Friday, Sept. 23, or early Saturday, Sept. 24, Eastern Daylight Time," NASA officials wrote. "Solar activity is no longer the major factor in the satellite's rate of descent."

The sun has had an extremely active week, one that has included several solar flares. High solar activity can cause the Earth's atmosphere to heat and expand, which can increase drag on a low-flying satellite like UARS, making it fall faster.

Late Wednesday (Sept. 21), NASA predicted that the UARS satellite would not be over North America when it finally plunged down to the Earth's surface. That scenario has changed now that the 20-year-old satellite's descent has slowed, the agency said.

But where the UARS spacecraft will fall still remains anyone's guess. NASA orbital debris experts have said the satellite could fall anywhere between the latitudes of Northern Canada and Southern South America, a region of Earth that encompasses much of the planet.

"It is still too early to predict the time and location of re-entry with any certainty, but predictions will become more refined in the next 12 to 18 hours," NASA officials wrote in the latest update.