Scientists are unable to pinpoint the time and place where the Upper Atmosphere Research Satellite, or UARS, will return to Earth due to the satellite's unpredictable tumbles and changes in the thickness of the atmosphere, fueled in part by a powerful solar flare on Thursday.
The solar flare released a blitz of highly charged particles called a coronal mass ejection in the direction of Earth. When the particles slam into the atmosphere, they cause it to heat up and expand, which in turn impacts the density of the air UARS is encountering as it tumbles uncontrollably in orbit.
Initially, scientists believed North America was out of the zone where up to 26 pieces of UARS debris, weighing a total of about 1,100 pounds (500 kg), would land or splash down.
There is now a low probability that debris will land in the United States, NASA wrote on its website on Friday.
"The satellite's orientation apparently has changed and that is now slowing its descent," NASA said.
"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."
NASA said the orbit of UARS was 100 miles by 105 miles (160 km by 170 km) at 10:30 a.m. EDT (1430 GMT) and re-entry is expected late on Friday or early Saturday Eastern Daylight Time.
"Solar activity is no longer the major factor in the satellite's rate of descent," NASA said.
With most of the planet covered in water and vast uninhabited deserts and other land directly beneath the satellite's flight path, the chance that someone will be hit by falling debris is 1-in-3,200, NASA said.
The satellite flies over most of the planet, traveling between 57 degrees north and 57 degrees south of the equator.
UARS was dispatched by a space shuttle crew in 1991 to study ozone and other chemicals in Earth's atmosphere. It completed its mission in 2005 and has been slowly losing altitude, pulled by the planet's gravity, ever since.
The satellite is one of about 20,000 pieces of space debris loitering in orbit around Earth. Something the size of the 13,000-pound (5,897 kg) UARS falls back into the atmosphere about once a year.
NASA held a news conference earlier this month about UARS' re-entry and has been posting daily updates on its website.