The 6-ton, bus-sized satellite is expected to break into more than a hundred pieces as it plunges through the atmosphere, most of it burning up.
But if you're hoping for a glimpse, the odds are slim. Most sightings occur by chance because the re-entry path can't be predicted early enough to alert people, said Canadian Ted Molczan, who tracks satellites for a hobby.
In all his years of monitoring, Molczan has witnessed only one tumble back to Earth — the 2004 return of a Russian communications satellite.
It "looked like a brilliant star with a long glowing tail," he said in an email.
The best guess so far is that the 20-year-old Upper Atmosphere Research Satellite will hit sometime Friday afternoon or early evening, Eastern time. The latest calculations indicate it will not be over the United States, Canada and Mexico during that time.
Until Thursday, every continent but Antarctic was a potential target. Predicting where and when the freefalling satellite will land is an imprecise science, but officials should be able to narrow it down a few hours ahead.While most of the satellite pieces will disintegrate, 26 large metal chunks — the largest about 300 pounds — are expected to survive, hit and scatter somewhere on the planet. With nearly three-quarters of the world covered in water, chances are that it will be a splashdown.
If the re-entry is visible, "it'll look like a long-lived meteor," said Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.Since the dawn of the Space Age, no one has been injured by falling space debris. The only confirmed case of a person being hit by space junk was in 1997 when Lottie Williams of Tulsa, Okla., was grazed in the shoulder by a small bit of debris from a discarded piece of a Delta rocket.
The odds of someone somewhere on Earth getting struck by the NASA satellite are 1 in 3,200. But any one person's odds are astronomically lower — 1 in 21 trillion."You're way more likely to be hit by lightning" than by the satellite, McDowell said.
NASA has warned people not to touch any satellite part they might chance upon. There are no hazardous chemicals on board, but people can get hurt by sharp edges, the space agency said.
The U.S. tracks the roughly 22,000 pieces of satellites, rockets and other junk orbiting the Earth. Nowadays, the world is more eco-conscious about what it puts up in space. Modern satellites must be designed to disintegrate upon re-entry or have enough fuel to be nudged into a higher orbit or steered into the ocean.
The satellite was launched in 1991 aboard the space shuttle Discovery to study the ozone layer, and back then there was no such rule. NASA used up the remaining fuel to put it into a lower orbit in 2005, setting the stage for its uncontrolled return. It will be the biggest NASA spacecraft to fall uncontrolled from the sky in 32 years.
It's not unusual for space debris to dive back to Earth. NASA's Orbital Debris Program Office estimates that medium-sized junk falls back once a week. Debris the size of the satellite due back Friday occurs less frequently, about once a year.
Harvard's McDowell noted that two massive Russian rocket stages have plunged back this year with little notice.