Traveling at 1,800 mph, experts say the 6-ton climate research satellite is expected to break into pieces as it re-enters the Earth's atmosphere sometime Friday afternoon or evening, Eastern Time. They say it's likely to come down somewhere over the Pacific Ocean.
There are many variables that can change the satellite's trajectory, and scientists are tracking them all very closely.
One variable is the thickness of the atmosphere, which can change depending on factors like solar activity.
"The variation in the atmosphere like a solar flare or day-night variations affect the density of these kinds of altitudes, so basically that really affects the orbits quite dramatically towards the end like this," said Bill Ailor, principal engineer at Aerospace Corp. in El Segundo. The institute is helping NASA track exactly where and when the satellite will fall. The company tracks thousands of satellites and other objects in orbit.
The satellite was launched on the back of the shuttle Discovery and has been orbiting Earth for 20 years.
An estimated 100 pieces, representing 1,200 pounds, are expected to survive the re-entry to Earth. The biggest chunk could be as large as 300 pounds.
Experts say they're not worried about falling debris hitting someone on Earth.
"The chances are very small that anybody will even see it quite honestly," said Ailor.
NASA says the chances of someone in the world getting hit by a falling satellite's debris are 1 in 3,200. But the odds of you personally getting hit are 1 in 21 trillion.
"Most of these things come down in the water, that's why we never see them," Ailor said.
The satellite will be the biggest NASA spacecraft to fall uncontrolled from the sky in 32 years.