"When they run out of fuel and they can no longer give themselves periodic kicks to stay in orbit, they begin to slowly drop," said Seth Shostak, Ph.D.
Shostak is a senior astronomer at the SETI Institute in Mountain View. He and his colleagues research life in the universe. Shostak says the GOCE satellite will continue to drop until it plunges into the Earth's atmosphere.
Pieces of the satellite will begin to burn up and essentially become asteroids. But a couple hundred pounds of it land here back on Earth.
Shostak stay it isn't likely anyone will be struck by a piece of the satellite.
The Eurpoean Space Agency says the crash is expected to occur between 1830 GMT Sunday and 0030 GMT on Monday (1:30 p.m. and 7:30 p.m. EST).
It says "with a very high probability, a re-entry over Europe can be excluded."
Spokeswoman Jocelyne Landeau said the satellite, GOCE, will mostly disintegrate as it comes down and "we will have only a few pieces which could be 90 kilograms at the most."
ESA said Friday that humans are 250,000 times more likely to win the lottery than to get hit by the debris.
GOCE was launched in 2009 to map the Earth's gravitational field.
(The Associated Press and ABC7 News reporter Ama Daetz contributed to this report)