The U.S. space agency NASA says a defunct, school bus-sized satellite is expected to crash to Earth on Friday, but it is uncertain where the debris will land.

NASA said Thursday that it has ruled out North America as a crash site for the 5.4-metric ton satellite, which will break up into pieces upon re-entry to the Earth's atmosphere. It said the location of the crash site will become clearer over the next day.

The agency says the odds are very low — one in 3,200 — that the satellite debris would injure anyone. It says the satellite pieces are most likely to fall over water or in a remote, uninhabited area.

Officials expect the debris footprint to be about 800 kilometers long.

NASA says as the satellite burns upon re-entry, it will be visible even during daylight hours.

The Upper Atmosphere Research Satellite was launched by the space shuttle Discovery in 1991 and has spent more than 7,300 days in space.

Designed to last only three years, the satellite measures various atmospheric conditions. The satellite was decommissioned in December 2005. Despite its impending crash, six of its ten instruments are still functioning.