The U.S. space agency NASA says one of its school-bus sized satellites is expected to crash back to Earth by week's end. But the agency is not sure where it will land.

The projected crash zone covers most of the inhabited portions of the planet, and officials expect the debris footprint to be about 800 kilometers long. But NASA officials say the odds of one person being hit by debris is one in 3,200.

Re-entry is expected this Friday, give or take a day, for the 5.4-metric ton satellite. NASA will be able to more accurately predict a potential debris zone as the week progresses.

Most of the the satellite is expected to burn up on re-entry, which NASA officials say will be visible even during daylight hours.

NASA officials expect approximately 26 pieces to re-enter Earth's atmosphere. The remaining debris will likely fall into the ocean.

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.

Space.com says satellites begin to be pulled down to Earth by gravity after their deactivation and as they lose fuel that boosts their orbit.