U.S. space agency NASA says that one of its decommissioned satellites is expected to crash back to Earth by week's end.

Re-entry is expected this Friday, give or take a day, for the school bus size, 5.4-metric ton satellite. NASA officials say approximately 26 pieces of the satellite will re-enter the Earth's atmosphere.

Most of the debris is expected to burn up on re-entry. NASA officials expect the burning debris to be visible even during daylight hours.

The projected crash zone covers most of the inhabited portions of the planet. NASA will be able to more accurately predict a potential debris zone as the week progresses.

NASA officials expect the debris footprint to be about 800 kilometers long.

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.