A six-tonne satellite is expected to splash down on Earth on Friday.

Experts say Friday remains the most likely day that the NASA research satellite will come crashing down through the atmosphere.

An estimated 26 pieces, weighing 544kg, are expected to survive.

NASA is anticipating a splashdown rather than a landing, as some three-quarters of the world is covered by water. The Aerospace Corporation in California, in fact, predicts that re-entry will occur over the Pacific late Friday afternoon, give or take 14 hours.

The 20-year-old Upper Research Atmosphere Satellite will be the biggest NASA object to fall uncontrolled from the sky in 32 years. Russia's old Mir space station came down over the Pacific, in a controlled re-entry, in 2001.

But one of its predecessors, Salyut 7, fell uncontrolled through the atmosphere in 1991.

The most recent uncontrolled return of a large NASA satellite was in 2002.