The US space agency has narrowed down its prediction of when a defunct six-ton satellite will crash back to Earth, saying on Wednesday that it is expected to land on September 23, US time.

"The time reference does not mean that the satellite is expected to re-enter over the United States. It is simply a time reference," NASA said on its website.

"Although it is still too early to predict the time and location of re-entry, predictions of the time period are becoming more refined."

NASA had previously said the satellite could hit Earth as early as Thursday, September 22 or as late as Saturday, September 24.

All but 26 pieces of the Upper Atmosphere Research Satellite (UARS) are expected to burn up on re-entry into Earth's atmosphere, but where exactly they will land remains a mystery.

Orbital debris scientists say the pieces will fall somewhere between 57 north latitude and 57 south latitude, which covers most of the populated world. The debris footprint is expected to span 500 miles (800 kilometers).

UARS is the biggest NASA spacecraft to come back in three decades, after Skylab fell in western Australia in 1979.

The risk to human life and property from UARS is "extremely small," NASA said, adding that in 50 years of space exploration no one has ever been confirmed hurt by falling space junk.

More frequent updates are scheduled for 12, six and two hours before it lands.

But even at two hours out, debris trackers will not be able to predict landing with an accuracy greater than 25 minutes of impact, or within a potential span of 7,500 miles (12,000 kilometers), NASA said.

"Part of the reason it is so uncertain is the spacecraft itself is rather unwieldy looking and it tumbles and we can't predict exactly how it is going to be tumbling," said Mark Matney, an orbital debris expert at NASA.

"Even as it tumbles that could change exactly where it is going to land."