The US space agency has so far pinpointed two windows of opportunity to land the shuttle back at Cape Canaveral, Florida, either at 10:00 am (1400 GMT) or 11:39 am (1539 GMT) on Friday.

But NASA spokeswoman Laurel Lichtenberger said "we may not know until the last minute, maybe not until tomorrow 9:00 am (1300 GMT)."

Atlantis and its seven-strong crew are returning from a successful 11-day mission to repair and restore the Hubble space telescope to allow it to continue its ground-breaking exploration of the universe for at least another five years.

All eyes were on the weather forecast Thursday, with a final decision on the return needed to be taken 90 minutes before the shuttle is due to land, to enable it to leave orbit safely and re-enter the Earth's atmosphere.

Heavy rains had stopped Thursday afternoon at the Kennedy Space Center in Florida, but the sky was completely clouded over and waves were being whipped up on the seas by strong easterly winds.

NASA has set several conditions for a landing: the cloud cover in the skies must not be more than 50 percent, visibility must be at least eight kilometers (five miles) and lateral winds must not be blowing at more than 28 kilometers an hour (17 miles an hour).

So far NASA has ruled out using Edwards Air Force Base in California as a back up landing point as it would mean flying the shultte back to Florida on a Boeing 747, involving substantial extra cost.

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