Low cloud cover over Cape Canaveral was considered a threat to visibility for landing at the Kennedy Space Center and the seven crew on Discovery were told to stay in orbit an extra 24 hours, extending their mission a second time to 14 days.

"We just can't get comfortable with the stability of the situation," ground control told the shuttle as a second landing time was put back.

"We are going to officially waive you off for 24 hours," Ken Ham at mission control told Discovery Commander Eileen Collins.

NASA now hopes Discovery will return at 5:08 am (0908 GMT) Tuesday. If the weather is again bad Discovery could land until Wednesday in Florida or at alternate runways in California and New Mexico.

Collins and Pilot James Kelly said earlier they had no worries about the nervewracking return to Earth even though Columbia burned up and disintegrated as it re-entered the Earth's atmosphere on February 1, 2003, killing all seven astronauts aboard.

"We're looking forward to coming home," Collins told mission control.

Hours before the first scheduled landing, the astronauts closed the massive bay doors of the payload area, and configured computers for the hourlong homeward journey.

Once they get the go-ahead, Collins and Kelly should fire up the two steering jets for three minutes to loose enough altitude to begin the plunge toward the Earth.

Discovery will be grounded with the rest of the fleet once it returns to Earth because the mission has exposed renewed problems with debris falling off shuttles -- the cause of the Columbia tragedy.

The mission was largely designed to test changes made to prevent insulation foam from breaking off upon launch.

"We have definitely accomplished our mission objectives," Collins told NBC television on Sunday. She stressed the crew would concentrate on getting home safely.

"We're going to have thoughts about Columbia, but we'll be very focused on the job at hand," she said.

The crew carried out emergency repairs in orbit and there were concerns that a tear to a thermal blanket just outside the cockpit could pose a new danger. But National Aeronautics and Space Administration officials said the return would be safe.

They also said the orbiter suffered no significant damage when foam insulation fell off its external fuel tank as the shuttle blasted off on July

The same problem doomed Columbia. Debris hit the orbiter's left wing, causing a crack that allowed superheated gases to penetrate the structure upon re-entry.

As it drops through the atmosphere, its speed drops from nearly 29,000 kilometers (18,000 miles) per hour in orbit to about 350 kph (220 mph) at touchdown.

The mission had been scheduled to last 12 days, but an extra day was added so the crew could transfer as much material and provisions as possible to the International Space Station (ISS), amid uncertainty over the date of the next shuttle flight.

The crew also retrieved waste and equipment to clear out space in the cramped orbiting lab.

During the mission, Stephen Robinson became the first astronaut to carry out a spacewalk under the shuttle, to extract two protruding pieces of fiber that risked overheating during re-entry.

In another two spacewalks, Robinson and Japanese astronaut Soichi Noguchi tested repair techniques adopted after the Columbia tragedy, and replaced one of the space station's four gyroscopes.

"It's been a great trip and we'll come back with a lot of stories," Noguchi told CBS television.

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