SPACE CENTER, Houston — After orbiting the Earth for nearly two weeks, astronauts aboard space shuttle Discovery were told to circle the planet for another day as bad weather in Florida forced NASA to delay today's scheduled landing.

The astronauts had powered up their spacecraft and were awaiting word from Mission Control to fire their braking rockets and head for home when controllers announced early today that low clouds over Cape Canaveral would postpone the landing.

"We've been working this pretty hard as I'm sure you can imagine from our silence down here," Mission Control radioed Discovery commander Eileen Collins. "We just can't get comfortable with the stability of the situation for this particular opportunity so we are going to officially wave you off for 24 hours."

When cloud cover still threatened after the second of two landing opportunities, NASA officials rescheduled the landing for tomorrow, when they would consider two alternative landing sites in addition to Florida's Kennedy Space Center.

"We will land somewhere on Tuesday," Flight Director LeRoy Cain said.

Before the weather deteriorated, Discovery had been set to land at Florida's Kennedy Space Center before dawn. Its return to Earth would have concluded the first shuttle flight since Columbia disintegrated while re-entering the Earth's atmosphere 2 1/2 years ago.

Discovery's 13-day flight to the international space station may be the last one for a long while. NASA grounded the shuttle fleet after a slab of insulating foam broke off Discovery's external fuel tank during liftoff — the very thing that doomed Columbia and was supposed to have been corrected.

After Discovery's July 26 launch, the shuttle spent nine days hitched to the space station, where astronauts resupplied the orbiting lab and removed broken equipment and trash.

Discovery was the first shuttle to visit the station since 2002.

During the trip, a pair of spacewalking astronauts replaced a failed 660-pound gyroscope, which controls the orientation of the station, and restored power to another. Yesterday was the first time in three years that all four of the station's gyroscopes ran simultaneously.

In a third unprecedented spacewalk, astronaut Stephen Robinson went beneath Discovery's belly to gently tug out two protruding thermal tile fillers. Engineers on the ground worried the material could cause overheating during re-entry and could lead to another Columbia-type catastrophe.

As a result of Columbia, Discovery's crew inspected their ship for damage on five different days during the mission and also tested repair techniques developed since the tragedy.

Columbia was doomed by a 1.67-pound piece of foam that broke free from an external fuel tank at launch. The foam pierced a hole in the ship's left wing and as the spacecraft re-entered the Earth's atmosphere, searing gases melted the wing from the inside, causing the ship to break apart.

NASA officials' excitement over Discovery's return to space was dampened by video that showed a nearly 1-pound chunk of foam — reminiscent of the one that doomed Columbia — breaking free from Discovery's external tank shortly after liftoff. The foam did not strike Discovery.

The agency quickly grounded future flights, saying that more work must be done.

Despite the setback, NASA says Discovery's flight has taught important lessons and overall been an "incredible" success.

"We've shown that we've been able to return the vehicle back to safe operational flight," astronaut Andrew Thomas said yesterday aboard Discovery. "There's a lot of success that goes with this mission that I think is going to be important for the long-term future and well-being of this flight program."