The US space agency will try to land Discovery and its seven-member crew on Tuesday.

The first opportunity will be at the Florida landing strip at 5.08am EDT (0908 GMT) with alternative sites in California and New Mexico available if Florida's weather is still unfavourable.

Discovery had been scheduled to land at 4.47am EDT (0847 GMT) on Monday, but Nasa managers were worried about clouds forming near the shuttle's 4.8-km landing strip and postponed the end of the first shuttle mission since the 2003 Columbia disaster.

"The one word that describes the situation is unstable," said astronaut Ken Ham, who radioed the shuttle astronauts from Mission Control in Houston to tell them of the delay.

The landing will bring to a close Nasa's first shuttle mission since Discovery's sister ship, Columbia, broke apart over Texas on 1 February 2003, 16 minutes from landing.

Damage

Nasa did not know that the ship's wing had been critically damaged during launch by a piece of falling debris.

The shuttle will be circling the
Earth before its return

As Columbia ploughed through the atmosphere 16 days later for landing, superheated gases blasted into the hole, melting the ship. The seven astronauts on board died.

Discovery's flight director, Leroy Cain, who also oversaw Columbia's failed landing attempt, acknowledged he would be nervous during the touchdown.

"If I came in to do an entry and landing on this flight or any flight and I didn't have butterflies in my stomach, I'd probably turn around and go back outside and find somebody else to do the job, quite frankly," he said.
 
"There are a lot of things to think about, a lot of things to worry about and that's what I get paid to do," he said.

While Discovery's descent will test the mettle of Mission Control for the first time in the 24-year shuttle programme, ground controllers know the condition of the shuttle's heat
shield.

Determining problem

After the accident, Nasa developed on-orbit laser imaging tools and inspection techniques, which were not only tested during Discovery's flight but were important in determining that an unplanned spacewalk was needed to make a minor but unprecedented repair on the ship's heat shield.

But the success of the inspection tools was overshadowed by the failure of the shuttle's fuel tank, the primary upgrade after the Columbia accident. Columbia's wing damage was caused by a piece of foam insulation that fell off the tank during launch.

A chunk of foam almost as large as the one that damaged Columbia flew off Discovery's tank as well. It did not strike the ship, but Nasa again suspended shuttle flights until the problem is solved.

Investigators are trying to determine if the 0.45-kg block of foam shed by Discovery's tank fell off because of pre-flight repairs and maintenance. If engineers can determine that the foam problem is specific to Discovery's tank, Nasa may be in a position to fly its second post-Columbia mission as early as September.

Next launch

Deputy programme manager Wayne Hale, however, said he did not believe the 22 September target launch date was realistic.

In addition to shaking out the remodelled fuel tank and other safety upgrades, Discovery spent nine days at the International Space Station for a critical servicing and resupply mission.

Since the Columbia accident, the station has had to rely solely on the smaller Russian Progress cargo vehicles to deliver cargo and Soyuz spacecraft for crew transfers.
 
In addition to replenishing the station's pantry, water supplies and other critical gear, Discovery delivered a new gyroscope to the outpost and revived a second failed device, restoring full service to the steering system for the first time in more than three years.