CAPE CANAVERAL, Fla. (Reuters) - Cloudy skies over its Florida landing site on Monday forced NASA to postpone the return to Earth of space shuttle Discovery on the first shuttle mission since the 2003 Columbia accident.
Although the clouds were dissipating and conditions appeared to be acceptable for a landing, NASA said it was taking no chances and delayed the shuttle's return for at least a day.
"We just can't get comfortable with the instability of the situation," astronaut Ken Ham radioed from Mission Control in Houston to the astronauts on the shuttle to tell them they would be staying in space another day.
Flight directors tried twice on Monday to bring Discovery back to Earth after 13 days in orbit but decided the weather was too unpredictable to be sure the spaceship's commander, Eileen Collins, would have a clear view of the 3-mile, canal-lined runway at the Kennedy Space Center.
Barring emergencies, NASA will only land the shuttle if there is good visibility for the approach to the runway and no rain or thunderstorms within about 35 miles (56 km).
Touchdown was retargeted for either 5:07 a.m EDT (0907 GMT) or 6:43 a.m. EDT (1043 GMT) on Tuesday in Florida, or 8:12 a.m. EDT (1212 GMT) at the primary backup site in California.
Shuttle program deputy manager Wayne Hale said NASA also would staff its second backup landing site in New Mexico. The ship has enough supplies to safely stay in orbit through Wednesday, if weather or technical issues prevent landing on Tuesday.
The landing will bring to a close NASA's first shuttle mission since Discovery's sister ship, Columbia, broke apart over Texas on Feb. 1, 2003, 16 minutes from landing.
NASA did not know that the ship's wing had been critically damaged during launch by a piece of falling debris. As Columbia plowed through the atmosphere 16 days later for landing, superheated gases blasted into the hole, melting the ship. The seven astronauts on board died.
While Discovery's descent will test the mettle of Mission Control, for the first time in the 24-year shuttle program, ground controllers know the condition of the shuttle's heat shield.
After the accident, NASA developed on-orbit laser imaging tools and inspection techniques, which not only were tested during Discovery's flight, but important in determining that an unplanned spacewalk was needed to make a minor but unprecedented repair on the ship's heat shield.
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 1-pound (0.45-kg) block of foam shed by Discovery's tank fell off because of preflight repairs and maintenance. If engineers can determine that the foam problem was specific to Discovery's tank, NASA may be in position to fly its second post-Columbia mission as early as September.
Hale, however, said he does not believe the Sept. 22 target launch date is realistic.
Discovery spent nine days at the International Space Station for a critical servicing and resupply mission.
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.