CAPE CANAVERAL, Fla. -- NASA yesterday pushed back the first post- Columbia space shuttle flight by at least two months, after last-minute analyses suggested that ice falling off the fuel tank could prove as catastrophic as the foam that doomed the last mission two years ago.
Discovery is now set to fly in mid-July, provided that engineers can perform the necessary repairs to eliminate the danger. The flight had been scheduled for late May.
The plan calls for installing a heater along a 70-foot liquid-oxygen pipe that runs along the tank -- unprecedented work that will require NASA to remove the shuttle from the launchpad and roll it back into the shop.
''We are not going to rush to flight," said Michael Griffin, NASA's administrator of just two weeks. ''We want it to be right, so we're doing what we need to do to ensure that."
The decision followed a flurry of launch-debris reviews in recent weeks that found that ice forming on certain unprotected spots on the tank could prove a deadly source of shrapnel during liftoff.
Because a 1Æ-pound chunk of fuel-tank foam insulation had damaged Columbia's wing and led to the spacecraft's destruction during reentry in February 2003, NASA had focused on keeping large pieces of foam from coming off during launch. Ice, the commonplace result of the tank being filled with super-cold fuel, was considered less threatening and therefore less important for analysis.
Once ice began looming as a bigger threat, NASA hurriedly conducted tests and data analyses that concluded, just in the past few days, that even small, slushy patches of ice could fly off the oxygen feed line and strike the shuttle with deadly force. Before, engineers did not believe that ice shards actually would hit the shuttle.
Through exhaustive research, NASA learned that pieces of ice forming on the expansion joints in the 70-foot feed line probably came off and smacked the shuttle in years past but caused only minor damage.
''That was the information that said, wait a minute, yes, maybe we've been a little lucky, maybe we don't understand this problem as well as we should," shuttle program manager Bill Parsons said.
Discovery and its crew of seven have from July 13 through July 31 to lift off. The two-week launch window is dictated by both the position of the space station and NASA's insistence on launching Discovery in daylight, to provide good photographs of the shuttle that can be examined for any liftoff damage.
If it misses the window, the 12-day supply and repair mission to the international space station will have to be delayed until September.
Griffin said he wants to launch as soon as possible. ''Schedule matters," he said, referring to the need to finish building the space station. But he added, ''It shouldn't matter to the point of causing people to do dumb things or to take ill-advised actions."
Station program manager Bill Gerstenmaier said the orbiting complex should be able to keep flying safely until Discovery gets there, even though the main oxygen generator is broken and the steering system is operating with the bare minimum number of working gyroscopes.
The shuttle fuel tank has already gone through extensive modifications since the Columbia catastrophe. NASA changed the way it applies some of the foam to the tank, and removed large sections of foam from certain trouble spots. Then heaters were installed to prevent ice from forming at those areas.
The shuttle team is dealing with a few other unrelated problems with Discovery involving balky engine-cutoff sensors in the fuel tank. The extra two months will give NASA time to solve those problems.