Super-cooling of the space shuttle's external fuel tank as it was filled with liquid propellant was the likely cause of insulating foam chunks falling from the spacecraft, NASA Administrator Michael Griffin said Friday.
"That's the closest thing to a smoking gun," he said during an interview at a convention in Los Angeles sponsored by the California Space Authority.
As recently as last week, shuttle program managers said they still weren't sure what caused the foam to come off the fuel tank.
But Griffin said that engineers wer now fairly certain that radical temperature changes brought on by filling the fuel tank with super-cooled liquid oxygen and liquid hydrogen led to cracking in the insulating foam, making it vulnerable to falling off during launch.
Insulating foam is applied to the fuel tank to prevent ice from forming on the surface of the tank.
"We have a good amount of data that supports the hypothesis," Griffin said.
A convincing piece of evidence came from a fuel tank that was filled with propellant and then emptied. Engineers later discovered a series of cracks near an irregularly shaped part of the tank known as the PAL ramp.
Griffin said the next step would be figuring out how to prevent the problem from recurring on future flights.
"We've accelerated the effort to make that determination," Griffin said.
A solution could require redesigning the foam application procedure or removing the foam altogether from the PAL ramp. It's still not clear, however, that removing it would not cause other problems during the launch, Griffin said.
A piece of falling foam damaged the wing of the shuttle Columbia in 2003, causing the craft's destruction on reentry and the death of all seven astronauts aboard. Another foam piece fell off during the flight of Discovery this summer, resulting in the grounding of the shuttle fleet.
Shuttle managers have said that if they were able to quickly solve the foam-shedding problem NASA could be in position to launch the next shuttle mission as early as May.
Griffin has been in charge at the nation's space agency for less than a year. During that time, he has been an advocate of President Bush's Vision for Space Exploration, which proposes to send humans back to the moon by 2020 and then on to Mars.
To accomplish those goals, Griffin has shifted NASA's focus from projects in low Earth orbit, such as the International Space Station and the space shuttle. He has promised to stop flying the shuttle in 2010.
Griffin has been critical of the nation's former low-Earth space strategy. He said the U.S. frittered away a 40-year lead in space exploration.
He said he was not surprised that other countries, particularly China, have made huge strides in catching up with America. Griffin said he believed the U.S. should remain at the forefront of space exploration.
"I want my nation to be a leader in all things important," he said.
Griffin recently testified before Congress that there was a deficit in the shuttle program, which he said Friday was in the range of "several billion dollars."
That deficit, he said, would require either spending more money on the shuttle, which could affect other programs, or curtailing shuttle flights.
He added, however, that he expected no new layoffs at the Jet Propulsion Laboratory in La Canada Flintridge, which recently cut its workforce by 5%.