The piece of foam that fell from Columbia during launch could not have inflicted enough damage on its own to cause the space shuttle to disintegrate upon re-entry from space, NASA said yesterday, backing away from early suspicions.
"It doesn't make sense to us that a piece of debris could be the root cause of the loss of Columbia and its crew," shuttle program manager Ron Dittemore said. "There's got to be another reason."
After days of analysis, he said, investigators are focusing on the desperate effort of the shuttle's automated systems to hold the aircraft stable, despite increasing resistance on the left wing.
During Columbia's final minutes, Mr. Dittemore said, the autopilot rapidly adusted the control mechanisms, then fired small rockets, in a losing effort to gain control of the yawing Columbia.
Final bits of data from the spacecraft show that "we were beginning to lose the battle," he said.
For that reason, his team is intensifying efforts to recover a final 32 seconds of data from the spacecraft.
NASA had latched on to the idea that a piece of foam that fell from the shuttle's big external fuel tank 81 seconds after liftoff caused enough damage to the vital heat-resistant tiles for the shuttle to burn up as it passed through the Earth's atmosphere on its return from space.
After looking at all the possibilities, however, it became clear that while the foam could have been a contributing factor, it could not have been the sole reason for the disaster.
The backtracking calls into question whether NASA should have been able to avert the disaster because of earlier research into the foam and the vulnerability of the tiles.
Canadian astronaut Colonel Chris Hadfield, NASA's top representative at Russia's space-training centre, said mission control could not have predicted the shuttle's demise.
"Everybody wants to find somebody who held up a piece of paper at a meeting six months ago and said 'I don't think [Columbia] should fly,' " he said. "I haven't seen anything that indicates anyone ever did that. No one thought [Columbia] had a serious problem. Unfortunately, we were tragically wrong.
"We need to find out why, and make sure this never happens again."
Col. Hadfield, a jet pilot all his adult life, said he has never seen any organization take safety as seriously as NASA does.
The Canadian Space Agency said yesterday that it will not allow its astronauts to fly on space shuttles until it determines the vehicles are safe.
Michel Vachon, director of the Canadian Astronaut Office, said the space agency will make its decision about the safety of the shuttles after NASA determines the cause of Saturday's tragedy.
"For us to decide that the Canadian astronauts go there, definitely we'll ask questions," Mr. Vachon said from Houston, where he was helping the six Canadian astronauts deal with the tragedy. "The decision is completely ours to decide that a Canadian will go or not go."
Col. Hadfield had just returned yesterday from a family memorial service for Col. Rick Husband, commander of the Columbia mission and a close friend of Col. Hadfield for 16 years.
When the seven astronauts died on Saturday morning, Col. Hadfield had just returned to the United States from Russia, where he has been for the past year and a half, and said he was eager to see Col. Husband and his other friends who were on the mission.
Col. Hadfield discovered that reunion would never take place when grief-stricken friends called his Houston house to tell him that the shuttle had been lost.
"It was a horrible, hollow, falling sensation of emptiness for me to realize something horrible that can never be undone had just happened," he said.