HOUSTON - Investigators have mounting evidence that a chunk of hard insulating foam hit the space shuttle Columbia, eventually causing it to break up while re-entering the atmosphere nearly three months ago.
Retired NASA engineers and program directors who helped develop the shuttle told the investigation board yesterday that hits from such debris were a concern early in the orbiter's history.
The shuttle "was never designed to stand a 3-pound mass hitting it at 700 feet per second," said Robert Thompson, former vice president of the space station program for McDonnell Douglas and a former manager of the shuttle program for NASA. "We worked very hard to make sure we didn't have a foreign object hit."
During yesterday's public hearing, the five gave the board a history lesson on how the orbiter was created and some of the problems with its design.
Air Force Maj. Gen. John Barry, a board member, said he was puzzled that while the shuttle's design specifications didn't allow for debris hits, almost every orbiter had 100 hits per mission on average.
"The design specification is in [contrast] to the reality," Barry said.
Aaron Cohen, a former director of the Johnson Space Center in Houston, said debris hits during orbit weren't discussed much in the shuttle's development phase because not enough was known at the time about the environment in space.
The tiles that protect the shuttle from the intense heat of re-entry are "actually pretty forgiving with reasonable types of hits, but you can't take large hits," he said.
The investigation board said Tuesday that evidence gathered since the accident on Feb. 1 pointed to damage from a chunk of foam that ripped off the shuttle's external fuel tank and hit a seal on the left wing during the shuttle's launch Jan. 16.
That would have created a slit large enough to let in hot atmospheric gases as the spacecraft re-entered the atmosphere on its return home two weeks later. Columbia's seven astronauts were killed when the shuttle broke up over Texas as it aimed for a Florida landing.
"For 11 weeks we have been saying we don't have any particular scenarios, any favorite scenarios," Harold Gehman Jr., a retired admiral heading the investigation board, told reporters. "I think 11 weeks into this, it's time that we attempted to see where the evidence was pointing us."
The board members said tests indicate an object that floated away from the shuttle in orbit was all or part of a T-seal - a reinforced carbon composite seal that fits between panels made of the same material that are designed to withstand temperatures of up to 3,000 degrees during re-entry. The seals and panels wrap around the leading edge of each wing.
Rear Adm. Stephen Turcotte, a board member, said he believes that some sort of "blunt-force trauma" and not simply wear and tear caused the seal to come off.
Physicist James Hallock, a board member in charge of the Transportation Department's aviation safety division, theorized that the hole created by the missing seal grew as hot atmospheric gases broke away adjoining wing panels.
Testing of a fuel tank similar to the one that propelled Columbia shows at least 74 defects in the insulating foam, many of which are air pockets. Engineers also discovered faulty bonding between some of the layers of foam.
Steven Wallace, a board member and the director of accident investigations for the Federal Aviation Administration, said he and his colleagues also found that seven of the 77 flight controllers involved in Columbia's flight were lacking some necessary certifications.
That did not contribute to the accident, but the board considers it a discrepancy that needs to be addressed, Wallace said.
The board's final report is not expected until midsummer.
"We're beginning a kind of a new phase in the board's work," Gehman said. "In the month of May, we'll spend more time deliberating and less time investigating."
The search for debris from the shuttle is winding down in Texas, where most of the debris fell. Air operations will end this week, said Mark Stanford, incident commander with the Texas Forest Service. The ground search is to end Wednesday.