Working hypothesis: wing hit by debris. (Columbia Disaster).

The independent board investigating the breakup of the space shuttle Columbia last week presented its first detailed account of what might have caused the Feb. 1 disaster that killed all seven crew members.

After 3 months of reviewing abundant evidence, including shuttle debris, wind tunnel measurements, telemetry, and videos, the Columbia Accident Investigation Board offered a scenario: The tragedy began during liftoff on Jan. 16, when debris from an external fuel tank struck Columbia's left wing. The impact apparently punched a hole just below the wing's leading edge. During the shuttle's reentry 16 days later, this breach permitted superheated gases to penetrate the wing's wheel well. That, in turn, caused the wing to deform, leading to the catastrophic breakup of the vehicle.

That "working hypothesis," as board chairman retired Navy Admiral Harold Gehman Jr. called it during a May 6 press briefing in Houston, fits with speculation that emerged just days after the disaster (SN: 2/8/03, p. 83). However, the board stopped short of blaming the catastrophe on pieces of foam insulation that broke from the external tank.

"The board ... is certainly suspicious that the foam had something to do with this" but is reserving judgment until further data are collected, Gehman told reporters. The group is still waiting for additional information, including results from tests at Southwest Research Institute in San Antonio. There, researchers are shooting chunks of shuffle foam out of a compressed-gas gun into panels of heat-resistant material known as reinforced carbon-carbon. These panels are similar to those that encased the shuttle's wings.

Ground-based video cameras recorded images of foam tumbling off one of Columbia's external fuel tanks and hitting the left wing just after liftoff. Foam has peeled off during other shuttle missions, and the board estimates that it occurs about once every 19 to 13 shuttle missions.

While Columbia was still in flight, NASA officials consulted with the agency's own engineers and contractors and concluded that the impact didn't pose a danger to the shuttle crew. NASA decided not to arrange for satellite images of the shuttle to look for damage. However, e-mails made public in March reveal that some NASA scientists remained worried.

The board now recommends that such images be a standard requirement for each shuffle flight and that NASA should develop a more comprehensive inspection plan for all of the shuttle's reinforced carbon-carbon components.

Nongovernment members were placed on the board after Congress expressed concern that the investigation wouldn't be impartial. Earlier this week, the Orlando Sentinel reported that these recent additions had been placed on the NASA payroll so that the board would contain only federal employees and can legally keep secret closed-door testimony or transcripts of its deliberations.

Gehman has argued that such a policy encourages witnesses to speak more openly. But some observers say the secrecy undermines the board's credibility.

"Congress and the public are going to question the force and validity of their procedural recommendations without access to the evidence that led them to their conclusions," asserts John Pike, a space-policy analyst with GlobalSecurity.org in Alexandria, Va.