"We have a very good understanding of what happened," investigation board chairman Harold Gehman said at a news conference. "The orbiter (space shuttle) was returning with a pre-existing flaw in the wing. ... The wing got heated from the inside."

However, Gehman said how the wing was damaged remains unproven, although a collision with foam insulation from the shuttle's external fuel tank during launch remains the most likely cause.

The shuttle came apart over Texas on Feb. 1 while streaking toward a planned landing at the Kennedy Space Center in Florida. Seven astronauts were killed and the shuttle fleet was grounded while investigators looked for the cause of the accident.

The board for weeks was unwilling to take a firm position on what happened even as evidence mounted that a hole in the left wing was at the heart of the disaster. Even Tuesday, the board called its conclusions a "working scenario."

But Gehman said "we made sure that there were no facts that contradicted our scenario."

"We now know enough," he said. "We're at the point where we should focus our efforts."

He said the board will be able to make a broad range of recommendations this summer to make the shuttle program safer.

Gehman said the board may never be able to prove the wing was damaged by the foam insulation, though technicians next month will test that idea by firing chunks of the material at thermal tiles. But Gehman said NASA already knows peeling insulation is a problem that must be fixed before the shuttle fleet flies again.

Also among the board's conclusions:

As Columbia was launched Jan. 16, photos showed that the leading edge of the left wing was struck by a large piece of foam insulation. There was no indication while the craft was in orbit that the wing had sustained serious damage.

On the shuttle's second day in orbit, Air Force radar detected an object drifting free of Columbia. Later analysis suggests, but does not prove, that the object was either a piece of reinforced carbon tile or a seal from the leading edge of the wing. It could have been broken by the foam during launch and then shaken free during a maneuver by Columbia.

When Columbia re-entered the atmosphere on Feb. 1, gases heated to several thousand degrees entered a hole in the wing and melted metal struts and wiring.

Sensors inside the wing detected rising temperatures within five minutes after the craft began its descent. Within six minutes, the sensors stopped sending data, suggesting wires were melting. In 15 minutes, all communication with Columbia ceased. Observers on the ground, from California to Texas, reported seeing burning debris falling.

Reinforced carbon panels from the leading edge of left wing, numbers 8 and 9, were eroded by extreme heat. Other pieces from the wing were splashed with molten aluminum, copper, nickel and other metals, indicating extreme heat.

Gehman said there is a "high level of agreement" on the scenario among board members, but that "we reserve the right to change any part of it" if new facts surface.

(China Daily May 7, 2003)