"We now know enough ... to the point where we should now focus our effort," said retired admiral Hal Gehman, who heads the Columbia Accident Investigation Board.

Gehman, speaking at a press conference broadcast from the Johnson Space Center in Houston, Texas, described the preliminary findings as "a working hypothesis" that was reached in the previous 10 days.

Columbia disintegrated on re-entering Earth's atmosphere on February 1, killing all seven astronauts on board, just 16 minutes prior to its scheduled touchdown in Florida.

The seal was likely damaged just 81 seconds after the Columbia blasted off on 16 January, when the shuttle's left wing was hit by a piece of insulating foam that that broke off from one of the shuttle's external fuel tanks.

Superheated gases seeped under entered the shuttle's heat-resistant external tiles upon re-entry and penetrated the spacecraft's cabin, resulting in disintegration.

Two days after the accident, as the Columbia and its seven astronauts were orbiting the earth, a North American Aerospace Defense system (NORAD) military radar detected a small object being detached from the shuttle.

The object remained close to the shuttle until it returned into the earth's atmosphere, Gehman said.

Tests carried out to identify the object showed it corresponded to "a T-seal between the RCC (reinforced carbon-carbon) tiles on the left wing lead edge" used to bind together the heat-resistant reinforced carbon tiles that protect the leading edge of the wings from re-entry heat, Gehman said.

These tiles can withstand heat of up to 1,650 degrees Celsius, he said.

Gehman said that the damage led to the wing's internal structure being exposed to hot gases during re-entry.

"There was a breach and the heat got into the area for about 3 minutes and began to heat its way through the spar," or superstructure of the shuttle, Gehman said.

"Once hot gases got into the spar, they began attacking the aluminum spar and the wire bundles," he added.

However, the wing's internal structure was not designed to withstand any heat over 350 degrees Celsius.

The enquiry will continue to look at NASA's safety procedures, and Gehman did not give any indication as to when space shuttle flights might resume. ft/ch/wf/aln

SPACE.WIRE