Shuttle explanation still sought

The Columbia space shuttle was lost because damage to a seal or panel on its left wing allowed the super-hot gases of re-entry to get inside the vehicle frame and break it apart.

Foam impact on launch is the primary candidate

This is the Columbia Accident Investigation Board's (CAIB) "working scenario" - a best assessment of what happened to the orbiter based on the evidence gathered so far.

The scenario contains few surprises, keeping to the facts known about the flight - but stopping short of pulling them together into an explanation of the accident.

The space shuttle broke up over Texas on 1 February killing all seven crew members. It had been on a 16-day mission in space to conduct science experiments in micro-gravity.

Testing panels

The board notes that foam from the orbiter's external fuel tank struck the left wing 81 seconds after launch - but it does not at this stage say the impact doomed the shuttle.

The Columbia broke apart minutes before the planned landing

"We're careful not to say the foam knocked a hole in the leading edge of the orbiter because we can't prove it," said retired admiral Harold Gehman, who heads the inquiry.

The board's best chance of getting proof that would identify the broken foam as the cause involves tests now under way at the Southwest Research Institute in San Antonio, Texas.

Samples of foam are being fired at actual shuttle parts to see what damage they cause.

Modelling conducted whilst Columbia was in orbit suggested no significant damage would have resulted from the impact. The Southwest Research Institute data will indicate if that modelling was flawed.

Radar images

The working scenario document says merely that foam hit Columbia in the vicinity of the lower left wing RCC (reinforced carbon-carbon) panels 5-9 and that analysis of telemetry and wreckage indicate Columbia had a wing defect centred on the RCC panel 8/9 area.

The suspicion is that the launch impact damaged a panel or T-seal, which sits between panels, so that it eventually came away, breaching the vehicle's critical heat-protection system - but the CAIB will not make that leap yet.

Radar images show an object moving in the vicinity of Columbia on day two of its mission. Analysis of the radar "signatures" of shuttle parts has eliminated all components apart from an RCC panel or a T-seal.

The scenario details the timings of significant events as Columbia raced through the atmosphere, how hot gases entered the left wing and began to the break the vehicle apart.

"Based on video imagery, main vehicle aerodynamic break-up occurred at 9:00:23 EST," it says.

"We now know enough... to the point where we should now focus our effort," Harold Gehman said.