The high-speed impact of a piece of foam insulation hitting Columbia's left wing is the likely cause of the space shuttle's destruction and the deaths of its seven astronauts, say investigators.
By Dr David Whitehouse
BBC News Online science editor
The foam impact caused a crack in the wing that allowed destructive superhot gas into the shuttle during re-entry on 1 February.
Columbia's left wing was fatally damaged during liftoff
In his most definite statement to date, Admiral Harold Gehman, who leads the accident inquiry board, says the foam is the most probable cause of the accident.
Roger Tretrault, who heads the technical team on the board, adds: "We have the photographic analysis and evidence which indicates that the foam struck on panel six through nine.
"When you put all of those pieces of Swiss cheese together, it's a pretty compelling story that, in fact, the foam is the most probable cause of the shuttle accident."
Flight resumption conditions
Roger Tretault says a fastidious analysis of debris resulted in a "strong indication" that a part of the shuttle's left wing had been pierced during take-off by a piece of insulation torn from the external fuel tank.
The shuttle's underbelly, nose and forward wing surfaces are protected by the thermal tiles from the searing heat produced when the shuttle hurtles into the Earth's atmosphere.
Admiral Gehman has four recommendations
Superheated gas entered the shuttle's superstructure through the crack in the left wing causing the wing to melt and Columbia to disintegrate.
Admiral Gehman listed four conditions the US space agency (Nasa) should insist upon before considering resumption of shuttle flights.
First, he says, is to "minimise or prevent" another detachment of fuel tank insulation on blast-off.
Second, the shuttles themselves must be reinforced to better withstand the shock of such an impact from insulation or other debris involved in blast-offs.
Third, Nasa must develop a method to inspect shuttles whilst in orbit and, if necessary, to undertake emergency repair missions while in orbit.
And fourthly, Nasa should improve its crew survival and rescue procedures.
'Force Nasa to do something'
The commission is to submit its report on the accident to Nasa at the end of July. Gehman expects to release an interim recommendation possibly as early as this week. Nasa has said work is already in progress to implement the recommendations.
"The investigation board is trying to craft words which will force Nasa to do something," he says.
It is expected that half the final report will focus on Nasa management and culture. The other half will involve technical matters, most notably the loss of foam from space shuttle external fuel tanks during every launch for the past 22 years.
A Nasa report released last month said Columbia could not have been saved once it began its return to Earth, not even by reducing its weight or adjusting its trajectory.
Columbia broke up 15 minutes before landing
Shuttles could be back in operation in about nine months, though Admiral Gehman says, "I don't speculate on the date of return to flight."
He adds: "I would say that having read every word of the draft report and having gone over what might be possible recommendations, I don't see any recommendations which are so difficult to accomplish that they shouldn't be able to return to flight in six to nine months."
Nasa is to release video and photographs of the crew salvaged from the wreckage showing the crew playing with objects in weightlessness and demonstrating daily routines like shaving and brushing teeth.