A catastrophic heat shield failure most likely caused the destruction of the space shuttle Columbia and the deaths of its seven crew members, writes Dick Ahlstrom, Science Editor
The seven astronauts on board the shuttle Columbia had no chance when their spacecraft disintegrated 207,135 feet up in the clear skies above Texas. The deceleration forces acting on their shuttle when travelling at 12,500 miles per hour ripped the 75-tonne craft to pieces and would have caused near instant death.
Now the search is on to try and find the cause of the disaster. Three separate investigations are under way - one by the US Congress, one by the shuttle's operators, the National Aeronautics and Space Administration (NASA), and an investigation by an independent team.
While NASA flight controllers have a good working theory about what happened to shuttle mission STS-107, they may never achieve a definitive answer. Columbia shattered to pieces and much of it would have burnt up during its chaotic descent through the atmosphere. Aviation specialists have been called in to try to reassemble what remains of the broken spacecraft, but it will take some luck to find the clues needed to prove what happened.
Columbia launched on January 16th and was only 16 minutes away from a triumphal landing in Florida, with a planned touchdown on Saturday at 2:16 p.m. Irish time. Family members were assembled along the runway to cheer home the seven astronauts.
Details provided by NASA show the first indication that something was amiss during an otherwise faultless flight came at about 1.53 p.m. Irish time, when a temperature sensor providing information about the craft's left-side hydraulic systems suddenly winked out.
"The first indications of a potential problem were of the loss of sensors, temperature sensors in the hydraulic systems on the left," according to Mr Ron Dittemore, NASA's space shuttle programme manager.
The shuttles are peppered with sensors that monitor conditions all across the spacecraft's skin and equipment, and send back a constant stream of computerised information to flight controllers. "We lost data and that's when we clearly began to know that we had a bad day," said Mr Dittemore.
At 1.56 p.m. sensors warned of high temperatures in the left-side landing gear assembly and at 1.58 p.m. more temperature sensors on the left wing unexpectedly failed. At about 1.59 p.m. sensors for left-side tyre pressures and temperatures went out - one of the last warnings of the impending disaster.
Flight commander Rick Husband and his crew had noted indications of some of these failures on their computer screens and in conversation with mission controllers, but there was no indication of trouble or concern. The final transmissions between the two were routine.
Mission Control: "Columbia, Houston, we see your tyre pressure messages and we did not copy your last."
It was 2 p.m. Irish time and Husband responds: "Roger, uh, buh", then nothing but silence as the disintegration of the shuttle began, and the craft and crew were lost. It is doubtful the astronauts had any consciousness of their impending doom.
These failures associated with the left wing provide the focus for the teams investigating the accident. The focus is sharpened by an incident that occurred during lift-off, almost 16 days previously, when the left-side wing was struck by flying debris.
The shuttles at launch are assisted into orbit by both a liquid fuel rocket and two solid fuel boosters.
The shuttle is attached to the huge liquid fuel tank and only disconnects for the final ascent into orbit. This tank is covered in a thick insulating foam to help keep its liquid oxygen and hydrogen fuel cold.
About one minute into the flight, a chunk of this insulation broke free and struck the shuttle's left wing. NASA acknowledged the incident on Friday at a briefing but didn't believe it represented a problem for the shuttle.
A similar incident had occurred last October during the lift-off of the shuttle Atlantis and NASA's assumption was that only minor damage would have occurred to either the wing or its critical heat protecting tiles.
"As we look at that now in hindsight we can't discount that there might be a connection," Mr Dittemore said over the weekend.
Much of a shuttle's structure is made of aluminium, which is protected by a layer of heat-resistant tiles. There are about 32,000 tiles covering the top and bottom of each shuttle, meant to last for at least 100 flights before replacement.
They are so effective that, while tiles along the shuttle's underside glow red hot during re-entry, the inside edge of the tile remains cool to touch. The integrity of these tiles is essential if the shuttle superstructure is to remain safe from re-entry heat.
The shuttle begins to heat up when it leaves orbit and re-enters the Earth's atmosphere. It begins to descend with its nose well up, exposing the tiled underbelly of the craft, which can withstand the searing 1,650 degrees centigrade heat generated as it crashes into the thin atmosphere at more than 17,000 miles per hour.
The assumption now is that the impact with the foam insulation must have caused more damage than originally suspected by the flight controllers.
Tiles could have been damaged or lost. This could have posed two serious risks. It would allow the heat to seep past the tiles and get at the aluminium framing behind. Equally, it could have caused a weak point in the wing surface with the high re-entry speed ripping more tiles free.
With the build-up of heat, any exposed aluminium would have softened like butter, causing complete failure of the wing. The craft would then have lost all stability, given its 12,500 m.p.h. speed at break-up, and would have disintegrated within seconds.
"As soon as there is a chink in the armour of the shuttle then it is likely to be a catastrophic failure. Once it has started to break up nothing can stop it," stated Prof Andre Balogh of Imperial College London over the weekend.
The crew could have done nothing to save themselves. The shuttle flies by computer right up until the final landing descent from 50,000 feet so Husband couldn't have intervened.
If the foam impact damage theory holds true, the crew were actually doomed from the day of launch on January 16th. There was no way for them to have returned home safely.