AS NASA sifts through the charred wreckage of the space shuttle Columbia, a picture of the events that may have led to its demise is slowly being pieced together.
Columbia began its mission from Cape Canaveral on the morning of Thursday 16 January. The lift-off seemed flawless, but the following day NASA engineers who reviewed film of the launch spotted a problem. About 80 seconds into the flight, a briefcase-sized chunk of foam insulation with a mass of just over a kilogram broke away from the shuttle鈥檚 giant external fuel tank and collided with Columbia鈥檚 left wing, shattering into a white cloud.
NASA hastily convened a team of engineers to study what problems, if any, the impact might have caused. But assessing damage from debris strikes is not easy. The team first calculated the energy that the foam could have imparted. If the debris had hit the underside of the wing, they reasoned, it could only have been a glancing blow 鈥� which transfers far less energy than a head-on collision.
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NASA knew that an almost identical incident with Columbia in 1992 had gouged a hole in the heat-shield tiles less than 3 centimetres deep and a dozen or so centimetres long, but had not penetrated the protective layer. On that occasion Columbia returned safely.
They assumed this latest impact was similar and modelled the degree of damage resulting from glancing blows at angles up to 16掳. The results showed that damage would have been minimal, and the team eventually agreed that the incident was 鈥渋nconsequential鈥�, says Ron Dittemore, manager of the space-shuttle programme.
Ground control informed the crew, orbiting 200 kilometres overhead on their scientific mission (快猫短视频, 1 February, p 10), that their flight had been given the all-clear. The mission then proceeded without incident until re-entry.
Whatever went wrong happened suddenly. On Saturday 1 February at 07:15 Central Standard Time, shuttle Commander Rick Husband and pilot William McCool began their one-hour journey home. They fired Columbia鈥檚 thrusters, slowing the craft for descent, then positioned it with its nose tilted up, ready for its usual computer-controlled re-entry.
The first sign of a problem appeared above California at 07:52. As Columbia streaked across the dark early-morning sky, Caltech astronomer Tony Beasley saw several small bright pieces coming away from the shuttle. A brighter piece followed moments later. And at Owens Valley Radio Observatory 20 kilometres away, astronomer Carmen Sanchez-Contreras saw it too. 鈥淚 saw a second bright spot, much smaller, that was suddenly left behind. It looked like something was separating from the shuttle,鈥� she told 快猫短视频.
At the same time, mission control in Houston received the first warning of unusual heating in temperature sensors in the shuttle鈥檚 left landing gear well. At 07:53, four temperature sensors on the trailing edge of the left wing failed suddenly.
At 07:54, sensors inside the fuselage above the left wing showed the temperature had risen 30 掳C in the previous five minutes 鈥� four times normal for that area. But the increase was small compared with the outside of the spacecraft, where air friction was heating tiles to 1100 掳C.
A minute later, a brake line on the left wing showed a significant temperature increase, and at 07:57 two more sensors failed. Then Columbia鈥檚 flight control system detected excessive drag on the left side of the vehicle and began to compensate using the elevons 鈥� flight-control surfaces on the rear of the delta wings 鈥� and, unusually, by firing two small thruster jets.
But the drag was getting progressively worse, and the flight computer appeared to be losing the battle for control, Dittemore says. The shuttle was still struggling when, over western Texas at 07:59, controllers alerted Columbia鈥檚 crew to the sensor failures. Rick Husband responded but was cut off in mid-sentence.It was the last anyone heard from the crew.
As the shuttle streaked over eastern Texas at Mach 18 and an altitude of 63 kilometres, people on the ground watched in horror as it broke up into multiple, tumbling smoke trails. When Columbia failed to arrive for its scheduled 08:16 touchdown, officials knew the craft was lost.
Speculation about the cause of the incident began immediately. Terrorism was all but ruled out, as the shuttle had been flying too high and too fast for any attack from the ground. And sabotage before launch seemed impossible. Some observers suspected an explosion in one of the on-board fuel tanks, while others point to the possibility, however remote, of a collision with space junk.
Other avenues of investigation will include the flight computer, which maintains the craft at a precise re-entry angle. This ensures the heat shield on the shuttle鈥檚 underside bears the brunt of the fierce incandescent plasma created on re-entry, shading the rest of the craft. And the unusual temperature rise inside the left landing-gear well has prompted the theory that the impact could have opened the landing-gear door, allowing the fiery plasma to invade the craft.
While at pains not to rule anything out, Dittemore points out that the temperature rise would have been much higher if plasma at over 1000 掳C had entered the wheel well. Instead, the excess heat in the wheel well and fuselage may have been the result of heat conducted from a much hotter part of the wing. What might make it so hot? One possibility is that the foam impact had inflicted far more damage than NASA thought.
The agency鈥檚 attention is now focusing on that impact. Despite the post-launch all-clear, could Columbia鈥檚 heat shield have been breached, exposing the aluminium airframe? The heat shield is a matrix of tiles made of low-density fused silica, which can withstand temperatures up to 1200 掳C. Although tough, these tiles are brittle 鈥� and perhaps susceptible to the impact of a chunk of foam at Mach 2, particularly if the foam contained ice. It is possible that the debris hit the wing not at a glancing angle underneath, as NASA had assumed, but full-on at the leading edge. The energy imparted would then be much greater than from a glancing blow. And if such an impact penetrated the reinforced carbon-composite blanket beneath the tiles, plasma could have heated the metal frame like a blowtorch.
With a melting point of only 660 掳C, aluminium could not have lasted long under the onslaught of the plasma. As the left wing buckled, tiles could have been ripped from the surface. The same buckling would also explain the rapid increase in drag experienced on the left of the vehicle. Within seconds the spacecraft would have disintegrated.
It is too early to say whether the foam impact theory will prevail. While all avenues continue to be investigated, one thing is clear: NASA鈥檚 ability to assess risk is now under scrutiny as never before.
