The investigation into catastrophic loss of the space shuttle Columbia and its crew of seven on Saturday morning is narrowing in on the spacecraft鈥檚 left side, which became abnormally hot just before contact was broken with mission control.
NASA stresses that the cause of the tragedy remains unknown. But the most probable explanation based on current knowledge is that heat-resistant tiles on the underside of the shuttle鈥檚 left wing were significantly damaged by debris during take off. This could have caused the intense frictional heat of reentry to the Earth鈥檚 atmosphere to penetrate the shield and weaken the wing. Travelling at about 20,000 km/h, the shuttle would have rapidly disintegrated.
Other less likely possibilities include structural failure, a collision with a piece of space junk or loss of control of the shuttle鈥檚 attitude. An act of terrorism is now thought to be an extremely remote possibility.
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Whatever went wrong, it went wrong suddenly. At 0815 EST on Saturday, Commander Rick Husband and pilot William McCool fired small on-board rockets to slow Columbia鈥檚 orbit and begin its final descent. They then turned the spacecraft around and tilted it so its undersurface, covered with the thick insulating ceramic tiles, would take the brunt of the heating.
By 0853, as the shuttle passed over California, the problems had begun. Caltech astronomer Carmen Sanchez Contreras, watching from Owens Valley, spotted Columbia apparently shedding debris.
鈥淲hen the shuttle was midway across the valley, I saw a second bright spot, much smaller, that was suddenly left behind by the shuttle,鈥� she told 快猫短视频 鈥淏ut it didn鈥檛 look like a big explosion to me.鈥� Contreras鈥檚 report is significant, said Ron Dittemore, space shuttle program manager, at a press conference on Sunday.
Signal cut
NASA鈥檚 first hint of any problem came at about the same time. Signals from four temperature sensors on the left wing stopped suddenly, 鈥渁s if someone had cut the wires鈥�, Dittemore said. Other sensors showed abnormal heating in the left wheel well.
A minute later, another sensor showed that the left side of the fuselage had warmed by 60 掳C in five minutes, compared to the normal 15 掳C rise on the right side.
At 0858, as the shuttle sped over New Mexico, the control system began tilting the shuttle, apparently in response to increased drag on the left side. Controllers alerted the crew, who responded 鈥渞oger鈥� 鈥� but were then cut off abruptly.
At almost the same time, people watching the shuttle streak over Texas saw pieces break off, followed by a flash and a thundering explosion. Debris began raining down from the sky.
A key point may be that the disaster occurred close to the most stressful point of the descent. Columbia was tearing through the air at 18 times the speed of sound and at 63 kilometres altitude, its exterior heated as high as 1600 掳C.
More than 1200 pieces of debris spread over a large area had been reported to authorities as of Sunday. NASA wants to recover every piece possible to ensure no clue escapes them.
Wing impact
The overheating of the left side suggests damage to the highly heat-resistant but fragile tiles. Tile loss and damage were problems on early shuttle launches, but NASA had thought they had solved the problem.
Investigators are paying special attention to an incident during Columbia鈥檚 launch on 16 January. About 80 seconds after lift-off a piece of insulating foam from the shuttle鈥檚 external fuel tank hit the shuttle鈥檚 left wing.
Mission controllers did not notice the impact until the day after launch, and their cameras did not record much detail. Dittemore said that, at the time, NASA engineers believed the incident was 鈥渋nconsequential鈥� and did not threaten flight operations or safety. They are now reassessing that possibility.
Yet he also admitted NASA could have done nothing if even they thought the damage was serious. Columbia was not equipped for spacewalking astronauts to examine the underside of the shuttle, and the crew had no way to repair the tiles. NASA had no other way to reduce heating on re-entry, and the astronauts had no other way home.
NASA hopes to get more information from a final 32 seconds of Columbia鈥檚 flight data, that its computers initially rejected as corrupted, as well as videotape and photographs of the shuttle coming apart.
The loss of Columbia leaves NASA facing a host of uncertainties: immediately, what went wrong; in the short term, how to service the International Space Station; in the long term, how and when to replace the ageing space shuttle fleet.
NASA has commissioned an independent panel headed by retired US Navy admiral Harold Gehman, with members from the military officers, the Federal Aviation Administration and the Department of Transportation. Their first meeting is to be on Monday.