Shuttle Repair Decision Didn't Come Easily
Wednesday, August 3, 2005
HOUSTON, Aug. 2 -- When NASA finally announced Monday that it would undertake an unprecedented spacewalk to repair the underside of the space shuttle Discovery, officials portrayed the job as almost routine. But the decision was anything but routine, and NASA needed four days to make it.
Even then, Discovery's astronauts had misgivings about touching the most sensitive and vulnerable part of the orbiter without being certain anything was wrong.
In the end, both Mission Control in Houston and Discovery's seven-member crew, flying 222 miles above Earth, agreed that the potential danger of the shuttle overheating during reentry outweighed the risk of making the first onboard, in-flight shuttle repair and of violating an agency taboo.
As spacewalks go, having Stephen Robinson pluck two pieces of protruding fabric from gaps in Discovery's thermal tiles was conceptually easy, NASA officials said Tuesday. It could probably be done in less than two hours by putting Robinson on board the international space station's 55-foot crane Wednesday morning and lowering him beneath the orbiter.
But the officials said the relative ease of the task did not carry any weight with decisionmakers at first. To do the job, Robinson would have to tinker with the orbiter's heat shielding, the shuttle's Achilles' heel. No one wanted to do that.
Discovery is built to withstand reentry temperatures of 2,300 degrees Fahrenheit on the belly tiles where Robinson will be sent, and much higher temperatures on the "reinforced carbon-carbon" that protects the leading edges of the orbiter's wings and its nose cap.
But no one would call it robust. Hit it with a hard object, or a fast-moving piece of insulating foam from the shuttle's external tank, and the shielding will shatter with stupefying results.
The shuttle Columbia disintegrated on reentry after a piece of foam hit heat shielding on its leading edge, causing NASA to ground the shuttle fleet for 2 1/2 years. Now the fleet is grounded again because Discovery's external tank lost a piece of foam during launch. It hurtled harmlessly into the void, but NASA officials acknowledged Discovery had dodged a bullet.
This was the conundrum facing engineers at Mission Control last Friday. New inspection equipment, installed after the Columbia disaster, had found two protruding "gap fillers," strips of ceramic-coated fabric, sticking out about an inch from cracks in the patchwork of thermal tiles beneath the orbiter.
Technicians had seen scores of protruding gap fillers before, but never in space, and seldom as long as an inch. NASA officials do not know if they are harmless shreds of insulation that can be ignored, or potential hazards that could interfere with the smooth cloak of white-hot gas that envelops the shuttle when it reenters Earth's upper atmosphere at 20 times the speed of sound.
"Trip" this "boundary layer" too early and the shuttle could build turbulent "hot spots" on the thermal undercoat, interfering with handling, or, worse, raising the heat of the shuttle's aluminum skin to unacceptable -- even catastrophic -- levels.
But in pre-inspection days, shuttles had flown with protruding gap fillers, and, as Paul Hill, Discovery's lead flight director, said Tuesday at a Johnson Space Center news conference, that appeared to be a good enough reason not to do anything. "You don't ever want to get close to the thermal protection system," he said.