Hours after spacewalkers replaced a broken gyroscope on the international space station, NASA on Monday gave them an even more challenging task: making the first-ever repair on the underbelly of a shuttle in orbit.
During a Wednesday spacewalk, Discovery's astronauts will fix two worrisome pieces of filler material protruding from the shuttle's protective tiles, deputy shuttle program manager Wayne Hale told reporters.
Astronauts have never ventured under an orbiting shuttle before, and have never attempted to fix their ship’s thermal shielding in flight. But Hale said experts had put together a "very simple plan" for making the fix.
The ceramic-fiber cloth that is used to fill the gaps between Discovery’s thermal tiles is sticking out about an inch (2.5 centimeters) or less in two spots. Some engineers worried that the material might disturb the air flow enough during Discovery's re-entry to cause a dangerous heat buildup beneath the spaceship. Others said the dangling material could burn off or roll up harmlessly, as it apparently has done on past flights.
"At the end of the day, the bottom line is that there is large uncertainty, because nobody has a very good handle on the aerodynamics at those altitudes and those speeds," Hale said. "Given that large degree of uncertainty, life could be normal during entry, or some bad things could happen."
Therefore, after three days of intense discussion, mission managers decided to go ahead with the operation to "set our minds at rest," he said.
Hale said the decision reflected the heightened safety awareness at "the new NASA," developed in the wake of the Columbia tragedy two and a half years ago.
"If we cannot prove that it is safe, then we do not want to go there," he said. "This exceeded our threshold and we needed to take action."
Risk of accidental damage
The astronauts will add the repair task to a previously scheduled spacewalk. It would be a largely unrehearsed operation, and a high-stakes one, too, with the risk that the astronauts might accidentally damage Discovery’s fragile thermal shield and make matters worse.
“The biggest thing that everybody’s concerned about is doing no harm,” said Mission Control spacewalk officer Cindy Begley.
Nevertheless, she said she was not overly concerned about the potential repair. “It’s not actually that bad,” she said.
The plan calls for astronaut Stephen Robinson to perform the operation while perched on the end of the space station’s 58-foot (18-meter) robot arm, which would bend and wrap around the side of Discovery to enable him to reach all the way underneath.
Robinson would first try to tug the dangling strips out with his gloved fingers. If that did not work, he would use a saw to cut them off while holding the material taut with forceps.
Discovery’s other astronauts and Mission Control would see him the whole time via robot arm cameras, but he would be out of sight of his spacewalking partner, Soichi Noguchi, who would be busy elsewhere doing other things. NASA decided two astronauts would be too many for the work site and might cause too much banging around.
NASA had planned to send Robinson and Noguchi out anyway Wednesday to perform some final space station work, including the installation of a stowage platform.
Discovery lifted off with a crew of seven July 26 on the first shuttle flight since the Columbia disaster 2½ years ago. Columbia broke up on re-entry after the wing was pierced during liftoff by a chunk of foam insulation that fell off the big external fuel tank, killing all seven of the shuttle's astronauts. The tragedy led NASA to ground the shuttle fleet for an extensive redesign of the tank and the orbiter.
But with so much focus on flyaway foam and possible nicks and gouges to the shuttle’s thermal shielding, protruding gap fillers were not “one of the big-ticket items” studied before Discovery’s flight, said mission operations representative Phil Engelauf.
The current mission has had foam problems as well: A large piece of insulation broke away from Discovery's fuel tank during its ascent, surprising the experts and leading NASA to put future shuttle flights on hold. NASA Administrator Mike Griffin has set up a "tiger team" of engineers to focus on the foam problem, and that team is expected to report back in three weeks.
During Monday’s seven-hour spacewalk, Robinson and Noguchi pulled out a 600-pound (273-kilogram) gyroscope that stopped working three years ago. They installed a new one in its place and, after two attempts to hook it up, got power flowing to the unit.
Until Discovery arrived last week, the space station had been flying with only two good gyroscopes, the bare minimum needed to keep the complex stable and pointed in the right direction.
During the mission’s first spacewalk on Saturday, the spacewalkers rewired a third gyroscope and got it working again, and so Monday’s swap left the station with four working gyroscopes. Engineers suspect a lubrication problem with the bearings may be plaguing the gyroscopes, but will know for sure once the broken unit is returned to Earth aboard Discovery next Monday.
This report includes information from MSNBC.com and the Associated Press.