NASA asked its orbiting astronauts Monday to take a closer look at the gears that control the international space station's solar wings to try to find out what's grinding inside and causing steel chips to clog the system.
The impromptu work — "exploratory surgery" as the station's program manager calls it — will keep shuttle Discovery in orbit an extra day.
During a spacewalk already planned for Tuesday, astronaut Scott Parazynski will inspect a good rotary joint that turns the space station's left set of solar wings toward the sun.
NASA wants to see what a perfectly running unit looks like to compare it to the one that is acting up on the opposite side.
On Sunday, spacewalker Daniel Tani found black dust that resembled metal shavings inside the motorized joint that controls the right set of solar wings. He said Monday that it was instantly apparent to him something was wrong.
The dust was everywhere, and the spinning mechanism was not shiny and clean like it should have been for something launched just four months ago, Tani said.
At least some of the shavings are steel and could be from the bearings inside the joint, said Mike Suffredini, NASA's space station program manager.
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At Suffredini's request, shuttle mission managers approved a more detailed inspection of the troublesome right joint. The time-consuming work will be conducted Thursday during the fourth spacewalk planned for Discovery's construction mission.
To make room for that inspection, managers had to drop a shuttle thermal tile repair demonstration that had been scheduled for that spacewalk. The test was hurriedly added to the mission after a piece of fuel-tank foam gouged Endeavour's belly on the last shuttle flight in August.
Discovery's commander, Pamela Melroy, said it's tough for the astronauts to make big changes like this it the last minute.
"There's a lot of choreography that goes into a spacewalk. We like to practice them and mentally rehearse them many, many times, and so it is difficult to put something together like this," Melroy said in a series of TV interviews. But she noted that her crew was ready to tackle the job.
Tuesday's inspection of the good left gear will take up just a small portion of the spacewalk. The primary goal will be to hook up a giant beam and its attached solar panels — folded up like an accordion — to their new location on the space station.
These panels, once unfurled, will be controlled by the left rotary joint.
Installing that beam and extending those solar panels to their full 240 feet has taken on added importance because of the rotary joint problem. A significant power shortfall would delay the arrival of European and Japanese laboratories on the verge of launching.
Thursday's inspection of the bad right joint will fill the entire spacewalk. Astronauts will have to remove all but one of 22 thermal covers in an effort to see where metal is grinding. Tani has already checked behind that one remaining cover.
The joint worked perfectly when it was installed during a shuttle visit in June. It began vibrating and experiencing electrical power spikes two months ago, and is now limited in use.
Any repairs would be put off until after Discovery departs.
"I don't think we're in any situation we can't recover from. It's just a matter of time," Suffredini said.
Discovery is now scheduled to undock from the space station on Monday and return to Earth on Nov. 7.