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NASA mission managers will add an extra day to the mission of the space shuttle Discovery so crewmen can do “exploratory surgery” on a malfunctioning part of the International Space Station’s power system, the space agency announced on Monday.

The problem concerns a rotary joint on the right side of the station that turns the station’s solar arrays so they face the sun during orbit. During an initial examination on Sunday, the spacewalking astronaut Daniel M. Tani collected what he called “metal-to-metal scraping” that peppered the inside of the joint mechanism. Tani picked up some of the fragments with tape and took them back into the space station.

Mission managers hoped the shavings were aluminized mylar, a foil used as backing on the insulating covers, and not steel, which would suggest that the mechanism is grinding against itself. They asked Peggy A. Whitson, the space station commander, to conduct an experiment of putting some of the filings on a piece of paper and running a magnet underneath.

Aluminum is not magnetic; steel is. The filings “followed the magnet around,” Michael T. Suffredini, the space station program manager, said on Monday morning in a briefing with reporters.

Mission controllers had known of the problem for several weeks, when a mission control employee contractor noticed an unusual vibration. Station managers have decided to lock the array in a position that allows it to collect a fair amount of sunlight until the problem can be fully investigated and resolved.

That move will keep the joint from literally grinding to a halt, but will limit the amount of electricity the station can generate for itself and an attached shuttle. That is not a problem for the current mission, Suffredini said, or for the next mission, in December, when a new science module will be brought up and attached.

But if the problem is not corrected and the rotation resumed, further construction could be constrained by the ability to provide power.

“I don’t think we’re in any situation we can’t recover from,” Suffredini said. “It’s just a matter of time.”

Suffredini announced two initial moves to get a better sense of the problem. On Tuesday, another spacewalker, Scott E. Parazynski, will peek inside the opposite rotary joint on the left side, which is running smoothly, to help engineers and scientists “figure out what normal means” and compare that with what they are seeing on the right side.

Then the fourth spacewalk, which was to be a demonstration of shuttle repair techniques, will be extended to six hours or more and devoted entirely to taking each of the 22 insulating covers off the right joint and examining the mechanism — a process that Suffredini called “exploratory surgery.”

The problems of the rotary joint came to the fore during a mission that has otherwise gone smoothly. The day’s activities included an agile robotic handoff between the space station and shuttle robotic arms of a solar array and truss to relocate the 17.5-ton assembly to its permanent home on the station’s left side.