(CBS/AP) NASA canceled a spacewalk Wednesday as it scrambled to deal with two power problems at the international space station.

The spacewalk set for Thursday was supposed to deal with a malfunctioning rotary joint for the solar wings on the right side of the space station. Instead, the astronauts were informed that their next spacewalk would be Friday - at the earliest - and involve work with the station's ripped solar wing.

Both issues are competing for the precious little spacewalking time that's left in Discovery's mission, which already was extended a day after the joint problem cropped up last weekend. The problems could delay future missions and make it even harder to finish building the orbiting outpost before the space shuttles must be retired.

The next shuttle mission is scheduled for the first week in December when Atlantis is expected to deliver a European research lab to the space station. CBS News space consultant Bill Harwood explained that extending the Discovery mission poses a special challenge for the space station's crew.

"Discovery's mission has already been extended one day," Harwood said. "They could extend it more to deal with this solar array problem, the new problem, but they don't want to do that if they don't have to. They really need to get Discovery down so the station crew can finish all this work that has to be done to prepare the station for the arrival of these new research labs."

Five spacewalks had been scheduled for Discovery's space station construction mission, the most ever attempted for a mission like this. In the 26-year history of space shuttle flight, five spacewalks have been performed only on Hubble Space Telescope repair missions.

At a news conference earlier Wednesday, shuttle commander Pamela Melroy said that her crew is ready to tackle whatever repairs are ordered - even if that means extending the mission again and adding another spacewalk.

"I think we're kind of in the groove right now, so if the ground decides that's the right thing to do and they ask us to do it, we'll be ready to support it," Melroy said.

Until they were informed of the switch in plans, astronauts Scott Parazynski and Douglas Wheelock were getting ready to spend the mission's fourth spacewalk Thursday thoroughly inspecting a malfunctioning rotary joint that keeps the station's solar panels turned toward the sun. A fifth spacewalk had been scheduled for Saturday for additional space station construction work, and will likely have to be put off as well.

NASA generally needs at least one day in between spacewalks to allow the astronauts to rest and get the space station prepared for the outings.

The solar wing damage occurred as it was being unfurled Tuesday. The tear forced NASA to halt the process before the wing was fully extended.

Until at least one of the problems is resolved, the station won't be able to generate enough power to support new equipment, such as a European lab that is supposed to be delivered by Atlantis in December. Delaying that mission would set back other deliveries, including the planned February installation of a new Japanese lab.

NASA is up against a hard 2010 deadline for completing the space station and retiring the three remaining shuttles.

The solar panel ripped just after Parazynski and Wheelock finished a seven-hour spacewalk to install the beam that holds the wings. Deploying the damaged wing's twin went off without a hitch.

Astronauts took hundreds of pictures of the wing tear, but NASA engineers couldn't tell what caused the damage, space station flight director Heather Rarick said late Tuesday.

"Until we know what we think the cause is, maybe until we get some better pictures, I don't think we really have any solid leads on how to fix it yet," Rarick said.

Astronaut Daniel Tani said he noticed a second, smaller tear near the 2½-foot rip while he was taking additional pictures Wednesday.

NASA also wasn't sure about the cause of the rotary joint problem. Steel shavings were found during a spacewalk over the weekend in the joint on the right side of the station. Until NASA figures out what's grinding inside the gears and fixes it, the right joint will remain in a parked position as much as possible, limiting power collection.

Discovery currently is scheduled to undock from the station on Monday and land on Wednesday.

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