CAPE CANAVERAL, Florida (Reuters) - A problem with a cooling system aboard the International Space Station may prompt NASA to extend shuttle Discovery’s mission for an unplanned fourth spacewalk, NASA officials said on Wednesday.

During three previous outings, spacewalking astronauts Rich Mastracchio and Clay Anderson installed a new 1,700-pound ammonia cooling system and packed up an old unit for return to Earth and refurbishment.

But when engineers tried to remotely activate the new device, they were stymied by a stuck valve which is preventing nitrogen from pressurizing the system. The station has two spare tanks of nitrogen, but they cannot be installed robotically.

Managers on Wednesday were considering whether to keep Discovery in orbit an extra day and dispatch Mastracchio and Anderson for another spacewalk, or have two of the resident station crew members tackle the job after the shuttle’s departure.

“Folks are continuing to troubleshoot the problem,” said station flight director Ron Spencer.

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The cooling system dissipates heat from the station’s electronics. The station has a second cooling loop, but it is not enough to maintain proper temperatures for the entire, fully operational station.

Without cooling, NASA would have to shut down half the station’s electrical system. That might affect whether all six of the station’s live-aboard crew could remain aboard.

A decision on whether to extend Discovery’s stay at the station is expected by Thursday. The shuttle and seven astronauts blasted off on April 5 for one of the U.S. space agency’s final major cargo hauls to the station before the shuttle fleet is retired at the end of this year.

The 13-day flight already was extended by a day so the crew could use the station’s communications system to send images of a final heat shield inspection down to ground control teams for assessment. Equipment on the shuttle needed for this transmission is not working.

The inspection is to check for any orbital debris damage to the heat shield before the shuttle flies through the atmosphere for landing. Heat shield inspections were implemented after the shuttle Columbia was destroyed on reentry in 2003.