This story was updated at 12:03 p.m. ET
NASA engineers are still perplexed by a stuck valve in the International Space Station's cooling system and may even add an extra spacewalk to fix it to the mission of shuttle Discovery astronauts, who celebrated a much-needed day off in orbit Wednesday while delivering tons of supplies to the orbiting lab.
The stubborn space valve is inside a new ammonia coolant tank delivered to the space station by Discovery's crew during three tricky spacewalks. Repeated attempts to open the valve using commands from Mission Control have been unsuccessful.
Space station flight director Ron Spencer said mission planners are contemplating extending Discovery's mission and adding an extra, fourth spacewalk to replace the faulty mechanism containing the valve.
A decision is expected sometime Thursday, Spencer said. Discovery is currently flying a 14-day mission to the space station and due to land April 19. The spacewalk repair would add another day to that schedule.
The space station uses nitrogen to pressurize its liquid ammonia coolant. Astronauts cannot fix the valve themselves because of its location inside the massive, refrigerator-sized tank.
Instead, they would have to replace the entire nitrogen tank portion of the ammonia coolant tank, which would require another spacewalk — either by Discovery's crew or the space station's own six-person crew, mission managers said. The space station has two spare nitrogen tanks on board.
NASA needs to fix or replace the nitrogen tank soon or else power down half of the space station's electronics to prevent them from overheating.
"It's actually half of the major power systems on the U.S. segment," station flight director Ed Van Cise said.
Shutting down half of the space station's power grid could force Mission Control to cut the number of astronauts aboard, from six astronauts to three, if the repair is not performed, Van Cise said. But it is far too early to discuss such a move, he added.
By late April, the space station's angle toward the sun will also be unfavorable for the radiators fed by the new ammonia tank, adding extra urgency to the repair, Spencer said.
Free time in space
While engineers work on the issue on the ground, the 13 astronauts aboard the linked shuttle Discovery and space station took a breather from their busy joint mission to delivery tons of supplies and equipment to the orbiting laboratory. They spoke with reporters on Earth and planned to take advantage of free time in space.
Astronaut Clayton Anderson, one of Discovery's two spacewalkers, said he hoped people on Earth tune into their flight even when there are no problems like the stuck nitrogen valve.
"What we do here is special. What we do here is difficult," Anderson said. "I'd really like to see everybody pay more attention to what we're doing up here regardless of whether things are going well or things are going poorly."
During the video link with reporters, the four women astronauts in the 13-spaceflyer group floated upside down above their crewmates. They are the most women in space at the same time.
"Really, it's a testament to the hard work and accomplishments of women, and we hope to inspire young women to follow in our footsteps and to pursue their dreams," said Discovery astronaut Stephanie Wilson.
Presidential speech on tap
But before the shuttle comes home, U.S. President Barack Obama will hold a space summit in Florida to discuss his new space exploration plan for NASA. The president initially cancelled the Constellation program building the replacement rockets and spacecraft to replace NASA's space shuttle after they retire later this year.
But he is expected to revive a stripped-down version of the Orion crew capsule developed in the Constellation program for use as an escape ship at the space station.
Discovery commander Alan Poindexter said his crew is trying to stay on top of the news from Earth, but remains focused on their mission to deliver tons of supplies and science equipment to the International Space Station. He wasn't sure if Mission Control would be able to beam President Obama's speech to the astronauts.
"We're staying abreast of most of the current news, but I don't know of any plans to tie us in for that at all," Poindexter said.
Discovery launched to the space station on April 5 and is due to leave the orbiting lab on Saturday, unless Mission Control decides to extend the flight for the extra spacewalk repair.
SPACE.com is providing complete coverage of Discovery's STS-131 mission to the International Space Station with Managing Editor Tariq Malik and Staff Writer Clara Moskowitz based in New York. Click here for shuttle mission updates and a link to NASA TV.