It was not immediately clear how much longer the crew could spend on the trouble, before having to quit for the day. By late morning, flight controllers still had no idea what was wrong.
Mission Control said there was still a possibility the cargo carrier could be unberthed from the space station and placed back aboard Discovery on Thursday. Otherwise, the job will have to be put off until Friday, which could keep the shuttle docked an extra day.
The cargo carrier, filled with trash and old equipment, needs to return to Earth aboard Discovery so it can be outfitted and fly back up in September on the last shuttle flight.
Earlier Thursday, Mission Control managed to clear another space station problem, at least for the time being. Managers ruled out the need for an emergency spacewalk by the shuttle crew to fix a stuck valve in a cooling loop; that would have required a flight extension for Discovery.
The valve repairs can wait until after Discovery leaves, engineers concluded following two days of analysis.
The newest problem is apparently with one of the control panels that send commands to release the bolts that lock the cargo carrier in place on the space station. Flight controllers noticed strange electrical signals coming from the panels and, indeed, the astronauts found a sheared screw. The astronauts taped the screw in place and double- and even triple-checked all the electrical connectors. Each connector was "rock solid," reported astronaut Clayton Anderson.
"Sounds like a Party, Dude"
Controllers huddled in Mission Control as the problem dragged on, apologizing to the astronauts for the slow step-by-step instructions.
"I'm going to let you know that in my line of sight right now, I have seven flight directors talking about this," Mission Control radioed.
"Wow, that sounds like a party, dude," Anderson replied.
The cargo carrier flew up on Discovery, filled with fresh station supplies and science experiments. The next time it flies, it's supposed to remain permanently at the space station and serve as an extra closet.
As for the jammed valve, engineers on the ground determined that the space station can operate fine for the next few weeks with the jammed nitrogen valve, a critical part of the cooling system. That gives them more time to figure out how to resolve the problem.
On Wednesday, Mission managers had considered adding a fourth spacewalk to Discovery's mission to address the problem.
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If flight controllers cannot get the valve to open by remote control, the space station crew will have to perform a spacewalk to replace the entire nitrogen assembly. Nitrogen is needed to pressurize the tank full of ammonia coolant that was installed by the shuttle crew earlier this week
Mission Control said it's too soon to know when those spacewalking repairs might occur. Space shuttle Atlantis is scheduled to arrive at the space station in mid-May with more spare parts and a small Russian lab. The valve job, if required, could take place before, during or even after that mission, officials said.
The valve failed to open Tuesday after spacewalking astronauts finished hooking up the new ammonia tank.
Discovery is scheduled to undock from the space station Saturday and return to Earth on Monday.
Back at NASA's launch and landing site, meanwhile, hundreds of journalists and dignitaries gathered for an afternoon visit by President Barack Obama.
Obama will pitch his post-shuttle plans for NASA, including development of a capsule for ferrying astronauts to the space station. That spacecraft originally was canceled by the president, but revived recently following an outcry from lawmakers and space enthusiasts. Obama also wants U.S. commercial rocket builders to become involved in space station operations.
Once Discovery lands, only three shuttle flights will be left. NASA hopes to wrap up the last mission along with space station construction by the end of September.
Poindexter said from orbit Wednesday that he knew of no plans to beam up the president's speech, real time, to the 13 space fliers.