Multiple computer glitches aboard the International Space Station triggered intermittent communications blackouts early Sunday, officials said, and while the three command and control computers seem to be working, engineers do not yet understand what has caused the machines to repeatedly transition from one to the other.

The station's command and control software was updated before the shuttle Endeavour's flight to account for the new Tranquility module and it's possible the computer failures, or transitions, are software related. Issues with computers in Tranquility also have been noted, but it's not yet clear whether they are related to the command and control issues.

During normal operations, one of the three C&C computers, known as a multiplexer/demultiplexer, operates as the "prime" machine, allowing station astronauts and ground controllers to send commands to various systems and providing critical telemetry.

A second C&C computer operates in backup mode, ready to take over if the prime computer suffers a problem, and the third machine operates in standby in a domino-like software architecture. The computer system is critical to all aspects of space station operation.

Early Sunday, the prime C&C computer ran into a problem, forcing a transition to the backup. A NASA source familiar with the troubleshooting said the backup "came on and was prime for about a second and then it failed a diagnostic, the equivalent of the blue screen of death on a Windows machine. The third one, which was in standby, it came up and was prime for about an hour and just a little while ago it also transitioned and stopped work. The first one that went down, the original prime, it's back up right now."

"But now that this has cycled through all three machines it is very clear there is some generic, probably software related problem," he said.

NASA mission control commentator Pat Ryan outlined a slightly different sequence of events, saying when the prime computer malfunctioned "the backup computer moved assume its responsibilities and then the standby computer moved to backup. In the process, Houston lost S-band communications with the crew on board."

Commander Jeff Williams called down at 9:55 a.m. EST "and confirmed there had been a primary and backup failure of the C&Cs," Ryan said. "However, two of the three computers were healthy. Later this morning, just before (11 a.m.), there was data here in mission control that indicated there had been another C&C transition, this time still with just two computers healthy.

"But after a third transition this morning, all three computers were indicating they were healthy, although they were now operating in different slots among the primary, backup and standby computers. About 20 minutes ago (just after noon EST), space station commander Jeff Williams indicated there had been another transition, this one only impacting the backup and standby computers.

"But we are still in a situation currently where all three computers are healthy but the team here in mission control is still scratching its head and trying to determine what's been causing the repeated transitions," Ryan said. "There has been no impact to station life support systems while this was going on and the crew is in good shape."

As of 12:30 p.m., command and control computer No. 3 was prime, with C&C No. 2 in backup and No. 1 in standby.

"We're thinking we might need another day off," Williams joked with mission control as the troubleshooting continued.

"Copy and concur, Jeff," replied Stan Love in Houston. "We are talking in the room, we still do not know what has been causing these transitions. We are toying with the idea there might be something related to commanding. But we are not sure, it's just speculation at this point."

Love later said engineers had detected a fault in a computer system in the Tranquility module, but it was not immediately known if that was related to the C&C issue.

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