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NASA mulls 'black box' failure aboard space station
Posted: April 11, 2014

A computer "black box" in the International Space Station's solar power truss that provides redundancy for robotic arm operations stopped responding to commands Friday, NASA said in a statement. If the component cannot be coaxed back to normal operation, the planned Monday launch of a SpaceX Dragon supply ship -- which must be captured by the station's robot arm -- could be delayed.

File photo of the International Space Station's robotic arm. Photo credit: NASA
The avionics box, known as a multiplexer-demultiplexer, or MDM, is mounted in the central S0 section of the station's power truss. Spares are available, but a spacewalk would be required to install a replacement, assuming flight controllers at the Johnson Space Center in Houston are unable to resolve the problem remotely.

"The mission control team at NASA's Johnson Space Center in Houston confirmed Friday night that a backup computer on the exterior of the International Space Station called a multiplexer-demultiplexer is not responding to commands," NASA said in a brief statement.

"The station's MDM computers control some systems associated with robotics aboard the space station. The primary MDM operating aboard the space station is functioning normally and there is no immediate impact to space station operations. The computer outage does not pose a risk to the six crew members aboard the space station."

Flight controllers are studying what might be needed to restore the unit to normal operation or what might be required to replace it if recovery efforts are not successful.

"Replacing the backup MDM, if needed, would require a spacewalk," the NASA statement said.

The apparent failure occurred Friday afternoon, amid work at the Cape Canaveral Air Force Station to prepare a commercial SpaceX Dragon cargo ship for launch to the space station atop a Falcon 9 rocket. Liftoff is targeted for 4:58 p.m. Monday and if all goes well, the uncrewed supply ship will reach the lab complex around 7:11 a.m. Wednesday.

But unlike supply ships built by the Russians and the European Space Agency, the Dragon is not able to dock on its own. Instead, the station's robot arm, operated by astronauts inside the space station, is used to lock onto a grapple fixture so the capsule can be moved into position for berthing at the Earth-facing port of the forward Harmony module.

While the Canadian-built robot arm remains fully functional, the loss of redundancy due to the apparent MDM failure could prompt NASA mission managers to consider a launch delay for the SpaceX/Dragon mission if the MDM cannot be restored to normal operation. But pending additional troubleshooting, the SpaceX launch remains on track for Monday, with forecasters predicting a 70 percent chance of acceptable weather.

"The backup MDM would provide redundancy for robotic systems that will be needed to attach the SpaceX Dragon spacecraft currently scheduled to launch on Monday and rendezvous with the ISS on Wednesday," NASA said in its statement. "NASA is continuing to work toward a Monday launch of the SpaceX cargo resupply mission pending further evaluations by the ISS Program."

The SpaceX cargo craft is loaded with 4,600 pounds of equipment and supplies, including spacesuit components intended to prevent any recurrence of a potentially catastrophic helmet water leak that occurred during a spacewalk last year.

If a spacewalk is required to replace the suspect MDM, two station astronauts, presumably Rick Mastracchio and Koichi Wakata, would use spacesuits currently aboard the station, following safety procedures implemented for two excursions last December to replace a faulty cooling system component.

Those spacewalks were successful, and there was no recurrence of the leak that occurred during a July 2013 spacewalk by European Space Agency astronaut Luca Parmitano.