NASA Engineers Ponder Temporary ISS Power Loss

NASA engineers are pondering the source of a day-long power loss aboard the International Space Station (ISS) Sunday that temporarily shut down science and communications equipment.

At 1:00 a.m. EST (0600 GMT) Sunday, a power channel in the station's Port 4 (P4) solar array electrical system shut down due to a glitch with an electrical distribution device known as a direct current switching unit, NASA officials said [image].

"The station's three crew members were not in any danger, but it did turn an off-duty day into a full work shift," the space agency said in a statement.

The power drop led to a temporary loss of ISS communications and the shutdown of heating units, some science equipment and one of three operating U.S. gyroscopes used for attitude control.

"Flight controllers restored power to nearly all affected systems and equipment by Monday morning," NASA officials said. "They are still investigating what caused the glitch, but believe it was an isolated event."

NASA officials said it took about 31 hours for flight controllers and the space station's Expedition 14 crew to restore ISS systems to normal.

"I wanted to congratulate you on a really well done job," Expedition 14 commander Michael Lopez-Alegria told flight controllers in a message rebroadcast on NASA TV after power was restored. "Hats off to you guys and thanks."

The ISS did fly in a two-gyroscope mode for attitude control, the minimum required to orient the station without consuming propellant for Russian thrusters, during the glitch. The space station has a total of four 620-pound (281-kilogram) Control Moment Gyroscopes, though one - CMG-3 - was powered down in October due to excessive vibration.

At no point did the ISS experience any loss of attitude control during the power loss, NASA said.

Delivered to the ISS in September 2006, the P4 solar arrays are the newest power-generating components aboard the orbital laboratory, and began feeding station systems in December during NASA's STS-116 shuttle flight to the orbital laboratory.

The arrays consist of two solar wings that reach out in opposite directions from their base on the P4 truss for a total wingspan of about 240 feet (73 meters). The arrays rotate to track the Sun and route electrical power through direct current switching units to the station's main bus switching units for distribution to ISS systems.

Each of the two P4 solar wings is designed to produce about 32.8 kilowatts of direct current power and together generate enough electricity to supply about 30 non-air conditions homes, according to a NASA description.

An older solar array segment, the Port 6 power module, currently extends above the ISS from atop the outpost's Unity node. Only one of that segment's two solar wings - the starboard array - is deployed. The port-reaching array was retracted during the STS-116 mission.

NASA's next shuttle mission - STS-117 aboard Atlantis - is slated to launch on March 15 to deliver the third of four solar arrays, part of its Starboard 3/Starboard 4 (S3/S4) payload, to the ISS.

When complete, the ISS will rely on four sets of U.S. solar arrays and eight power channels to supply space station systems with about 84 kilowatts. The final configuration will employ eight direct current switching units routing power through the station's four main bus switching units for distribution to power-hungry systems.

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