Spacewalk underway to repair electrical system

Astronaut Barry "Butch" Wilmore, left, and Reid Wiseman, right, suited up for a planned 6.5-hour spacewalk Wednesday to replace a critical electrical component on the space station's solar power truss. NASA TV

Astronauts Reid Wiseman and Barry "Butch" Wilmore floated outside the International Space Station Wednesday to replace an electrical component for one of the lab's eight solar power channels. They also plan to remove a broken camera, install a replacement at a different location and relocate a support mast and wireless transmitter assembly in preparation for spacewalks next year.

U.S. EVA-28 got underway at 8:16 a.m. EDT (GMT-4) Wednesday when Wiseman and Wilmore, floating in the Quest airlock compartment, switched their spacesuits to battery power. This is the second spacewalk in a week for the U.S. crew after an EVA last Tuesday to relocate a broken cooling pump, to replace a camera light and to install a component to provide backup power to the lab's robot arm transporter.

For identification, Wiseman, call sign EV-1, is wearing a suit with red stripes while Wilmore, EV-2, is using an unmarked suit. This is the 183rd spacewalk devoted to station assembly and maintenance since construction began in 1998, the sixth of seven EVAs planned for this year, the second for Wiseman and the first for Wilmore, a former shuttle pilot.

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The primary objective of the six-and-a-half-hour excursion is to replace an electrical component known as a sequential shunt unit, or SSU, that failed earlier this year, knocking out one of the eight solar power channels in the station's NASA-built electrical grid.

The space station is equipped with eight 110-foot-long solar wings that provide power to eight distribution channels. Each array is equipped with a sequential shunt unit to regulate voltage as the station moves into and out of Earth's shadow. The SSU in power channel 3A failed May 8, forcing flight controllers to re-route power from channel 3B to keep a variety of components in operation.

The EVA timeline is set up to ensure that Wiseman and Wilmore can remove the faulty SSU during a night pass when the arrays are not generating power. Once the replacement SSU is in place, flight controllers will reconfigure the electrical grid for normal operation across all eight power channels.

The astronauts, meanwhile, will press ahead with work to move a camera support mast from the lower side of the port-1 solar array truss segment to the top of the forward Harmony module. The relocation is required to clear a path for work next summer to robotically move a storage compartment from the bottom of the central Unity module to the forward-facing port of the left-side Tranquility module.

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Spacewalk underway to repair electrical system