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Spacewalk underway to repair electrical system
Posted: October 15, 2014

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.

Astronauts Reid Wiseman and Butch Wilmore will conduct Wednesday's spacewalk. Photo credit: NASA
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.

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.

First, Wiseman will remove and temporarily stow a camera on the stanchion that has a stuck zoom lens. The stanchion then will be disconnected, carried by Wilmore to the upper side of the Harmony module and installed at camera port 11.

While the stanchion is being moved, Wiseman will disconnect a wireless transmitter assembly from the top of the left-side truss at camera port 8 so it can be attached to the stanchion installed atop Harmony by Wilmore. The move will improve communications during a series of planned spacewalks next year to prepare forward ports for dockings by new commercial crew ferry craft.

Wiseman then will carry the camera with the stuck zoom lens back to the airlock, retrieve a fresh camera and install it on the top of the left-side truss where the wireless transmitter had been located.

The spacewalk includes a half hour for a variety of so-called "get ahead" tasks that could be carried out if time is available. The get-aheads include preparations for solar array battery replacements, retrieval of a camera light, retrieval of storage bags and ethernet cable routing.

The year's seventh and final planned spacewalk is scheduled for Oct. 22 when station commander Maxim Suraev and Alexander Samokutyaev plan to jettison no-longer-needed gear and carry out a detailed photo survey of the Russian segment's exterior.

NASA plans eight to 10 station spacewalks next year to prepare the lab for dockings by Boeing and SpaceX commercial crew ferry craft starting in 2017.

When the work is complete, the U.S. segment of the station will be able to accommodate two commercial crew vehicles and two U.S. cargo ships at the same time, giving NASA fully independent access to the orbiting laboratory for the first time since the shuttle's retirement in 2011.