(CBS News) KENNEDY SPACE CENTER, FL--Floating in the Quest airlock module, space station astronauts Sunita Williams and Akihiko Hoshide switched their spacesuits to battery power at 8:16 a.m. EDT Thursday to begin a planned six-and-a-half-hour spacewalk.
The primary goals of the excursion -- the first NASA-orchestrated spacewalk since the shuttle's retirement last year -- are to replace a balky power switching unit on the station's solar array truss and to install cables needed by a Russian lab module scheduled for launch next year. If time is available, the astronauts will carry out a variety of "get-ahead" tasks, servicing external cameras and installing a protective shield over the lab's forward docking port.
For identification, Williams, call sign EV-1, is wearing a spacesuit with red stripes while Hoshide, call sign EV-2, is wearing an unmarked suit. Williams is responsible for the cable installations while Hoshide, who will spend most of the spacewalk anchored to the station's robot arm, will focus on the replacement of main bus switching unit No. 1.
This is the 164th spacewalk devoted to station construction and and maintenance since assembly began in 1998, the third so far this year, the fifth for Williams and the first for Hoshide. Going into the EVA, 108 astronauts and cosmonauts have logged 1,027 hours and 38 minutes -- 42.8 days -- of station EVA time.
"There (are) a couple of things that are really critical on the spacewalk," Williams said in a NASA interview. "One of them is replacing an MBSU, main bus switching unit. We have four of them on the space station, one of them hasn't been working quite a hundred percent for probably the last eight or nine months.
"We've been talking about trying to get this guy replaced. It's nothing critical at the moment, it just decreases some of our redundancy, and, of course, with a humungous space station that we have and all the laboratories that are running and all the power that's coming from the solar arrays, we like to have as much flexibility as possible. So we'd like to replace that MBSU."
Power generated by the station's eight solar array wings, four on each end of the lab's main truss, is routed to four main bus switching units, all located in the central S0 truss segment. Two power channels feed into each 220-pound MBSU, which in turn deliver 160-volt array power to a pair of DC-to-DC converter units. The DDCUs step the primary power down to the 124 volts used inside the station.
The MBSUs are critical to station operation, providing grounding and allowing flight controllers or station astronauts to crosstie power channels or to isolate them as needed when problems develop. MBSU No. 1 is delivering power, but it no longer responds to commands or provides detailed diagnostic information.
The second major objective of Thursday's spacewalk is to route cables from the U.S. segment of the station to the interface with Russia's modules that will provide power from the station's U.S. solar arrays to a Russian laboratory module scheduled for launch late next year.
The multi-purpose laboratory module will be attached to the Earth-facing port of the Zvezda command module after the Pirs docking compartment currently attached is jettisoned next year.