Two station astronauts set for NASA spacewalk

CBS News

Space station astronauts Sunita Williams and Akihiko Hoshide wrapped up final preparations Wednesday for a planned six-and-a-half-hour spacewalk Thursday to replace a balky power switching unit on the lab's solar array truss and install cables that will route electricity to a Russian lab module scheduled for launch next year.

"This particular (power distribution) box has failed in such a way that it's passing power, but it can no longer be switched," said spacewalk officer Kieth Johnson at the Johnson Space Center in Houston. "In order to do future operations, we need to take that box out and put one in that allows us to do that.

"Another task we're doing on the EVA is cable routing, two primary and redundant power cables to the Russian multi-purpose laboratory module. We start on the U.S. segment and run it over to the interface between the Russian and U.S. segments. Then the Russians later on will run their half of that cabling."

For identification Williams, call sign EV-1, will be wearing a spacesuit with red stripes while Hoshide, call sign EV-2, will wear an unmarked suit. The excursion is scheduled to begin around 8:15 a.m. EDT (GMT-4) Thursday when the astronauts, floating in the U.S. Quest airlock module, switch their suits to battery power.

In this computer graphic, Akihiko Hoshide, anchored to the end of the space station's robot arm, pulls a malfunctioning electrical distribution box -- MBSU 1 -- from its mount on the forward face of the lab's solar power truss. The robot arm and its mobile transporter are anchored to work site No. 4. (Credit: NASA)
This will be 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.

Williams, Hoshide and Yuri Malenchenko were launched to the station aboard a Russian Soyuz spacecraft on July 14, joining Expedition 32 commander Gennady Padalka, Sergei Revin and NASA astronaut Joseph Acaba.

A veteran Navy helicopter pilot, Williams will replace Padalka as commander of the outpost later this fall. She spent 195 days in space during a 2006-2007 stay aboard the outpost, logging 29 hours and 17 minutes of spacewalk time during four EVAs. Hoshide, an astronaut with the Japan Aerospace Exploration Agency, visited the station during a 13-day shuttle mission in 2008.

"There's 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.

Williams will focus on the cable installation work while Hoshide, spending most of his time anchored to the end of the station's robot arm, replaces MBSU No. 1. If time is available, the astronauts will remove a balky camera from the robot arm, replace another camera on a Japanese experiment platform and install a protective cover over the station's forward docking port.

"So the spacewalk is a little bit split, it's actually two spacewalks, which we combined into one," Williams said before launch. "One spacewalker will be primarily doing the MBSU ... and then the other crew member will be laying the cables from the S0 (truss segment) all the way back toward the FGB (Russian Zarya module). So it'll be great if we can get all that work done."

Johnson said Williams and Hoshide practiced most of the work in NASA's giant EVA swimming pool near the Johnson Space Center that simulates the weightless environment of space. Even though the training took place months ago, and even though some of the details have changed, Johnson said the astronauts are up to speed.

"The crew has seen this in the water several times, so they've got good training," he said. "But what ends up happening, we change priorities and we change the tasks that they looked at in their training a while ago. But we think that by getting them the right information and talking with them before the EVA that we've minimized that aspect of the EVA."

Even so, he said, "we want to make sure we don't let the crew run long on a task. We've timelined it so that we can stop at various points to get them inside. We're comfortable with that."

Williams will exit the Quest airlock first and Hoshide will pass out tool bags and tethers. External stowage platform No. 2, where the spare MBSU is mounted, is just forward of the airlock hatch. After attaching tool bags to a tether, Williams will attach a handling aid to the replacement MBSU, fold insulation blankets back and use a power tool to loosen two bolts holding it in place.

She then will take the cable bundle and make her way forward to the interface between the Destiny laboratory module and the multi-hatch Unity compartment where cables carrying power from the U.S. solar arrays converge before entering the pressurized modules. She then will attach one end of a cable and route the rest aft toward the Russian segment.

Hoshide, meanwhile, will carry tools up to the forward face of the solar array truss where MBSU No. 1 is mounted and loosen the two bolts holding it in place. He'll then attach a handling fixture, connect a foot restraint to the station's robot arm and "climb aboard," anchoring his feet.

In a final test, flight controllers will power the MBSU down and back up to help diagnose its current condition. Then it will be powered down and Hoshide will remove it from its mounting plate.

Acaba, operating the arm from the multi-window cupola module, will move Hoshide and the MBSU you down to external stowage platform No. 2

Williams, meanwhile, plans to attach the second MLM cable and route it forward to the solar array truss, floating into the interior and plugging it into a power socket. By that point, Hoshide should be back at ESP 2, where he plans to temporarily stow the failed power unit before removing the replacement from its mounting plate.

Acaba will drive Hoshide and the replacement MBSU back up to the forward face of the S0 truss. Williams, after wrapping up work inside the truss, will join him for the actual installation, helping Hoshide line up the big box and bolt it down.

If time is available, Hoshide will remove a balky camera from from a mast on the arm's mobile base system and get off the arm before replacing a failed camera on the Canadian-built space crane. Williams, meanwhile, will be working to finish mounting MBSU No. 1 on the external stowage platform.

The astronauts also hope to replace another camera on a Japanese experiment platform and install a debris shield over the station's forward docking port.

Here is a timeline of major EVA events (in EDT and event elapsed time):
08:15 AM...00...00...Spacesuits to battery power
08:20 AM...00...05...Egress and setup
09:00 AM...00...45...EV1: Spare MBSU prep/worksite setup
09:00 AM...00...45...EV2: SSRMS (robot arm) setup
09:45 AM...01...30...EV1: Route Russian lab (MLM) power cable 413 (PMA-Node 1)
10:10 AM...01...55...EV2: Prep and remove failed MBSU
10:30 AM...02...15...EV1: Route MLM power cable 413
11:05 AM...02...50...EV2: SSRMS maneuver; retrieve/install spare MBSU
11:30 AM...03...15...EV1: Assist with spare MBSU install
11:55 AM...03...40...EV1: Route MLM power cable 651
12:25 PM...04...10...EV2: SSRMS cleanup and boom prep
12:40 PM...04...25...EV1: Assist EV 2 with CLPA replacement
12:55 PM...04...40...EV2: CLPA R&R
01:00 PM...04...45...EV1: Install failed MBSU on ESP-2
01:40 PM...05...25...EV2: Get aheads
01:45 PM...05...30...EV1: Get aheads
02:00 PM...05...45...Cleanup and airlock ingress
02:45 PM...06...30...Airlock repressurization