The spacewalk marks the first time that Russian Orlan space suits have been used to work on a U.S. segment of the craft, and also the first time mission controllers in Houston and Moscow will hand off ground support responsibilities.
"This is something that we'd always envisioned we'd like to do to give us added redundancies during spacewalks," said Mike Suffredini, station manager of integration and operations, during a briefing on the upcoming event held today at NASA's Johnson Space Center in Houston.
The job
During their spacewalk, Padalka and Fincke will replace a device called a remote power controller module (RPCM), which serves as both power source and circuit breaker for one of four gyroscopes used to orient the space station. That module failed on April 21, less than a day after the Expedition 9 crew arrived, and cut off power to its gyroscope.
Among the three other gyroscopes, one is already broken and must be replaced, leaving only the minimum two needed to keep the station oriented without relying on Russian thrusters.
"If we lose one more gyroscope we'll have to use a lot of [thruster] fuel," Fincke explained from his orbiting home during a June 15 mission update. "We really don't want to get into that situation, so we're going outside."
Using those thrusters could also be hazardous for the Expedition 9 crew during two additional spacewalks planned for later in their mission.
"We don't want to have to fire those jets while the crew is out there doing these spacewalks," said John Curry, NASA's ISS EVA flight director, during the briefing.
A dress rehearsal
To prepare for what mission planners expect to be a six-hour spacewalk, Padalka and Fincke donned their Russian-built Orlan space suits for about three hours today inside the station's Russian Pirs docking compartment.
The dress rehearsal gave the two astronauts a chance to practice manipulating the U.S. tools they will use during their repair, including a pistol-grip tool to unscrew bolts and a body restraint tether that has not been used on Russian space suits before.
To provide better lighting during their spacewalk, ISS astronauts have installed lights on their Orlan helmets that are normally used on U.S. space suits. The astronauts will also use an Orlan tether adapter to carry American tools, Fincke said.
Spacewalk planners originally hoped to use U.S.-built Extravehicular Mobility Unit (EMU) space suits for this week's repair job. But during checkouts last month, Fincke and Padalka found that two out of the three EMUs had coolant system problems.
"Those suits were past their expected lifetimes and needed to be refurbished, and we just couldn't get them up and running in time," Fincke said.
Suffredini said that EMU coolant-system replacement parts are expected to be sent aboard the next supply ship, Progress 15.
EVA outline
Under the guidance of Russian mission controllers, Padalka and Fincke plan to step outside the station's Pirs docking compartment at about 5:50 p.m. EDT (2150 GMT), traveling along the outstretched Russian Strela crane to reach the U.S. section of the sprawling complex. There, U.S. station ground controllers will monitor the Expedition 9 crew as they make their way, hand-over-hand, to the truss housing the faulty RPCM.
"They will be some 40 meters away from the station's service module," Curry said. "That will be the furthest away from that area we've been."
Orlan space suits require a clear line of sight with the Russian-built Zvezda service module to provide communications not only between astronauts and the ground, but between the astronauts themselves. With so much of the station between the Expedition 9 crew and Zvezda, it is possible that Padalka and Fincke may lose communications during their spacewalk.
But ground controllers and the astronauts have adopted a series of hand gestures to use in case they suffer radio silence. An area nearby their work site - with a clear line of sight to Zvezda module - has also been set aside as a communications outpost between the crew and the ground, NASA officials said.
Once the faulty RPCM has been swapped out with a replacement and U.S. mission controllers have verified it is working properly, Fincke and Padalka will make their way back to the Pirs docking component and should reenter the ISS at about 11:50 p.m. EDT (0350 a.m. June 25 GMT).
"We feel that everything we're doing is very safe and that the station will be okay," Fincke said.
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