Astronauts Set for Toughest Spacewalk in Station Fix

Two astronauts will step outside the International Space Station on Saturday on what's expected to be the longest spacewalk of their mission, this one completely dedicated to cleaning metal grit out of a crucial solar array-turning gear.

Endeavour shuttle astronauts Heide Stefanyshyn-Piper and Steve Bowen will head out on the complicated job at about 1:45 p.m. EST (1845 GMT) and expect to spend at least seven hours working outside the space station.

"Right now, it looks like it's going to be pretty challenging tomorrow," Stefanyshyn-Piper told reporters Friday. "We have a lot of work to do."

Today's spacewalk will be the third of four aimed at cleaning the massive gear during Endeavour's 15-day mission to the space station. The shuttle also delivered a new crewmember, a recycling system that converts urine into drinking water and other vital equipment to help double the station's crew size up to six people next year.

While the mission's last two spacewalks included a variety of station maintenance tasks, today's excursion is reserved solely for the tune-up of a 10-foot (3-meter) wide gear designed to turn the space station's starboard solar wings so that they always face the sun to maximize power.

The gear hasn't worked properly for more than year due to damage from metal shavings caused from the gear grinding on itself.

Stefanyshyn-Piper and Bowen plan to replace five of the 12 large bearings that are spaced out around the gear's metal race ring surface. They also plan to wipe the ring surface clean of metal shavings and add a layer of grease to replace the lubricant lost over time. One of the bearings was replaced last summer, while five others were swapped out in spacewalks earlier this week, mission managers said. Another bearing will be removed to allow gear cleaning and then reinstalled, they added.

"Steve and I are going to be very, very busy out there cleaning the rest of the race ring and replacing the remaining trundle bearings," Stefanyshyn-Piper said.

The spacewalkers will also use a few unexpected tools, including greased-up wipe cloths and a makeshift grease gun cannibalized from the shuttle heat shield repair kit aboard Endeavour. The tools will make up for equipment lost during a Tuesday spacewalk when a $100,000 bag escaped from Stefanyshyn-Piper while she tried to clean the mess left from a leaky grease gun.

"I think the greased-up wipes seems to be the most practical way to go," said Stefanyshyn-Piper, the lead spacewalker for Endeavour's mission. "It requires a little bit more meticulous work because you don't have much grease there, but it is sufficient to get by."

Mission Control asked the spacewalkers to take a caulk gun-like tool from Endeavour and outfit it with a grease cartridge to hold in reserve to the greased-up wipes and remaining grease gun. The tool was initially designed to squirt out a black, heat-resistant material called NOAX to fill in dings and divots on a space shuttle's fragile wing-mounted and nose cap heat shielding.

"We would only use the NOAX gun that has been converted to a grease gun as a backup," space station flight director Ginger Kerrick radioed up to the crew from Mission Control in Houston.

Today's spacewalk will be the fifth career excursion for Stefanyshyn-Piper and the second for Bowen. Bowen and Endeavour crewmate Shane Kimbrough will perform the mission's fourth spacewalk on Monday.

NASA is providing live coverage of Endeavour's STS-126 mission on NASA TV. Click here for SPACE.com's mission coverage and NASA TV feed.

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