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updated 7/12/2011 10:48:10 AM ET 2011-07-12T14:48:10

This story was updated at 9:22 a.m. EDT.

HOUSTON – Two space station astronauts floated outside of the International Space Station today (July 12) to perform the only planned spacewalk during the shuttle Atlantis' final visit to the orbiting outpost.

Today's excursion is the final spacewalk conducted with a visiting shuttle crew present, but it is being performed by space station residents Ron Garan and Mike Fossum. This will be the final time for the spacewalkers to enjoy views of the shuttle Atlantis parked at the complex during their time outside the station.

The last spacewalk taken by members of a shuttle crew occurred during the last shuttle mission, Endeavour's STS-134 mission, which landed on June 1 after its own final visit to the station.

Garan and Fossum began today's spacewalk at 9:22 a.m. EDT (1322 GMT), which was slightly behind schedule in order to accommodate all the preparations. The outing is expected to last approximately 6 1/2 hours, NASA officials said. [ Photos: NASA's Last Shuttle Mission in Pictures ]

This is the 160th spacewalk conducted in support of space station assembly and maintenance. Garan and Fossum are both veteran spacewalkers, and previously performed three spacewalks together during the space shuttle Discovery's STS-124 mission in June 2008.

Shuttle pilot Doug Hurley and mission specialist Sandra Magnus will conduct robotic operations from inside the complex using the space station's robotic arm.

Last spacewalk during a shuttle mission

The shuttle Atlantis launched into space on July 8, and the crew is almost midway through a 13-day mission to deliver critical supplies and spare parts to the space station. Yesterday (July 11), mission managers added an extra day to the flight to give astronauts enough time to unpack all their cargo and complete various other maintenance tasks around the orbiting outpost.

During today's spacewalk, Fossum is wearing the spacesuit with a solid red line, making him distinguishable from Garan, who is wearing an all-white suit.

The spacewalkers will begin the day with the most crucial task of the outing: retrieving a broken cooling pump module from a storage platform on the exterior of the station and installing it inside the shuttle Atlantis' payload bay.

While Fossum prepares the pump module to be moved, Garan will strap his feet onto the space station's robotic arm so that his hands will be free to grasp the large module and carry it over to Atlantis while riding the robotic appendage.

Hurley and Magnus will fly Garan to the shuttle's payload bay, where he will be flipped upside down to install the faulty pump into the aft section of Atlantis' cargo bay.

The pump module malfunctioned on July 31, 2010 and required three emergency spacewalks to swap out the failed component for a spare that was already onboard the station. Atlantis will bring the faulty pump back to Earth to allow engineers to troubleshoot the malfunction.

"We need to understand why exactly that pump module failed," space station flight director Jerry James said in a news briefing Monday (July 11). "In my mind, that's probably the most important portion of [the spacewalk]."

Installing experiments on the station

Next, the spacewalkers will remove an experiment, called the Robotic Refueling Mission (RRM), from Atlantis payload bay and prepare to install it on the outside of the space station. The Robotic Refueling Mission is an experimental gas station that will test technology that could one day allow satellites to be refueled in space.

Fossum and Garan will switch places, and Fossum will ride the robotic arm while carrying the RRM experiment to the exterior of the Destiny laboratory, where it will be installed near the station's two-armed Dextre robot.

Once these primary tasks are completed, Garan will work on setting up a materials science experiment, called MISSE 8, which was originally installed, but not deployed, during Endeavour's STS-134 mission.

The spacewalkers will then install a protective cover over a docking port on the station's Tranquility node. If all these tasks are completed efficiently, mission managers may elect to have the spacewalkers complete a few so-called "get-ahead" tasks, which consist of general station maintenance activities.

NASA is aiming to wrap up today's spacewalk at around 3:14 p.m. EDT (1914 GMT).

You can follow SPACE.com Staff Writer Denise Chow on Twitter@denisechow. Visit SPACE.com for complete coverage of Atlantis' final mission STS-135 or follow us @Spacedotcom and on Facebook.

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