Astronauts aboard the International Space Station took an extra spacewalk on Tuesday to wrap up some unfinished business outside the orbiting laboratory.

Space station commander Michael Fincke of NASA and Russian cosmonaut Yury Lonchakov donned their Orlan spacesuits and stepped outside the space station to install a stubborn European-Russian experiment and finish some other orbital chores left over from their mission's first spacewalk in December.

The spacewalk began around 12:20 EDT (1620 GMT) and was set to last just over five hours. It comes just one day before NASA's planned launch of the space shuttle Discovery toward the station on Wednesday night.

Discovery is due to dock at the orbiting laboratory on Friday to deliver a new station segment, U.S. solar arrays and swap out one space station crewmember.

"We're looking forward to the big day tomorrow," Fincke radioed down to Mission Control in Houston on Monday.

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Chief among the tasks for Fincke and Lonchakov is the installation of EXPOSE-R, a collection of nine separate materials exposure experiments assembled by Russia's Federal Space Agency and the European Space Agency.

The experiment suite will expose seeds, spores and other samples to the space environment for about 18 months before being retrieved for the return trip home.

Lonchakov, who celebrated his 44th birthday aboard the station last week, has been preparing the EXPOSE-R experiment for the spacewalk.

Fincke and Lonchakov attempted to attach the experiment platform to the hull of the station's Russian segment in late December, but a connector failed to transmit telemetry back to mission control on Earth. Stymied, the spacewalkers hauled EXPOSE-R back inside the station for repairs.

"They determined that it had been improperly configured," NASA spokesperson Kelly Humphries of the Johnson Space Center told SPACE.com on Monday. "So they've got it configured properly now and they're going to go out and install it. They're expecting it to work."

In addition to EXPOSE-R's installation, Fincke and Lonchakov have a list of other chores to perform outside the station's Russian segment.

The astronauts are planning to remove some unneeded straps from equipment, relocate a micrometeoroid impact experiment, reinstall insulation on the station's Zvezda module and conduct a photographic survey of the outpost's Russian segment, NASA officials said.

While Fincke and Lonchakov toil outside the space station, their crewmate Sandra Magnus of NASA will remain inside the orbiting laboratory.

Magnus is preparing to return to Earth later this month aboard the space shuttle Discovery. Her replacement, Japanese astronaut Koichi Wakata, is due to arrive aboard Discovery and stay aboard until June.

Wakata, a veteran spaceflyer for the Japan Aerospace Exploration Agency, is Japan's first long-duration astronaut.

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