NASA astronaut Sunita Williams (left) and Japanese astronaut Akihiko Hoshide (right) begin a planned 6.5-hour spacewalk outside the International Space Station on Aug. 30, 2012, in this still from a NASA TV broadcast.

Two astronauts ventured outside the International Space Station today (Aug. 30) to perform maintenance tasks and upgrades to their home in orbit.

NASA spaceflyer Sunita Williams and Japanese astronaut Akihiko Hoshide officially began today's spacewalk at 8:16 a.m. EDT (1216 GMT), after switching on the batteries on their bulky white spacesuits. The pair is expected to spend roughly 6.5 hours carrying out repairs on the exterior of the International Space Station.

This morning, to prepare for the spacewalk, Williams and Hoshide donned oxygen masks for an hour before the airlock was depressurized to the levels typically used in U.S. spacesuits. Once they had put on their spacesuits, the two astronauts spent 100 minutes breathing oxygen at the spacesuit's pressure level.

During this time, Williams and Hoshide engaged in 50 minutes of light, in-suit callisthenic exercise, alternating with 50 minutes of in-suit breathing while resting. This procedure, called In-Suit Light Exercise (ISLE), was first introduced last year as a better way for astronauts to prepare their bodies for spacewalks.

Traditionally, astronauts camped out overnight in the Quest airlock, and the inside pressure was lowered to purge nitrogen from the spacewalkers' bloodstreams. This helps the astronauts reduce the risk of decompression sickness, or what is commonly known as "the bends." [Gallery: Spacewalking Astronauts Fix Up Space Station]

Today's excursion is NASA's first spacewalk in more than a year. Former space station astronauts Mike Fossum and Ron Garan previously completed maintenance tasks in the vacuum of space in July 2011, while four other NASA astronauts were visiting the station during the agency's final space shuttle mission.

The primary objectives for Williams and Hoshide will be to replace a faulty power box on the space station's backbone-like truss, install a series of cables, and remove a camera that recently broke on the station's 57-foot-long (17-meters-long) Canadarm 2 robotic arm.

The power box, which is known as a main bus switching unit, helps relay power throughout the orbiting complex.

"It's passing power, but can no longer be switched," Kieth Johnson, lead U.S. spacewalk officer at NASA's Johnson Space Center in Houston, told reporters in a news briefing on Aug. 14. "In order to do future operations, we need to take that box out and put one in that allows us to do that."

After switching out the box with a spare, the defunct unit will be temporarily stored on a cargo platform attached to the exterior of the space station.

Next, Williams will get to work installing cables for a new Russian laboratory module that is expected to launch to the orbiting outpost in 2013, Johnson said. Part of this wiring work will require her to climb inside a section of the station's truss.

Once that is complete, Hoshide will remove the failed camera from the Canadarm 2 robotic arm, and replace it with a new one.

If there is extra time, the spacewalkers may also install a micrometeoroid debris shield over part of an American module, remove a camera located on the exterior of the Japanese Kibo laboratory, and troubleshoot power and data wires on the Russian Zarya module, Johnson added.

Today's excursion will be Williams' fifth spacewalk and Hoshide's first time working in the vacuum of space. The outing is the third this year, but only the second spacewalk of the station's current Expedition 32 mission. Two Russian cosmonauts previously performed a nearly six-hour spacewalk outside the space station on Aug. 20.

In addition to Williams and Hoshide, the station's six-person crew includes Russian cosmonauts Gennady Padalka, Yuri Malenchenko and Sergei Revin, and American Joe Acaba.

Today's spacewalk is being broadcast live on NASA Television here: http://www.nasa.gov/ntv

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