Live in orbit: Fourth Spacewalk Under Way

Two American astronauts are outside the International Space Station today, headed off on a spacewalk aimed at repairing one giant solar wing rotary mechanism on the outpost and doing a little preventative maintenance on another.

Endeavour mission specialists Shane Kimbrough and Steve Bowen switched their spacesuit batteries to internal power at 1:24 p.m. EST, marking the official start of the fourth and final spacewalk planned during the shuttle's stay at the station.

The spacewalk started 21 minutes ahead of schedule as the joined shuttle-station complex was soaring 223 statute miles above the South Pacific.

The excursion is the 118th to be performed in the assembly and maintenance of the station since the first two outpost building blocks were linked in orbit 10 years ago next month.

Kimbrough is answering to the radio call sign "EV-3" and is wearing the spacesuit with broken red stripes on its legs. Bowen is "EV-2" and is wearing an all-white spacesuit.

You'll be seeing shots from the spacewalkers' helmet-cams throughout the spacewalk. A faint numeral 16 in the lower righthand corner of a view indicates the shot is coming from Kimbrough's helmet-cam. An 18 shows the view is from Bowen's helmet-cam.

Shuttle pilot Eric Boe is directing the spacewalking activity from a window seat inside the shuttle-station complex.

Kimbrough was the first to exit the airlock; Bowen followed shortly thereafter. Their first order of business will be to gather up tool bags and secure safety tethers before headong out to the portside end of the station's central truss.

Once there, the astronauts will remove thermal covers from the Solar Alpha Rotary Joint, a 10-foot-wide mechanism that turns two massive American solar wings like paddlewheels so they can constantly track the sun as the station orbits Earth.

The two will be using a special space grease to lubricate the joint -- preventative maintenance aimed at avoiding the type of trouble encountered with the so-called SARJ mechanism on the starboard side of the central truss.

That joint has not been operating properly since September 2007, and the astronauts aim to finish up some unprecedented repair work on it later in today's spacewalk.

You can watch the action unfold right here in The Flame Trench. Simply click the NASA TV box on the righthand side of the page to launch our NASA TV viewer and 24/7 coverage of the STS-126 mission.

And be sure to refresh this page for periodic updates.