Live in orbit: Spacewalking inspections on tap

Discovery's astronauts are boning up for a spacewalk aimed at inspecting a fouled-up rotary joint that is critical to maximizing electrical output from one of three massive sets of American solar wings at the International Space Station.

Mission specialists Scott Parazynski and Douglas Wheelock are scheduled to venture outside the U.S. Quest airlock at 4:58 a.m. Thursday, although an earlier start is possible.

The job at hand: Inspecting a 10-foot wheel-shaped joint designed to turn the station's starboard solar wings so they can constantly track the sun, maximizing the collection of solar energy and electrical output.

Engineers on the ground, meanwhile, are analyzing a separate solar power problem: a torn blanket on one of two solar wings deployed on the port side of the station on Tuesday. Discovery's astronauts might be asked to attempt repairs to the blanket during another spacewalk on Saturday.

You can watch live coverage of all mission operations here in The Flame Trench. Simply click the link below the image above to launch our NASA TV viewer and round-the-clock coverage of NASA's 120th shuttle mission, the 23rd devoted to station assembly and maintenance.

The spacewalking inspections on Thursday will involve removing 21 of 22 thermal covers that protect the station's starboard Solar Array Rotary Joint -- known by the acronym SARJ (pronounced "sarge") -- from extreme temperature swings in low Earth orbit: from 250 degrees Fahrenheit to Minus 250 degrees Fahrenheit.

Engineers over the past two months have noted high current levels and vibrations when the joint turns. Spacewalker Dan Tani removed a thermal cover and spotted widespread metal filings during a quick-look Sunday, an indication that internal gears or bearings are being sheered when the joint turns. Mission managers decided to park the wings in a stationary position to prevent permanent damage, a move that significantly reduces the amount of electricity the arrays can generate. NASA needs to sort out both solar power problems before the planned Dec. 6 launch of Atlantis and the long-awaited Columbus science laboratory.

Click here to see the preliminary timeline for the fourth of five planned spacewalks during Discovery's stay at the station: EVA%204%20Timeline.pdf. It contains the step-by-step details on the spacewalking inspections and the tools Parazynski and Wheelock will use during the excursion.

"Our activities tomorrow will be primarily just to investigate -- to see if we can find the smoking gun that will lead to probably another spacewalk, or maybe even a series of spacewalks, to clean up the debris and changeout whatever element failed in the system," Parazynski told reporters today in a space-to-ground news conference. "So we'll see how that goes. I don't know where this story is going to end, but it's going to be very interesting."

The timing of major mission milestones is detailed in the latest version -- Rev G -- of the NASA TV schedule. Check it out here: tvsked_revg.pdf.

You can also see the detailed timeline for the day today as well as messages sent up to the astronauts in the Flight Day 9 Execute Package: FD09%20Execute%20Package.pdf.