Discovery has displaced its two spacewalkers, Michael Fossum and Ronald Garran, for their first mission outside the International Space Shuttle during Discovery’s mission. The two astronauts will have to retrieve the Orbiter Boom Sensor System from the station’s starboard truss to the shuttle, prepare the Kibo laboratory for installation and test a method of cleaning a rotary joint that has worked poorly since October.

The first objective is to transfer the OBSS left after the previous shuttle mission from the station’s truss to space shuttle Discovery. 

Most of the tasks will fall to Garan, who will be releasing the stanchions holding the boom to the truss and removing a bag that has been protecting the boom’s sensor package.

Just before the station’s robotic arm takes control of the boom and hands it off to the shuttle’s robotic arm, Fossum will detach the keep-alive umbilical that has been providing the boom’s systems with power while stowed. 

The boom will be used later in the mission to inspect the shuttle’s heat shield.  Next, the spacewalkers will prepare the Kibo laboratory for installation.  While Garan is working on the boom, Fossum will inspect the Harmony Node’s left side active common berthing mechanism to ensure that it’s ready to attach to Kibo. 

After the boom work is done, Garan and Fossum will work together in the shuttle’s cargo bay to remove contamination covers from the surface where the module will connect to Harmony.  Fossum also will disconnect the heater cables connecting the module to the shuttle and remove three bolts that lock the shutters of Kibo’s forward window in place for launch. 

The spacewalkers’ final tasks of the spacewalk will take them to the station’s starboard solar alpha rotary joint.  The 10-foot-wide rotary joint, which allows the station’s starboard solar arrays to rotate and track the sun, began experiencing increased vibration and power usage in the fall of 2007. Inspections turned up metal shavings inside the joint. 

One of the joint’s 12 trundle bearing assemblies, which allow the joint’s outboard ring to rotate around its inboard ring, was removed.  Garan will install a replacement for that trundle bearing assembly.  Meanwhile, Fossum will inspect a potentially damaged area on the joint to determine whether there is debris sitting on the surface of the metal, or a divot in the metal. 

A similar inspection was performed during STS-123, but Fossum will use sharper tools to give him more tactile feedback.  Fossum also will try out techniques for cleaning the surface of the joint’s race ring.  First, he will try removing a section of the debris, using a putty knife as a scraper.  Next, he will apply grease to the surface and then try the scraper again.

While Fossum and Garan work outside the orbital outpost, mission specialists Karen Nyberg and Akihiko Hoshide are using the station’s robotic arm to remove the JPM from the shuttle’s payload bay and install it on Harmony.