Spacewalk experts presented a plan to mission managers in Monday's Mission Management Team meeting. Space Shuttle Deputy Program Manager Wayne Hale, in a Monday afternoon briefing, said that with the level of uncertainty involved in flying a reentry with protruding gap fillers it was an easy decision to move ahead with a well-understood process for removing them.
Early Monday, Robinson and Noguchi replaced a 600-pound gyroscope on the International Space Station, leaving the orbiting laboratory with a complete functional set of four. Called control moment gyros, or CMGs, the 600-pound devices maintain the Station's orientation in space, the way it is pointed and which part faces the Earth as it orbits the planet.
The 7-hour, 14-minute spacewalk began at 3:42 a.m CDT. After leaving the Discovery airlock, Noguchi and Robinson made their way hand-over-hand to the Station's Z1 Truss, atop the Unity Node where the four CMGs are housed. There Noguchi, of the Japan Aerospace Exploration Agency, attached himself to a foot platform at the end of the Station's Canadarm2, operated by Discovery Pilot Jim Kelly and Mission Specialist Wendy Lawrence.
Coached and monitored by Mission Specialist Andy Thomas on Discovery's aft flight deck, the spacewalkers removed CMG-1, which had failed in June 2002. Noguchi held it as the arm took him back to the rear of Discovery's cargo bay, where he and Robinson, who had moved back on his own, temporarily stowed it. They then took the new CMG from its cradle, and Noguchi held it while the arm moved him back to the Z1 Truss.
There he and Robinson installed it in the space vacated by the failed device. That completed, flight controllers began the hours-long process of checking out the new CMG-1 and spinning it up to its 6,600 rpm operating speed.
On the mission's first spacewalk on Saturday, Noguchi and Robinson had rerouted CMG-2's power supply. A faulty circuit breaker had interrupted that power supply in March. The two spacewalks leave the Station with four functioning CMG's. The station can hold its attitude on two, but more will be required as it grows.
Discovery Commander Eileen Collins and Mission Specialist Charlie Camarda, along with the Station's Commander Sergei Krikalev and NASA Science Officer John Phillips, worked Monday on transferring cargo to and from the Station. The 3,768 pounds of up-bound cargo from the Multi-Purpose Logistics Module Raffaello, which came to the Station in Discovery's cargo bay and then was attached to a docking port on the Unity Node, has been transferred to the Station. Work continues to stow it and to reload Raffaello with equipment and trash to be returned to Earth.