Canadarm2 experienced an electrical problem in March in one of two redundant power and data channels which command the operation of the seven-jointed apparatus. It was determined that a short created inadvertent commanding in the arm's primary channel which resulted in the unexpected activation of the arm's brakes. The backup commanding channel has functioned perfectly. Even though the arm has continued to operate flawlessly through a software modification, the replacement of the problematic wrist joint was added to the STS-111 mission.
Inside the shuttle, Pilot Paul Lockhart will choreograph the planned seven-hour spacewalk while Commander Ken Cockrell will use the shuttle's robotic arm to provide television views of the spacewalk activity. Inside the station, Endeavour Astronaut Dan Bursch and Expedition Five Commander Valery Korzun will conduct a checkout of the health of the arm once the new joint is installed. Expedition Five Flight Engineers Peggy Whitson and Sergei Treschev will continue cargo transfer activities throughout the day.
Each joint of Canadarm2 is attached to the next joint by six bolts and an additional bolt that disconnects power, data and video connections. The first task for the spacewalkers is to remove the latching end effector (LEE), essentially the hand of Canadarm2, leaving the faulty wrist roll joint exposed. Next they will disconnect the wrist roll joint and Perrin will carry the failed unit to Endeavour's payload bay to temporarily store it next to the new joint. Chang-Díaz will assist in removing the new joint from its launch carrier and Perrin will bring it up to Canadarm2. The spacewalkers will align the new component with the wrist yaw joint at the end of the arm, tighten the six bolts and turn the final bolt to connect power, data and video lines. They will reinstall the LEE and power will be turned back onto Canadarm2.
Endeavour's astronauts - Cockrell, Lockhart, Chang-Díaz, Perrin, Bursch, Yury Onufrienko and Carl Walz - were awakened just before 4 a.m. Central time this morning to the song, "On the Road Again," by Willie Nelson, selected for Walz by his family. Walz is returning to Earth after a six-month stay in orbit.
All systems on both Endeavour and the International Space Station continue to function normally as the two craft orbit the Earth every 90 minutes at an altitude of 240 statute miles.