With more time now needed to complete an analysis of the communications problems between the arm's shoulder pitch joint in its redundant or backup mode, and the arm's backup computer unit, Shuttle and Station program managers decided to postpone the launch of Atlantis on the STS-104 mission to deliver the Joint Airlock to the ISS until no earlier than early July. The next ISS assembly flight after that, STS-105 aboard Discovery, was also delayed until no earlier than early August to accommodate the Airlock mission and to allow managers to consider a replacement of the shoulder joint on the August flight during one of two spacewalks planned by Dan Barry and Patrick Forrester. No final decision on replacing the joint has yet been made.
The recently installed Canadarm2 has fully redundant computer control systems. All of the arm's joints are functioning perfectly through its primary channel, but the backup channel has run into a series of recent problems, including the unexpected activation of a switch for the arm's brakes earlier this month in the backup mode of operation during a test of the arm's wrist. That problem has not reoccurred in subsequent testing. The shoulder pitch joint problem cropped up last week during other arm checkouts when the joint experienced intermittent dropouts in communicating with the backup computer unit.
It had been hoped that a software patch developed by Canadian engineers who designed the arm would clear the communications dropouts, but it did not resolve the problem, resulting in the need for further testing and analysis. Robotics experts suspect that the problem resides in the shoulder pitch joint hardware, not the software or the computers associated with its operation. Expedition Two Flight Engineers Jim Voss and Susan Helms had hoped to perform a "dry run" of the Airlock installation procedures today, but that simulation was postponed.
The Canadarm2 is required for the grapple of the Airlock in Atlantis' cargo bay and its installation on the starboard docking port of the Unity module of the ISS. The 12 ½ ton Airlock will enable future Station spacewalks to be conducted in either U.S. or Russian spacesuits rather than from the Russian Zvezda module. The Shuttle robot arm cannot reach the Airlock installation location.
In the meantime, ISS managers will decide Friday whether to ask Voss and Expedition Commander Yury Usachev to add the replacement of the arm computer unit to a previously scheduled "internal" spacewalk in the Zvezda's spherical transfer compartment on June 8. That first ISS-based spacewalk is designed to reposition a docking mechanism in preparation for the arrival of a Russian docking module later this year. If approved, the computer replacement task would involve Usachev and Voss venturing outside the Station in Russian suits for their spacewalk before reentering Zvezda's docking compartment for the mechanism repositioning work.
Engineers are also assessing the performance of a motor on one of the two wings of the P6 solar array truss structure that enables the solar mast to track the sun as the ISS orbits the Earth. Although there is more than ample power being generated for all Station systems, the motor is generating higher than normal electrical currents and may need to be replaced on a subsequent Shuttle assembly flight.
Meanwhile, Usachev, Voss and Helms completed the unloading of an unmanned Russian Progress resupply vehicle which arrived at the ISS last week loaded with 3100 pounds of supplies, food, clothes and spare parts. A spare computer hard drive brought to the ISS on the Progress was installed in one of three central Station computers, bringing the outpost's computers back to full functionality.