Atlantis' Commander Jim Halsell and Pilot Scott Horowitz tested the navigation equipment and flight controls they will use late tonight to rendezvous with and then dock to the station. They also fired Atlantis' engines periodically to adjust the rate at which the Shuttle is closing in on the station. Atlantis is 430 statute miles behind the station now, closing in 30 statute miles with each orbit of Earth. Atlantis will continue that closing rate throughout the day while the crew sleeps.
During an engine firing Friday evening using both large orbital maneuvering system (OMS) engines on Atlantis, flight controllers saw an indication that one of two propellant valves for the left OMS engine may have failed to close. The engine's backup valve did operate normally, and the engine remains in good operating condition. However, as a precaution, the left engine will not be used again until Atlantis is ready to deorbit at the completion of the flight. Controllers are continuing to analyze the information and are uncertain if the valve actually did not operate properly or whether the indication may have been an errant sensor reading. The right OMS engine alone can be used for large rendezvous engine firings with no impact on the mission.
Astronauts Jeff Williams and Jim Voss checked the spacesuits and equipment they will use during the upcoming spacewalk, finding them in good shape. Three suits -- one for Williams, one for Voss and a spare, which could be used by either -- were checked out aboard Atlantis. Williams and Voss were assisted in the checks by Horowitz, who will coordinate the spacewalk activities from within the cabin. Later, fellow crew member Mary Ellen Weber successfully tested the Shuttle's robotic arm. Weber will operate the arm during the spacewalk to maneuver Williams and Voss between Atlantis and the station. Weber uncradled the arm this morning and used it to survey the Shuttle's payload bay. Later, Weber powered on the docking system Atlantis will use to attach to the station and extended the system's docking ring. The ring latches onto the station when the Shuttle makes contact and is then retracted to secure the two spacecraft together.
The crew will begin a sleep period at 8:11 a.m. and awaken at 4:11 p.m.. At 8:39 p.m., with Atlantis trailing the International Space Station by about 50,000 feet, Halsell will fire the Shuttle's engines to initiate the final phase of the rendezvous. Just before 10 p.m., as Atlantis reaches a point about a half-mile below the station, Halsell will take over manual control of the approach. He will reach a distance of about 170 feet from the station at 10:39 p.m. Docking is expected at about 11:31 p.m. Atlantis is in an orbit with a high point of 204 miles and a low point of 199 miles, circling Earth every 90 minutes.