Fourth Spacewalk Begins at 9:54 a.m. EDT

Spacewalkers Chris Cassidy and Tom Marshburn began the STS-127 mission’s fourth spacewalk at 9:54 a.m. EDT when they switched their spacesuits to battery power.

They will replace four of the remaining Port 6 truss batteries in a planned seven and a half hour spacewalk. Two of the six original P6 batteries were changed out during the mission’s third spacewalk on Wednesday before work was cut short because of anomalous carbon dioxide levels in Cassidy’s suit. The lithium hydroxide canister that scrubs CO2 from the suit was replaced for today’s spacewalk.

The new batteries are stored on the Integrated Cargo Carrier – Vertical Light Deployable, or ICC-VLD positioned near the Port 6 truss. Cassidy and Marshburn will work together to remove insulation from the old Port 6 batteries, install scoops to gently remove them, pass the batteries back and forth to a stowage location on the ICC-VLD, and repeat the process to replace them with the new batteries.

Each new battery assembly consists of 38 lightweight Nickel Hydrogen cells and associated electrical and mechanical equipment. Two battery assemblies connected in series are capable of storing a total of 8 kW of electrical power. This power is fed to the space station via the Battery Charge/Discharge Unit and Direct Current Switching Unit respectively. The batteries have a design life of 6.5 years and can exceed 38,000 charge/discharge cycles at 35% depth of discharge. Each battery measures 40” by 36” by 18” and weighs 375 pounds.

STS-127 Mission Specialists Chris Cassidy and Tom Marshburn will tackle a challenging 7 ½-hour spacewalk today to finish swapping out batteries for the International Space Station’s oldest set of solar arrays.

Their outing will be devoted entirely to finishing the work started on the third spacewalk of the mission – removing old batteries from the Port 6 truss structure and transferring new batteries from the Integrated Cargo Carrier on the end of the station’s robotic arm to the empty sockets on the truss.

Pilot Doug Hurley and Mission Specialist Julie Payette will position Canadarm2 near the truss for the spacewalk and, once all of the battery swaps are complete, maneuver the carrier back into Endeavour’s cargo bay. That maneuver will require them to hand off the carrier to the shuttle’s arm for re-berthing by Hurley and Commander Mark Polansky.