Astronauts David Wolf and Christopher Cassidy are preparing for a challenging spacewalk today, working at the far left end of the International Space Station's long power truss to replace four of six aging batteries in the lab's oldest set of solar arrays.

An artist's concept of spacewalkers Chris Cassidy (left) and Dave Wolf (right) working to swap out batteries on the Port 6 truss. Credit: NASA TV

If one visualizes a football field, the station's airlock is on the right side of the 50-yeard line while the port-six, or P6, solar array is deep in the left end zone. Along with working far from the safety of the airlock, the spacewalkers must manually swap out 375-pound batteries without damaging fragile cooling fins.

"EVA 3 brings its own challenges," Wolf said in a NASA interview. "Chris, Navy Seal, he and I will go out several hundred feet, multiple tether lengths out. There's no way to get back quick. The (space station) robot arm will place these large solar array batteries reasonably near where they will be installed. But we'll be handling these heavy batteries, each the size of a small refrigerator, each with rows of delicate cooling fins on the back.

"And we will pull those out of the Vertical Cargo Carrier, hand them back and forth until we get them in position and mate those fins with the opposing fins on the space station. Those are cooling fins so those batteries won't overheat in use. We'll do four of those batteries on EVA-3. We'll do two more EVA-4 and we consider this one of the highest critical pieces of work to do on this mission."

The final two P6 batteries will be replaced during a spacewalk Friday by Cassidy and Thomas Marshburn.

Today's spacewalk was scheduled to begin at 10:58 a.m. EDT, but the astronauts were running ahead of schedule in their preparations and could start early. This will be the 128th EVA devoted to station assembly and maintenance since construction began in 1998, the ninth so far this year and the third of five planned by Endeavour's crew. It will be the seventh spacewalk for Wolf and the first for Cassidy, a former Navy Seal.

For identification, Wolf will be wearing a spacesuit with solid red stripes around the legs while Cassidy will use a suit with diagonal stripes.

"P6 is the truss segment that's been up there the longest and the batteries are therefore the oldest so those batteries need to be changed out and there's six of them total," Cassidy said in a NASA interview. "We've learned in our training that it's unrealistic to get six completed in one spacewalk. So Dave and I will tackle four and they need to be done in pairs. You know, when you read the instructions on your flashlight and it says, 'Don't mix an old one and a new one together,' I never knew why. Well, the same thing applies to space batteries. You want the pairs to be both new batteries. We can't mix old ones and new ones.

"So consequently if we do two and we try to do the third, we're committed to the fourth. I think we could probably do five in one EVA, but it doesn't make sense from a hardware perspective. They're done in sets. So we're doing these first four batteries."

Each nickel-hydrogen battery measures 40-by-36-by-18 inches and weighs 375 pounds. Two batteries in series can store 8 kilowatts of power. The batteries have a design life of 6.5 years and can endure more than 38,000 charge/discharge cycles.

"P6 is a long ways away from the airlock," Cassidy said. "I think it's the farthest you could get from the airlock hatch because the airlock's on the starboard side. We're going port, and it's way out there. We have safety tethers that allow us to go 55 or 85 feet. There's two different sizes and so we have to stack ... some together to give us twice the distance.

"The task in and of itself, of physically pulling out a battery and putting it in, is not necessarily the hard part," Cassidy said. "The hard part is the choreography with the folks moving the arm, Doug (Hurley) and Julie (Payette). As we grab the battery, they'll guide the pallet, the ICC-VLD, away from us, essentially pulling the battery out of the carrier. And then Dave and I have to ... do a series of hand offs. He grabs it and then I move and then I'll grab it and he lets go and then he moves and we kind of do this hopping motion until we get to the spot where the battery's going to go."

Here is an updated timeline of today's activity (in EDT and mission elapsed time; includes revision K of the NASA television schedule):

EDT........DD...HH...MM...EVENT

07/22
06:03 AM...06...12...00...STS/ISS crew wakeup
06:43 AM...06...12...40...EVA-3: 14.7 psi repress/hygiene break
07:28 AM...06...13...25...EVA-3: Airlock depress to 10.2 psi
07:53 AM...06...13...50...EVA-3: Campout EVA preps
08:03 AM...06...14...00...ISS daily planning conference
09:23 AM...06...15...20...EVA-3: Spacesuit purge
09:38 AM...06...15...35...EVA-3: Spacesuit prebreathe
10:28 AM...06...16...25...EVA-3: Crew lock depressurization
10:33 AM...06...16...30...SSRMS ICC maneuver to EVA work site
10:58 AM...06...16...55...EVA-3: Spacesuits to battery power
11:03 AM...06...17...00...EVA-3: Airlock egress
11:13 AM...06...17...10...EVA-3: Setup
11:28 AM...06...17...25...EVA-3: EV1: JLE PL preps
11:28 AM...06...17...25...EVA-3: EV2: JEF MLI removal
11:58 AM...06...17...55...EVA-3: P6 battery replacement (4 of 6)
04:43 PM...06...22...40...EVA-3: Cleanup and ingress
04:48 PM...06...22...45...SSRMS maneuvers to clear SARJ
05:28 PM...06...23...25...EVA-3: Airlock repressurization
05:38 PM...06...23...35...Spacesuit servicing
06:18 PM...07...00...15...Evening planning conference
07:30 PM...07...01...27...Mission status briefing
09:03 PM...07...03...00...ISS crew sleep begins
09:33 PM...07...03...30...STS crew sleep begins
10:00 PM...07...03...57...Daily highlights reel

Additional coverage for subscribers:
VIDEO: STEP-BY-STEP SUMMARY OF SPACEWALK NO. 3 PLAY

VIDEO: LEFT-HAND BOOSTER CAMERA LOOKING UPWARD PLAY
VIDEO: LEFT-HAND BOOSTER CAMERA LOOKING INBOARD PLAY
VIDEO: LEFT-HAND BOOSTER CAMERA LOOKING DOWNWARD PLAY
VIDEO: RIGHT-HAND BOOSTER CAMERA LOOKING UPWARD PLAY
VIDEO: RIGHT-HAND BOOSTER CAMERA LOOKING INBOARD PLAY
VIDEO: RIGHT-HAND BOOSTER CAMERA LOOKING DOWNWARD PLAY

VIDEO: TUESDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: CREW ANSWERS PUBLIC'S QUESTIONS FROM SPACE PLAY
VIDEO: EXPOSED FACILITY RECEIVES EXPERIMENT CARRIER PLAY
VIDEO: JAPANESE EXPERIMENTS UNBERTHED FROM SHUTTLE PLAY
VIDEO: OVERVIEW OF FLIGHT DAY 7 ACTIVITIES PLAY
VIDEO: TUESDAY MORNING FLIGHT DIRECTOR INTERVIEW PLAY

VIDEO: FLIGHT DAY 6 HIGHLIGHTS MOVIE PLAY
VIDEO: MONDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: REPLACEMENT COOLING PUMP PUT ON STATION PLAY
VIDEO: SPARE COMMUNICATIONS ANTENNA TRANSFERRED PLAY
VIDEO: SPACEWALKER HOPS ABOARD STATION ARM PLAY
VIDEO: WOLF AND MARSHBURN BEGIN EVA NO. 2 PLAY
VIDEO: OVERVIEW OF FLIGHT DAY 6 ACTIVITIES PLAY
VIDEO: STEP-BY-STEP SUMMARY OF SPACEWALK NO. 2 PLAY
VIDEO: MONDAY MORNING FLIGHT DIRECTOR INTERVIEW PLAY
VIDEO: OVERVIEW OF FLIGHT DAY 6 ACTIVITIES PLAY
VIDEO: STEP-BY-STEP SUMMARY OF SPACEWALK NO. 2 PLAY

VIDEO: FLIGHT DAY 5 HIGHLIGHTS MOVIE PLAY
VIDEO: CANADIANS IN SPACE PLAY
VIDEO: SUNDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: PALLET HANDED OFF TO SPACE STATION PLAY
VIDEO: CARGO CARRIER UNBERTHED FROM SHUTTLE BAY PLAY
VIDEO: SUNDAY MORNING FLIGHT DIRECTOR INTERVIEW PLAY
VIDEO: OVERVIEW OF FLIGHT DAY 5 ACTIVITIES PLAY

VIDEO: FLIGHT DAY 4 HIGHLIGHTS MOVIE PLAY
VIDEO: SATURDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: UPDATE ON ENDEAVOUR'S HEAT SHIELD CHECKS PLAY
VIDEO: JAPANESE OUTDOOR SCIENCE PLATFORM ATTACHED PLAY
VIDEO: SHUTTLE AND STATION ARMS HANDOFF SCIENCE DECK PLAY
VIDEO: JAPAN'S EXPOSED FACILITY UNBERTHED FROM SHUTTLE PLAY
VIDEO: THERMAL COVER JETTISONED FROM THE KIBO MODULE PLAY
VIDEO: HELMET-CAMERA VIEWS AS KOPRA CLIMBS TO SHUTTLE PLAY
VIDEO: TIM KOPRA EMERGES FROM AIRLOCK FOR EVA NO. 1 PLAY
VIDEO: SATURDAY MORNING FLIGHT DIRECTOR INTERVIEW PLAY
VIDEO: OVERVIEW OF FLIGHT DAY 4 ACTIVITIES PLAY
VIDEO: STEP-BY-STEP SUMMARY OF SPACEWALK NO. 1 PLAY
VIDEO: NARRATED TOUR OF ENDEAVOUR'S PAYLOAD BAY PLAY

VIDEO: FLIGHT DAY 3 HIGHLIGHTS MOVIE PLAY
VIDEO: FRIDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: SHUTTLE CREW WELCOMED ABOARD STATION PLAY
VIDEO: ENDEAVOUR DOCKS TO THE SPACE STATION PLAY
VIDEO: ASTRONAUTS' ONBOARD CAMCORDER FOOTAGE PLAY
VIDEO: SHUTTLE FLIES OUT IN FRONT OF STATION PLAY
VIDEO: ENDEAVOUR PERFORMS 360-DEGREE BACKFLIP PLAY
VIDEO: BEAUTIFUL VIEWS OF ENDEAVOUR APPROACHING PLAY
VIDEO: STATION CAMERA CATCHES ENDEAVOUR'S "TI BURN" PLAY
VIDEO: NARRATED PREVIEW OF RENDEZVOUS AND DOCKING PLAY

VIDEO: FLIGHT DAY 2 HIGHLIGHTS MOVIE PLAY
VIDEO: TUESDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: PREVIEW ANIMATION OF HEAT SHIELD INSPECTIONS PLAY

VIDEO: FLIGHT DAY 1 HIGHLIGHTS MOVIE PLAY
VIDEO: THE FULL STS-127 LAUNCH EXPERIENCE PLAY
VIDEO: INSIDE MISSION CONTROL DURING LAUNCH PLAY
VIDEO: LAUNCH REPLAY: VAB ROOF PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PRESS SITE PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PAD PERIMETER PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: BEACH TRACKER PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: UCS-23 TRACKER PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PLAYALINDA BEACH PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PATRICK AIR FORCE BASE PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: BANANA CREEK VIP SITE PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PAD FRONT CAMERA PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: WEST TOWER PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: CAMERA 070 PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: CAMERA 071 PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: CAMERA CS-1 PLAY
VIDEO: LAUNCH REPLAY: CAMERA CS-2 PLAY
VIDEO: LAUNCH REPLAY: CAMERA CS-6 PLAY

VIDEO: SHUTTLE ENDEAVOUR BLASTS OFF! PLAY | HI-DEF
VIDEO: STS-127 POST-LAUNCH NEWS CONFERENCE PLAY
VIDEO: FOOTAGE OF FUEL TANK AFTER JETTISON PLAY
VIDEO: ASTRONAUTS REACH PAD 39A PLAY | HI-DEF
VIDEO: CREW DEPARTS QUARTERS FOR LAUNCH PAD PLAY | HI-DEF
VIDEO: ASTRONAUTS DON SPACESUITS FOR LAUNCH PLAY | HI-DEF

VIDEO: WEATHER SCRUBS LAUNCH AGAIN PLAY | HI-DEF
VIDEO: CREW ARRIVES AT PAD 39A ON MONDAY PLAY | HI-DEF
VIDEO: CREW DEPARTS QUARTERS ON MONDAY PLAY | HI-DEF
VIDEO: ASTRONAUTS SUIT UP ON MONDAY PLAY | HI-DEF

VIDEO: THUNDERSTORMS SCRUB SUNDAY'S ATTEMPT PLAY | HI-DEF
VIDEO: CREW DEPARTS QUARTERS ON SUNDAY PLAY | HI-DEF
VIDEO: ASTRONAUTS SUIT UP ON SUNDAY PLAY | HI-DEF
VIDEO: NARRATED SUMMARY SHUTTLE'S PREPARATIONS PLAY | HI-DEF
VIDEO: NARRATED SUMMARY PAYLOADS' PREPARATIONS PLAY | HI-DEF

VIDEO: LIGHTNING STRIKES POSTPONE ENDEAVOUR LAUNCH PLAY
VIDEO: FRIDAY'S PRE-LAUNCH NEWS CONFERENCE PLAY
VIDEO: THURSDAY'S STATUS REPORT ON THE COUNTDOWN PLAY
VIDEO: COUNTDOWN PREVIEW AND WEATHER BRIEFING PLAY
VIDEO: ASTRONAUTS RETURN TO CAPE FOR LAUNCH PLAY | HI-DEF

VIDEO: NO LEAKS FOUND DURING SPECIAL TEST PLAY
VIDEO: EXPLANATION OF THE HYDROGEN LEAK AND THE REPAIR PLAY

VIDEO: POST-SCRUB NEWS BRIEFING PLAY
VIDEO: FIRING ROOM UPDATE WITH LAUNCH DIRECTOR PLAY
VIDEO: SCRUB NO. 2 DECLARED DUE TO HYDROGEN LEAK PLAY

VIDEO: SUNDAY'S UPDATE FROM MISSION MANAGEMENT TEAM PLAY
VIDEO: LEAK POSTPONES SHUTTLE ENDEAVOUR LAUNCH PLAY

VIDEO: TIME-LAPSE OF SUN SETTING OVER PAD 39A PLAY
VIDEO: ANOTHER TIME-LAPSE OF GANTRY RETRACTION PLAY
VIDEO: TIME-LAPSE OF LAUNCH PAD TOWER ROLLBACK PLAY

VIDEO: PRE-LAUNCH INTERVIEW WITH MARK POLANSKY PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH DOUG HURLEY PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH CHRIS CASSIDY PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH JULIE PAYETTE PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH TOM MARSHBURN PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH DAVE WOLF PLAY

VIDEO: THE STS-127 MISSION PREVIEW MOVIE PLAY
VIDEO: THURSDAY'S PRE-LAUNCH NEWS CONFERENCE PLAY
VIDEO: JAPANESE SCIENCE FACILITIES ABOARD STATION PLAY
VIDEO: COUNTDOWN BEGINS TICKING FOR SATURDAY'S LAUNCH PLAY
VIDEO: LAUNCH COUNTDOWN PREVIEW BRIEFING PLAY
VIDEO: CREW ARRIVES JUST BEFORE MIDNIGHT PLAY | HI-DEF
VIDEO: GET TO KNOW ENDEAVOUR'S ASTRONAUTS PLAY

VIDEO: ASTRONAUTS PRACTICE EVACUATION OF SHUTTLE PLAY | HI-DEF
VIDEO: CREW BOARDS SHUTTLE FOR PRACTICE COUNT PLAY | HI-DEF
VIDEO: ASTRONAUTS SUIT UP FOR DRESS REHEARSAL PLAY | HI-DEF
VIDEO: CREW INSPECTS CARGO IN THE PAYLOAD BAY PLAY | HI-DEF
VIDEO: TRAINING SESSIONS AT LAUNCH PAD AND BUNKER PLAY | HI-DEF
VIDEO: INFORMAL CREW NEWS CONFERENCE AT LAUNCH PAD PLAY
VIDEO: ASTRONAUTS ARRIVE FOR PRACTICE COUNTDOWN PLAY

VIDEO: FLIGHT READINESS REVIEW SETS LAUNCH DATE PLAY

VIDEO: PAD 39A GANTRY ENCLOSES SHUTTLE PLAY | HI-DEF
VIDEO: ROLLAROUND MOVES ENDEAVOUR TO PAD 39A PLAY | HI-DEF
VIDEO: ENDEAVOUR IS HAULED OFF LAUNCH PAD 39B PLAY | HI-DEF
VIDEO: TIME-LAPSE OF SHUTTLE'S LAUNCH PAD SWITCH PLAY

VIDEO: SHUTTLE AND STATION PROGRAM UPDATE PLAY
VIDEO: THE STS-127 MISSION OVERVIEW BRIEFING PLAY
VIDEO: PREVIEW BRIEFING ON MISSION'S SPACEWALKS PLAY
VIDEO: THE ASTRONAUTS' PRE-FLIGHT NEWS BRIEFING PLAY

VIDEO: PAD 39B AND ITS LAST SPACE SHUTTLE PLAY | HI-DEF
VIDEO: ENDEAVOUR'S SUNRISE ARRIVAL AT PAD 39B PLAY | HI-DEF
VIDEO: MIDNIGHT ROLLOUT FROM ASSEMBLY BUILDING PLAY | HI-DEF
VIDEO: ENDEAVOUR HOISTED FOR ATTACHMENT TO TANK PLAY | HI-DEF
VIDEO: CRANE ROTATES ENDEAVOUR VERTICALLY PLAY | HI-DEF
VIDEO: ENDEAVOUR MOVES TO ASSEMBLY BUILDING PLAY | HI-DEF
VIDEO: ORION AND ARES ROCKET PROGRESS REPORT PLAY
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