Spacewalkers To Make 2nd Attempt To Extract Faulty Pump

Spacewalking astronauts will venture outside the International Space Station early Wednesday and make a second attempt to remove a faulty ammonia pump that failed late last month, knocking out half the cooling capability on the U.S. side of the outpost.

U.S. flight engineers Douglas Wheelock and Tracy Caldwell-Dyson will start their excursion an hour later than initially planned. The additional time will enable more prep time for robotic arm support operations. It also will give flight controllers in Mission Control a little extra time to rest. The teams have been working long hours to stabilize the station and plan spacewalking repairs since the pump failed July 31.

A jammed connector and a leak of toxic ammonia coolant prevented the two astronauts from removing the faulty pump during an initial spacewalk last Saturday.

Wheelock and Caldwell-Dyson now are scheduled to exit the U.S. Quest airlock around 7:55 a.m. Wednesday. You can watch NASA TV coverage of the excursion here in The Flame Trench beginning at 7 a.m. Wednesday. Wheelock will be wearing a spacesuit with red stripes around its pant legs and he'll be answering to the radio call sign "EV-1." Caldwell-Dyson will be "EV-2" and she'll be wearing an all white spacesuit.

The two astronauts will isolate the jammed connector by closing nearby valves. Then they'll use a tool to remove the connector and vent any residual ammonia remaining in the failed ammonia pump module, which weighs 780 pounds and is about as large as a bathtub.

Once that work is done, the astronauts aim to disconnect five electrical cables from the pump module and unfasten four bolts holding it in place on the starboard side of the station's central truss. A grapple bar will be used to extract the pump module from inside the S1 truss segment, and then it will be temporarily stowed on the station's mobile rail cart.

If all goes well, a spare pump will be installed during a third spacewalk tentatively set for Sunday. The failed pump ultimately will be stowed on a pallet-like platform that's also located on the truss.

The spacewalk will be the fifth for Wheelock and the second for Caldwell-Dyson. It will be the 149th to be carried out in the assembly and maintenance of the station since its first two building blocks were linked in late 1998. Nearly 1,000 hours of spacewalking time has been accumulated since then.

IMAGE NOTE: Click to enlarge the Associated Press image of Wheelock, right, and Caldwell-Dyson trying to remove a faulty ammonia pump during a spacewalk that stretched more than eight hours last Saturday. It was the longest such excursion in the 148 spacewalks conducted since station assembly began in late 1998. Photo credit: AP via NASA TV.