Orbital Maintenance Workers Start Spacewalk At Station

Spacewalking astronauts Doug Wheelock and Tracy Caldwell-Dyson are heading outside the International Space Station today on an excursion aimed at restoring full cooling capability to the U.S. side of the outpost.

The two U.S. astronauts switched their spacesuits to battery power at 8:27 a.m., marking the beginning of the 149th spacewalk performed at the station since assembly began in late 1998.

Wheelock will be the first out of the U.S. Quest airlock. He is wearing a spacesuit with a red stripe on its pant legs and is answering to the radio call sign "EV-1." Views from his helmet-cam are distinguished by a faint No. 19 in the lower right-hand corner.

Caldwell-Dyson is "EV-2" and she is wearing an all-white spacesuit. Views from her helmet-cam have the No. 20 in the lower right-hand corner.

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The first order of business for the astronauts will be to gather up tools needed during the planned six-hour spacewalk.

The prime goal is to extract a failed ammonia coolant pump from the starboard side of the station's central truss. A jammed coolant line connector and a toxic ammonia leak prevented the two astronauts from completing the job during a spacewalk last Saturday.

Caldwell-Dyson will be crawling hand-over-hand to the juncture between the first and second segments of the starboard truss -- S1 and S3 -- where she will close a valve to halt the flow of ammonia through the fluid line with the jammed connector -- M3 in the graphic..

Wheelock then will make another attempt to release the connector, which is one of four coolant lines hooked up to the faulty pump. The three others were successfully disconnected on the first spacewalk.