Doug Wheelock, left, and Tracy Caldwell Dyson struggle with cooling system valves during Aug. 7 spacewalk Photo Credit/NASA TV
The International Space Station’s mission management team signed off Tuesday on two new spacewalks by NASA astronauts Doug Wheelock and Tracy Caldwell Dyson in their bid to recover a vital external cooling system.
The first of the two six to seven hour excursions is scheduled to get under way Wednesday at 7:55 a.m. EDT, and the third in a series of spacewalks is expected no earlier than Sunday. The start of the Wednesday’s excursion was pushed back an hour late Tuesday to accommodate last minute planning.
The pair was stalled Saturday when they attempted to begin the replacement of a bulky pump motor on the right side of the station’s 357-foot-long solar power truss.
During a grueling eight hour spacewalk, they succeeded in unfastening three of four “quick disconnect” valves, the first steps in the process of removing the motor.
However, a fourth quick disconnect leaked ammonia during several attempts by Wheelock and Caldwell Dyson to separate the final cooling line from the pump. Finally, NASA’s Mission Control directed the spacewalkers to re-connect the valve to prevent a significant ammonia loss and permit around-the-clock re-planning.
The system circulates toxic ammonia through external radiators on the truss to dissipate the heat generated by internal avionics, life support equipment and science gear.
The motor shorted out on July 31, forcing the station’s six U.S. and Russian astronauts to reduce their use of electricity and curtail science experiments.
During Wednesday’s spacewalk, the pair plans to close two quick disconnects inboard of the pump in an effort to isolate the failed motor from most of the cooling system. Some residual ammonia is expected to seep out. However, the pressure in the cooling line was lowered late Monday with commands from NASA’s Mission Control to reduce the loss.
If they succeed in freeing the final quick disconnect, the astronauts will loosen five electrical connectors and four bolts securing the pump to the solar truss.
As the outing winds down, Wheelock will transfer the old pump to the Mobile Transporter, a rail car that rides along the truss for temporary storage.
If some of the ammonia sticks so their space suits, Wheelock and Caldwell Dyson will spend time allowing it to evaporate before they re-join their U. S. and Russian colleagues inside the station.
During a Sunday spacewalk, the astronauts would install a replacement pump that is currently secured to a stowage platform outside of the station’s Quest airlock, re-connect the coolant lines and restore electricity..
Fellow NASA astronaut Shannon Walker will serve as the robot arm operator during the spacewalks. Wheelock will work from the tip of the 58-foot-long robot arm.
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