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ISS Crew Preps For Final Spacewalk.

NASA is preparing for a third - and hopefully final - spacewalk to repair the coolant pump on the International Space Station that failed two weeks ago.

The last spacewalk, conducted Wednesday, involved disconnecting the failed pump and stowing it on an external platform on the outside of the station, while inserting the replacement. The next excursion is scheduled for Monday.

Astronauts Doug Wheelock and Tracy Caldwell Dyson had originally tried to disconnect and remove the failed pump on Aug. 7. But as the two tried to disconnect one of the ammonia lines, the ammonia in the coolant system began leaking out.

Ground crews and the crew of the ISS lowered the pressure in the system, which seemed to solve the problem, said NASA spokesperson Nicole Cloutier-Lemasters. In their last spacewalk on Wednesday, they were able to disconnect the ammonia and power lines without a problem.

Cloutier-Lemasters said that one problem with ammonia is that in space the ammonia initially freezes, and settles on the astronauts' suits, like snow. They then have to wait in the sunlight for it to \"bake off\" - or turn into a gas --before they can re-enter the station. The closed environment in the station means that unlike the ammonia in a household cleaner, there is nowhere for it to dissipate to.

The astronauts are scheduled to be on the spacewalk for about six hours. In that time they should be able to reconnect the ammonia and power lines to the replacement pump and starting the system up again.

The coolant system is important for maintaining the temperature of both the station and many of the instruments and equipment on the outside of the structure. The ammonia coolant system is one of two \"loops\" in the system. The other uses water and keeps the habitat comfortable, while the ammonia loop transfers heat from the inside to the outside.

The station's temperature can be maintained with only one functioning loop, but the lack of an ammonia loop means that certain kinds of experiments can't be done, and some of the equipment in the station that generates a lot of heat can't be run at full capacity.
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Publication:International Business Times - US ed.
Date:Aug 13, 2010
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