WASHINGTON - Two International Space Station astronauts were prepared Saturday to set out on the first of two spacewalks to fix a cooling pump that dramatically failed last week.

ISS astronauts will need at least two spacewalks to remove the failed ammonia pump unit and replace it with a new one after the device failed Saturday.

Astronauts Doug Wheelock and Tracy Caldwell Dyson, both flight engineers, are scheduled to head into space to start the repairs.

The first spacewalk will get under way at 1055 GMT on Saturday. In the second excursion on Wednesday, the crew is expected to complete connecting fluid ammonia lines to the replacement pump.

In the meantime the station "remains in a stable configuration," NASA said. "The crew is on a normal sleep shift, and supporting a normal workday, but most of the planned activities" for the week "have been cancelled or deferred in order to support spacewalk preparations."

The first spacewalk will focus on getting the failed unit out of the starboard truss on the ISS, which poses a few technical challenges including releasing lines that are pressurized with ammonia which is usually pumped into the cooling system, when the module is working, NASA said.

Astronauts ahead of the spacewalk were "taking a look at the activities to be performed during the excursion as well as familiarizing themselves with the steps needed to decontaminate the spacesuits should ammonia crystals seep out of the pump module connectors," NASA said.

Once the failed unit has been removed, the two astronauts will have to move a 780-pound (355 kilograms) spare unit around 30 feet (10 meters) from the opposite side of the truss for insertion into the gap left by the defective pump module.

"This is a big, unwieldy object, so maneuvering it around and handing it off to crew members... could take some time and a lot of focus," Courtenay McMillan, the spacewalk flight director for the expedition, told reporters earlier in the week.

But robotics experts have devised procedures that will be used by station crew member Shannon Walker to guide the station’s robotic arm, Canadarm2, from the robotics workstation in the Destiny Laboratory.

She will move Wheelock into position to swap the failed unit with the spare unit, stored on an external stowage platform.

That spare parts carrier is attached to the Quest airlock that Wheelock and Caldwell Dyson will use to exit and reenter the station.

The crew faces a very tight lead time for such a tricky spacewalk -- less than a week when NASA usually takes two weeks to prepare for a spacewalk to fix a "Big 14 failure," when a major unit stops working.

If the second of the two ISS cooling units fails -- a highly unlikely scenario, according to NASA -- then the ISS astronauts would no longer be able to cool most of space station components.

The crew would not be in danger however because they could move to the Russian segment of the ISS, which has its own cooling system.

Astronauts tried to reactivate the pump module after Saturday`s failure, but the circuit breaker tripped, NASA said.

BDST: 12:48 HRS, August 07, 2010