Spare coolant pump bolted into place on station

NASA is awaiting the "moment of truth" during today's spacewalk, when ground controllers will determine if a newly installed coolant pump is working properly.

Astronauts Doug Wheelock and Tracy Caldwell Dyson installed the pump less than three hours into a spacewalk this morning, their third to recover a crucial station coolant loop.

Caldwell Dyson helped Wheelock guide the spare pump module -- the size of a large kitchen appliance -- onto rails on the starboard side of the station's main truss.

"Like butter," she said as the 780-pound module slid into place.

Wheelock fastened four bolts to secure the connection. He had held the bulky module while standing on the end of the station's robotic arm.

Caldwell Dyson then began to connect five electrical cables to the module to give it power. Ground controllers will briefly run the pump to ensure its motor is functioning as expected.

"Then the moment of truth as the thermal systems officer monitors his data to ensure that the new spare is alive and well," said NASA TV commentator Rob Navias.

If that's the case, Wheelock will proceed to connect four ammonia fluid lines to the new pump, a task that again poses a risk of the kind of ammonia leaks seen when the spacewalkers removed the failed pump earlier this month.

The spare was flown to the station four years ago during shuttle Discovery's STS-121 mission.