After an exhaustive effort to come up with a workaround to fix a balky valve in one of the International Space Station's two coolant loops, NASA managers decided Tuesday to change gears and press ahead with at least two and possibly three spacewalks to replace a refrigerator-size ammonia pump module.

Astronauts Rick Mastracchio, a spacewalk veteran, and first-time flier Mike Hopkins are scheduled to begin the first spacewalk, or EVA, Saturday around 7:10 a.m. EST (GMT-5). The second EVA is planned for Monday with the third, if necessary, on Christmas day.

The decision to carry out multiple spacewalks to repair coolant loop A means a delay for the planned launch of an Orbital Sciences Corp. Antares rocket carrying the company's Cygnus cargo capsule. Orbital engineers rolled the rocket to its seaside pad at the Mid-Atlantic Regional Spaceport at NASA's Wallops Island, Va., flight facility early Tuesday for a possible launch try Thursday night.

The flight now will be put on hold until next month.

"Orbital Sciences Cygnus spacecraft, atop its Antares rocket, now will launch no earlier than Jan. 13," NASA said in a statement. "The postponement of the Antares launch will allow ample time for the station crew to focus on repairing a faulty pump module that stopped working properly on Dec. 11."

While the station's six-member crew is not in any danger because of the coolant system problem, research activities have been curtailed and, more important, the lab has lost redundancy in a critical system. If a problem took down coolant loop B, the crew could be forced to evacuate and return to Earth aboard their Soyuz ferry craft.

"Our best position to be in is to have both those loops up and running and available to us," Kenny Todd, a senior space station manager at the Johnson Space Center in Houston, said earlier. "While we're sitting at one loop, I think we're somewhat vulnerable, and so clearly, from a program perspective, our intention would be to move sooner rather than later to recover that functionality."

The space station is equipped with two independent external coolant loops that use ammonia to dissipate the heat generated by the lab's electrical systems. Cooling is critical for station operations and while one loop can support critical systems and keep the lab operational, both are needed to avoid powerdowns of non-essential equipment.

The problem with coolant loop A developed last week when the flow control valve inside the station's starboard/loop A ammonia pump module malfunctioned, allowing the temperature of the coolant to drop below safety limits.

That did not affect loop A's ability to cool major electrical components mounted in the station's solar power truss. But to carry away heat generated by systems inside the station's habitable modules, where ammonia is not allowed, the coolant must flow through "interface heat exchangers" where it picks up heat carried by water that flows through cold plates where electrical gear is mounted.

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NASA plans spacewalks to repair International Space Station