After prepping their patient — a $2 billion cosmic ray detector — during two earlier spacewalks, two space station astronauts ventured back outside for a third outing Monday to carry out what amounted to transplant surgery, installing replacement coolant pumps in a bid to revive the costly instrument and extend its life.

The work required European Space Agency astronaut Luca Parmitano and NASA crewmate Drew Morgan to attach the 350-pound pump module to the 7.5-ton Alpha Magnetic Spectrometer, plug in power and data cables and then splice together eight existing coolant lines that were cut during the crew's second spacewalk.

The AMS was not designed to be serviced in orbit and connecting, or "swaging," the small, relatively fragile coolant lines while working in pressurized spacesuits was considered an especially challenging task, rivaling work to repair some of the Hubble Space Telescope's most delicate systems.

But four years of planning and training paid off, and the astronauts had no problems working through their repair checklist, completing all of the required connections. They returned to the International Space Station's Quest airlock at 12:33 p.m. EST to wrap up a six-hour, two-minute excursion.

"You guys, I have to say, made that look easy today," Canadian astronaut Jeremy Hansen radioed from mission control in Houston. "A really nice job. We're in a good posture to get AMS up and running on the next EVA (spacewalk)."

The crew reported a small amount of water in Parmitano's spacesuit at the end of the outing, a reminder that he was forced to make an emergency return to the airlock during a 2013 spacewalk when his helmet flooded with water from the suit's cooling system. It was not immediately known where the water observed Monday came from or what impact, if any, it might have on the crew's plans for a fourth and final AMS repair spacewalk.

A final excursion is required to make sure the AMS coolant system is leak-free and to reinstall insulation before the instrument can be reactivated.

But the crew's December schedule is jam-packed, with the arrival of U.S. and Russian cargo ships and Boeing's Starliner commercial crew capsule, which is scheduled for launch to the station on an unpiloted test flight around December 17. Whether the crew can squeeze in another spacewalk between now and the end of the year is not yet known.

But the crew was thrilled getting through the first three spacewalks without any major problems, crediting their training teams for the successful work.

"Hopefully with the next EVA, we'll find out all this work was put to good use," Parmitano radioed.

Floating in the Quest airlock compartment, Parmitano and Morgan switched their spacesuits to battery power at 6:31 a.m. EST to officially kick off the 11th spacewalk so far this year, the 224th devoted to station assembly and maintenance since construction began in 1998.

Launched to the space station in 2011, the AMS was built to study high-energy cosmic rays to glean clues about what happened to the antimatter presumably created during the big bang in equal measure with normal matter. Matter and antimatter annihilate on contact, and it's not clear why the universe is dominated by normal matter today.

The AMS also may shed light on the nature of the unseen dark matter permeating the universe and the mysterious dark energy that appears to be speeding up the expansion of the cosmos.

To achieve the required sensitivity, the AMS detectors must be chilled using carbon dioxide coolant pushed through the instrument in thin lines the width of a pencil. Originally designed to operate for just three years, the AMS chalked up eight years of operation before being hobbled by coolant pump failures.

To fix the system, engineers came up with a four-spacewalk plan to install a custom-built module containing four powerful pumps and a reservoir of fresh carbon dioxide coolant. Because the AMS was not designed to be serviced, engineers had to come up with a variety of innovative tools and techniques, requiring years of planning, development and training.

"Something that is really, really cool for an astronaut is to actually be part of the development of an EVA," Parmitano said during pre-flight training. "I've been lucky enough to have been part of the development team from the beginning, initially just as a consultant and then ... as a test subject for some of the tools.

"We're going to perform what could be considered open heart surgery on this amazing experiment. We're going to cut tubes, and install a completely new pump to help the refrigeration work, keeping the magnet cold so the the Alpha Magnetic Spectrometer can work. This is really the first time any of these actions have been attempted."

During spacewalks November 15 and 22, Parmitano and Morgan removed a protective panel from the AMS, exposing the thermal control system. Then they cut zip ties and pulled back insulation, cut through a line to vent residual coolant overboard and then cut eight coolant circulation lines, setting the stage for Monday's spacewalk.

"The third EVA is when we bring out the new pump system, we install that, and then we've got ... eight tubes that we're connecting," said Brian Mader, an engineer at the Johnson Space Center who helped design the repair work. "It's a piece of art. It's really amazing how ... the engineers figured out how to coil these tubes on the box."

The pump module contains four small pumps, a spherical carbon dioxide tank and data and power cables. To splice it into the AMS thermal control system, Parmitano and Morgan connected the eight previously cut coolant lines, one by one, effectively splicing, or "swaging," them together using custom tools.

While the work appeared to go smoothly, the system will not be pressurized until the fourth spacewalk when Parmitano and Morgan will be able to tighten fittings as required to ensure leak-free performance. Once the final spacewalk is complete, engineers will press ahead with re-activating the AMS and resuming science operations.