Astronauts set for third instrument repair spacewalk

After prepping their patient — a $2 billion cosmic ray detector — during two earlier spacewalks, two astronauts are set to operate during a third outing Monday, performing what amounts to transplant surgery by installing replacement coolant pumps.

The work requires European Space Agency astronaut Luca Parmitano and NASA crewmate Drew Morgan to attach an intricate pump module to the 7.5-ton Alpha Magnetic Spectrometer and then splice it into eight thin coolant lines. A fourth spacewalk will be needed to check for leaks and to reattach insulation.

The equipment was not designed to be serviced in orbit and the work is considered the most challenging project since spacewalks to repair and upgrade the Hubble Space Telescope.

Floating in the Quest airlock compartment, Parmitano and Morgan are expected to switch their spacesuits to battery power around 6:50 a.m. EST to officially kick off the 11th spacewalk so far this year. It will be the 224th spacewalk devoted to International Space Station assembly and maintenance and the third of four needed to repair the Alpha Magnetic Spectrometer.

For identification, Parmitano, call sign EV-1, was assigned a suit with red stripes along with helmet camera 11. Morgan, EV-2, will be wearing an unmarked suit with “helmetcam” 18.

Launched 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.

The AMS may also shed light on the nature of the unseen dark matter permeating the universe, possibly helping to explain the motions of galaxies and galactic clusters, and the 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 was still collecting data after eight years, at which point a fourth and final coolant pump began failing.

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 develop a variety of innovative tools and techniques to complete the job, requiring years of planning and training.

“Something that is really really cool for an astronaut is to actually be part of the development of an extravehicular activity (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,” Parmitano explained. “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 on November 15 and 22, Parmitano and Morgan removed a protective panel from the AMS, exposing the thermal control system. They then cut zip ties, pulled back insulation, cut through a line to vent residual coolant overboard and then cut eight coolant circulation lines. That permanently disabled the AMS and set 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 module contains four small pumps, a spherical carbon dioxide tank, data and power cables. To splice it into the AMS thermal control system, Parmitano and Morgan will connect the eight previously cut cables, one by one, effectively splicing, or “swaging,” them together using custom tools.

During a fourth spacewalk, the astronauts will carry out leak checks, make any adjustments that might be needed and then re-install insulation.

“The fittings that we're using have a built-in leak check mechanism,” said Mader. “If it's leaking, it has a visual indicator that will show that. So, we come out and we look at those leak indicators. If there is a leak, then we take action. Hopefully, that's not going to happen. But you know, that's why it's there. And then we close everything out on that EVA.”

Flight planners have not yet set a date for the fourth spacewalk.