Spacewalking astronauts plugged a leak and finished fixing a detector.

Spectrometer walks described as complex

CAPE CANAVERAL, Fla. — Spacewalking astronauts plugged a leak in a cosmic ray detector outside the International Space Station on Saturday, completing a series of complex repairs to give the instrument new life.

The $2 billion Alpha Magnetic Spectrometer could resume its hunt for elusive antimatter and dark matter by midweek.

Team members around the world expressed relief as NASA’s Andrew Morgan and Italy’s Luca Parmitano wrapped up work on the spectrometer. It was their fourth and final spacewalk since November to revive the instrument’s crippled cooling system.

“Congratulations … the AMS pump system is now leak tight,” tweeted the European Organization for Nuclear Research, or CERN, which helps run the spectrometer.

Mission Control cautioned it was too soon to declare success with the space station’s premier science instrument, but noted: “It still has a good heartbeat.”

Last month, Morgan and Parmitano installed new coolant pumps on the spectrometer. They went back out Saturday to check for any leaks in the plumbing.

Parmitano quickly discovered a leak in one of the eight coolant lines — the first one he tested — and tightened the fitting.

The line still leaked after a mandatory one-hour wait, and Parmitano tightened it again. Finally, success — the leak was gone.

Mission Control acknowledged the leak added some unwanted “drama” to the spacewalk. “Everybody’s hearts stopped,” Mission Control told the astronauts.

“It was hard fought today, really well done. Cool heads prevailed,” Mission Control said as the spacewalk drew to a close.

Barring further trouble, the spectrometer — launched to the space station in 2011 — will have its coolant lines filled with more carbon dioxide Sunday.

One pump will be turned on as early as Monday and the remaining three Tuesday. That could lead to the resumption of science observations by Wednesday.

NASA described the spectrometer spacewalks as the most complicated since the Hubble Space Telescope repair missions a couple of decades ago.

Unlike Hubble, this spectrometer was never intended for astronaut handling in orbit, and once it started faltering in 2014, it took NASA years to devise a repair plan.

NASA In a photograph taken from NASA’s Andrew Morgan’s helmetcam, Italian astronaut Luca Parmitano works outside the International Space Station during a spacewalk Saturday. The astronauts worked to complete repairs to a cosmic ray detector.