CAPE CANAVERAL, FLORIDA - Spacewalking astronauts plugged a leak in a cosmic ray detector outside the International Space Station on Saturday, bringing it another step closer to new life. 

It was the fourth spacewalk since November for NASA's Andrew Morgan and Italy's Luca Parmitano to fix the Alpha Magnetic Spectrometer. They installed new coolant pumps last month to revive the instrument's crippled cooling system and needed 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. ``Our day just got a little more challenging,'' Mission Control observed. 

The line still leaked after a mandatory one-hour wait, and Parmitano tightened it again. Finally, success — the leak was gone. ``Let us all take a breath,'' Mission Control urged. By then, the astronauts were already halfway into their planned six-hour spacewalk. 

Mission Control acknowledged the leak added some unwanted ``drama'' to the spacewalk. ``Everybody's hearts stopped,'' Mission Control told the astronauts. Parmitano wondered aloud what the flight surgeon in Houston saw when the leak erupted — he said his heart rate ``either flat-lined or spiked, one of the two.'' 

Back to work soon

Barring further trouble, the $2 billion spectrometer — launched to the space station in 2011 — could resume its hunt for elusive antimatter and dark matter next week, according to NASA. 

NASA has described the spectrometer spacewalks as the most complicated since the Hubble Space Telescope repair missions a few decades ago. Unlike Hubble, this spectrometer was never intended for astronaut handling in orbit, and it took NASA years to devise a repair plan. 

Despite their complexity, the first three spacewalks went well. Morgan and Parmitano had to cut into stainless steel pipes to bypass the spectrometer's old, degraded coolant pumps, and then splice the tubes into the four new pumps — no easy job when working in bulky gloves. The system uses carbon dioxide as the coolant. 

Besides checking for leaks Saturday, the astronauts had to cover the spectrometer with thermal insulation. 

``We are very excited for you to be finishing off all of the amazing work that you've already put into this AMS repair,'' astronaut Jessica Meir radioed from inside as the spacewalk got underway, ``and I think everyone's excited to the prospects of what AMS has to offer once you guys finish off the work today.'' 

The 7½-ton (6,800-kilogram) spectrometer was launched to the space station on NASA's next-to-last shuttle flight. Until it was shut down late last year for the repair work, it had studied more than 148 billion charged cosmic rays. The project is led by Samuel Ting, a Nobel laureate at the Massachusetts Institute of Technology. 

Good for as much as 10 years

The repairs should allow the spectrometer to continue working for the rest of the life of the space station, or another five to 10 years. It was designed to operate for three years and so already has surpassed its expected lifetime. 

NASA's two other astronauts on board, Meir and Christina Koch, performed two spacewalks over the past 1½ weeks to upgrade the space station's solar power system. 

Altogether, this station crew has gone out on nine spacewalks. 

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