CAPE CANAVERAL, FLA. -- Spacewalking astronauts ventured out Monday to install new pumps on a cosmic ray detector outside the International Space Station.

It was the third spacewalk in nearly three weeks for Italy's Luca Parmitano and NASA's Andrew Morgan. And it marked the culmination of years of work to repair the Alpha Magnetic Spectrometer.

The astronauts need to connect four new pumps to the spectrometer, which is crippled without proper cooling. The $2 billion instrument has been up there hunting for antimatter and dark matter for 8 1/2 years.

NASA compares the series of spacewalks -- the most complex since the Hubble Space Telescope missions -- to heart bypass surgery. The spectrometer was never meant for hands-on repairs like this and was designed to last just three years.

Once the new pumps are connected to the spectrometer -- bypassing the old, degraded pumps -- the spectrometer should last the entire life of the space station. That's another five to 10 years.

Given the high stakes, Mission Control urged the spacewalkers to "take good care" of the pumps. Parmitano clutched the 350-pound (159-kilogram) box of pumps, a bulky 3 1/2 feet (107 centimetres) by 2 1/2 feet (82 centimetres), with both hands as he headed toward the spectrometer.

Led by a Nobel laureate, the spectrometer flew up on space shuttle Endeavour's last mission in 2011. It's since studied more than 148 billion charged cosmic rays.