Spacewalking astronauts have ventured out to install new pumps on a cosmic ray detector outside the International Space Station.
Monday’s spacewalk is the third in nearly three weeks for Italy’s Luca Parmitano and Nasa’s Andrew Morgan.
And it is the culmination of years of work to repair the alpha magnetic spectrometer.
The astronauts must connect four new pumps to the spectrometer, which is crippled without proper cooling.
The two billion US dollar spectrometer has been up there hunting for antimatter and dark matter for eight and a half years.
Nasa compares the series of spacewalks, the most complex since the Hubble Space Telescope missions, to heart surgery.
The spectrometer was never meant for hands-on repairs like this.
Mission Control urged the spacewalkers to “take good care” of the pumps, as they headed out.