Astronauts have begun an extraordinarily complicated series of spacewalks to fix a cosmic ray detector at the International Space Station.

Italian astronaut Luca Parmitano and US astronaut Andrew Morgan ventured out on Friday with dozens of tools and four new pumps for the Alpha Magnetic Spectrometer.

Nasa considers these spacewalks the most difficult since the Hubble Space Telescope repairs a few decades ago.

Unlike Hubble, the spectrometer was never meant to undergo space surgery. After more than eight years in orbit, its cooling system is almost dead.

The astronauts will go out at least four times this month and next to revitalise the instrument.

Nasa has described the work as a heart bypass.

The spectrometer is searching for elusive antimatter and dark matter. It has already studied 148 billion cosmic rays.