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Josh Cassada and Frank Rubio leave the International Space Station on Saturday for an outside mission.
The NASA astronauts install a solar panel - the first of six so-called iROSA modules.
The new solar system should generate 120 kilowatts on a space day, according to Nasa the ISS should then have 20 to 30 percent more energy.
A SpaceX space transporter brought the individual parts of the system into space on November 27th.
When fully unfolded, the iROSA collectors will be 19 meters long and six meters wide.
The two astronauts were not as alone as it seems during their mission: NASA astronaut Nicole Mann and her Japanese colleague Koichi Wakata directed them from inside the ISS.