Crucial ISS spacewalk underway
Russian Yury Malenchenko is monitoring their work from inside the station.
The astronauts can only work during 33-minute time intervals while the sun is blocked by the Earth.
The astronauts will try to replace a broken motor drive, which helps transmit electricity from the solar arrays to the station.
The task is considered especially dangerous as the astronauts will expose themselves to electrical current.
Their previous attempt last December failed. If they don't manage this time, the scheduled additions to the ISS will be put in question.