MOSCOW (Reuters) - Two Russian cosmonauts left the Mir orbiting station on Friday on their fourth space walk this month to carry out repair work, a Mission Control spokeswoman said. "Talgat Musabayev and Nikolai Budarin began the space walk at 11.40 a.m.(0740 GMT)," she told Reuters by telephone. She said Musabayev had opened a hatch and told Mission Control: "We are begining work." The cosmonauts intend to do work needed to enable a new external propulsion unit to be installed during another space walk, scheduled for next week. They are expected to spend six hours outside Mir. The third crew member, NASA astronaut Andrew Thomas, remained inside Mir. During their last space walk less than a week ago, Musabayev and Budarin detached and jettisoned a propulsion unit attached to Mir by a 14-metre (45-foot) boom which helps keep the station aligned to the sun and gathering solar power. The cosmonauts have been carrying out regular repairs on Mir, which has been in space 12 years and has suffered a number of mishaps in the past year, including a collision last June which was the worst accident since it was launched.