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The astronauts will exit the station's Quest airlock to replace an avionics box that routes command and data information to external science experiments.
Today's milestone spacewalk marks the 200th time ISS astronauts exited the station to go outside, into open space. The spacewalk is expected to last around 6,5 hours. Any duration between five and eight hours is the norm.
Current world record for the largest number of spacewalks is held by Russian astronaut Anatoly Solovyev, who has been on 16 spacewalks and spent more than 82 hours outside in space.