Two Nasa astronauts are making the first ever all-female spacewalk.
Christina Koch and Jessica Meir left the International Space Station's Quest airlock on Friday to replace a failed power control unit.
They were expected to spend five and a half hours replacing the power regulator with a spare.
Ms Koch had already carried out four spacewalks but it was the first for MS Meir, who Sbecame the 15th woman to walk in space, Nasa said.
Ms Koch, an electrical engineer, and Ms Meir, who has a doctorate in marine biology, stepped outside in their Nasa spacesuits at 11:38 GMT (07:38 EDT). They made their way to a location called the Port 6 truss structure to replace the a battery charge-discharge unit (BCDU).
Once the task is complete, Ms Koch and Ms Meir will return to the airlock with the failed BCDU. The device will subsequently be loaded on to the next SpaceX Dragon resupply ship for inspection on Earth.
Nasa had announced in March that Ms Koch would take part in the first all-female "extra-vehicular activity" (EVA) with colleague Anne McClain. But the spacewalk was called off because a medium-sized suit wasn't available in the near-term for McClain./BBC news