Peggy Whitson, a 56-year-old American astronaut, has become the oldest woman to perform a spacewalk.
She was already the oldest woman to have ever entered space.
Over six and a half hours, Whitson and fellow astronaut Shane Kimbrough, 49, performed a “power maintenance spacewalk”, NASA said.
The spacewalk, which started at 12 noon GMT, involved replacing the ISS’s lithium-ion batteries, which store solar energy used to power the station.
Ahead of the walk, Whitson told the Associated Press she had adapted to life onboard the ISS faster than during her previous visit.
“I love being in space,” she said. “I sleep like a baby. I mean, it is amazing the great sleep. Nothing hurts when you’re lying in bed. You can sleep the whole night through. It’s great.”
While the majority of the work was carried out by a remote control robot, another spacewalk is scheduled to complete the installation.
On 13 January, European Space Agency astronaut Thomas Pesquet and Kimbrough will wrap up the mission.