"I'm sure that they'll sit back and reflect on it, as we all will. We will all celebrate that," McArthur said.

Koch and Meir, a marine biologist who arrived at the orbiting lab this week, are both members of NASA's Astronaut Class of 2013, the first and only one with an even split between men and women. They're also both making their first space flights.

Koch, an electrical engineer, is more than 200 days into an approximately 300-day mission, which will set a record for the longest single spaceflight by a woman.

"In the past, women haven't always been at the table," Koch said during a televised interview earlier this week. "And it's wonderful to be contributing to the human spaceflight program at a time when all contributions are being accepted, when everyone has a role, and that can lead, in turn, to increased chance for success."

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Since the world's first spacewalk in 1965, only 14 women have done them, versus 213 men, according to NASA.

Expect more women spacewalking together on the horizon.

"It turns out that over the next couple years, we're having a lot of medium suit people fly," NASA's space station program manager, Kirk Shireman, said.

Koch will serve as the lead spacewalker for Sunday's excursion with Andrew Morgan, her US male crewmate. There are 11 spacewalks coming up in the next few months - 10 US and one Russian. Only one is of two women.