Many firsts for women in a very male orbit.

In 2007, Peggy Whitson set off on her second trip to the space station and soon became its first woman commander. At the time, she was one of only three women to have lived on the I.S.S. Sunita Williams later commanded it in 2012, and Dr. Whitson had command again in 2017. They are the only women ever to take command of the station.

Both women are amazing, but why did it take so long to have a woman commander. And why have there been just two?

Partly it’s statistics: Only 66 women have been into space, compared to just over 500 men. When approximately 90 percent of space travelers are men, it’s easy for years to pass in which the space station is populated by all-male crews.

The record for the number of women in space simultaneously is four; that was set when the Space Shuttle Discovery visited the I.S.S. in 2010. At the time, there were nine men and four women in space. That year also was the first time that two women served on the space station simultaneously — but it took another decade for the feat to be repeated.

In 2019, Anne McClain and Christina Koch were scheduled to conduct the first all-woman spacewalk — until NASA discovered that there weren’t two spacesuits that fit both astronauts. Spacesuits are generally too large for many of the women astronauts, who are then unable to participate in all aspects of a mission.

That limitation, in turn, reduces the number of appropriate slots for women on missions, so fewer women fly, which leads to fewer commanders. Even so, Ms. McClain accomplished a first on that mission: She became the first woman to serve in two different crews with other women.

Pause for a moment to unpack that. Until last year, with that one exception in 2010, every woman who has lived on the I.S.S. did so with an otherwise all-male crew.

Months later, in October 2019, that all-woman spacewalk finally happened. But while Jessica Meir and Ms. Koch were the only people outside the space station, they were still outnumbered inside by men.

NASA has been working to achieve gender parity in its astronaut classes. But even if it balances out the American astronaut corps, an all-female I.S.S. crew remains a distant dream, because our international partners fly almost exclusively men.

Now the retirement of the station is on the horizon. Will a third woman ever command it?