When the Apollo astronauts went to space in the 1960s, Mae Jemison was a little girl in Chicago, watching the historic launches along with the rest of the country. She remembers being irritated that the crew members all looked the same: They were all white men. Where were the women, she wondered, or anyone of color?

“I thought that was the most absurd thing in the entire world,” Jemison says. “I just thought, well, would the aliens actually think this is all there is to humanity?”

Jemison made her own trip to space three decades later. She flew aboard the Space Shuttle Endeavor in September 1992 for a weeklong mission, becoming the first African-American woman in space. By that time, the United States had been sending women to space for about 10 years, starting with Sally Ride’s mission in 1983. American astronauts were starting to look less and less like the pioneers that first pierced the boundary between the atmosphere and what lies beyond. As more and more women went up, people stopped, for the most part, wondering how they’d handle their periods in space, or asking them how many tampons they’d need for a weeklong mission.

Today, if aliens dropped into low-Earth orbit, as Jemison imagined as a young girl, they’d find Peggy Whitson in command of the International Space Station.

In the 1950s, before any American had been to space, women were considered good potential candidates for spaceflight. As NASA’s first, all-male astronaut class was undergoing rigorous medical and physiological tests to make sure they could survive the shakes and sounds of a rocket launch, Randolph Lovelace, the doctor in charge of the examinations, suspected women might do as well or even better than the men. After all, women are, on average, lighter and smaller than men, and require less oxygen. So he started testing female pilots at his clinic in New Mexico in 1960, subjecting them to the same tests the male candidates faced. Thirteen of 19 women passed the tests, compared to 18 of 32 men. The women did particularly better than the men in isolation tests.

Lovelace may sound progressive for his time, but his reasoning for including women wasn’t. Lovelace imagined a future full of space stations circling Earth, carrying people busy at work. “He was thinking that if you’re going to have dozens of people on space stations, then you’re going to need secretaries, you’re going to need telephone operators, you’re going to need lab assistants—and that means you need women,” says Margaret Weitekamp, a curator in the space history department at the Smithsonian’s National Air & Space Museum and the author of Right Stuff, Wrong Sex: America’s First Women in Space Program. “So he’s thinking about the women for very much traditional, pink-collared, gendered jobs.”

Despite the promise of Lovelace’s tests, none of the participants ended up flying, and women would wait two more decades before getting their chance to blast off into space. After the Russians launched Sputnik in 1957, the space race that ensued became another proxy battle in the conflict between the United States and the Soviet Union, turning astronauts into de facto soldiers. Men, as they had always done, would go to the front lines. Women, the thinking went, were to be protected and kept away from the battlefield, even if it was inside a cramped capsule.

“The men go off and fight the wars and fly the airplanes and come back and help design and build and test them. The fact that women are not in this field is a fact of our social order,” said John Glenn, the first American to orbit the Earth, at a 1962 congressional hearing on gender discrimination at NASA, convened after much door-knocking in Washington by one of the members of Lovelace’s program, Jerrie Cobb. When the hearing took place, hiring discrimination on the basis of gender was two years away from being prohibited by law.

Some critics back then wondered how women could handle menstruation in microgravity, and whether exposure to radiation would affect fertility. The one and only report to come out of Lovelace’s program, published in 1964, noted that, although many women had passed strenuous testing, there were concerns about “the potential for the menstrual cycle to alter performance during space flight.” Spoiler: it doesn’t.

Other critics just made some really terrible jokes. “As for the ladies’ alleged ability to withstand boredom and confinement better than the man, I think there might be a number of harried husbands who have sat through long evenings listening to the wife’s recitations of the day’s activities, who have credentials in the area of tolerance and boredom,” said one NASA psychologist, according to Martha Ackmann’s book The Mercury 13: The True Story of Thirteen Women and the Dream of Space Flight. Wernher von Braun, a former Nazi and a NASA rocket engineer, said that male astronauts “are all for” allowing women to participate. He cited a quip from Robert Gilruth, the director of NASA’s aptly named Manned Spaceflight Center at the time. “As my friend Bob Gilruth says, we’re reserving 110 pounds of payload for recreational equipment.”

A shift came in the late 1970s, when NASA was preparing to launch the first mission of the Space Shuttle program. The shuttle could carry more passengers than ever, which meant not everyone onboard had to be a military pilot. Scientists and doctors were welcome, too. There was a sense, Weitekamp says, that space was now for everyone, which meant that the shuttle crews needed to better reflect the country’s population. NASA selected its first female astronauts in 1978, and Ride made her historic trip five years later. But old habits die hard. When Ride returned from her historic mission, someone tried to hand her a bouquet of roses. She declined. “She was not interested in being treated differently than the rest of the members of her crew because she was the one woman on that mission,” Weitekamp said.

In the years since, research has shown that women may be more susceptible to certain health risks than men in space. Women are more likely to get urinary tract infections, and have a lower threshold for exposure to radiation, due to the risk of breast, ovarian, and uterine cancers. None of these factors have anything to do with their brains, temperaments, or abilities—the qualities that critics once cited as disqualifying.

To date, nearly 60 American women have flown in space. The latest class of NASA astronauts, selected in 2013, includes four women and four men. What’s most interesting,  Weitekamp says, is not the gender parity. Two of the women have something the earliest female astronauts couldn’t: military backgrounds. One of them, Anne McClain, is an Army major who flew helicopters during combat missions in Iraq.

“She has a resume that puts her on par with what male candidates would have had access to a generation or so earlier,” Weitekamp said.