For Americans looking to reach the stars, there’s only one possible career that leads there: astronaut. On June 7, 2017, NASA revealed a new class of astronaut candidates, picked from a record-breaking 18,353 applications. In the 56 years of human spaceflight, only 338 other men and women have earned the rank of astronaut at NASA. So, how were these few selected?

The answer isn’t quite black-and-white—the process has changed drastically from the start of the space program to today. In fact, many of today’s astronauts would have been eliminated from consideration had they applied in 1959, when the first search commenced.

“I couldn’t have been an astronaut way back in the early days,” says former NASA astronaut Dr. Michael Massimino, who flew on two shuttle missions in the 2000s. Massimino is an engineering specialist who has twice repaired the Hubble Space Telescope, and became the first person to use Twitter in space on the daring final service mission, in 2009.

But Massimino’s engineering background would not have helped him half a century earlier: the original recruits all had to be military pilots who had flown at least 1,500 hours. Additionally, he would have been ruled out on the basis of height. “I’m 6 foot 3 inches tall,” he says. At the time, astronauts could be no more than 5 feet 11 inches tall, as the capsules could not fit taller men.

The rationale for choosing military test pilots for the astronaut program came from President Dwight D. Eisenhower, who reasoned that pilots were used to flying new, powerful technology. The records of 508 experienced pilots were screened in early 1959 by NASA’s Space Task Group (STG) for several criteria: The candidates must be under 40 years old, have graduated from test pilot school, be in top physical health, be qualified to fly jets and have a bachelor’s degree. This resulted in a pool of 108 men from the Air Force, Marines and Navy.

Sixty-nine arbitrarily selected men from this initial pool were invited to Washington, D.C., to undergo a series of interviews and briefings by the STG. Though the STG expected many candidates to back down once they learned of the mission, few did. Those who went forward with the process underwent a series of examinations—written, psychological, medical and others—of which 32 men passed and accepted astronaut candidacy.

At the 2017 International Space Development Conference, General Thomas P. Stafford, a former Air Force test pilot who became an astronaut in 1962, was asked why he pursued such a risky career. “I always wanted to go higher and faster,” he said. “People always ask ‘Were you scared?’ And the answer is no.”

The 32 men who had been chosen as astronauts were sent to the Lovelace Clinic in Albuquerque, New Mexico for even more thorough medical testing. Those who passed this round were then sent to Aeromedical Laboratory of the Wright Air Development Center in Dayton, Ohio, for a set of rigorous examinations designed to test the candidate’s physiological and psychological responses to situations expected in spaceflight. Following these tests, 18 men were recommended, and the STG narrowed it down to a final seven: Scott Carpenter, Gordon Cooper, Virgil “Gus” Grissom, John Glenn, Wally Schirra, Alan Shepard and Donald “Deke” Slayton.

Just six of these men would be successfully sent into space at that time. Slayton developed an irregular heart rhythm, grounding him until a 1975 Apollo-Soyuz mission. Soon, the U.S. set its sights on a new target—the moon—and NASA needed more astronauts. The agency put out an open call for applications, looking for generally similar criteria with a few tweaks: The height restriction was raised to 6 feet tall; the required hours of flight time was lowered to 1,000; the education requirement mandated a degree in physical science, biological science or engineering; the maximum age was lowered to 35; and, perhaps most importantly, civilian pilots—including women—were allowed to apply. Of approximately 250 applications, nine men, including Neil Armstrong, were selected to become part of Astronaut Group 2 in 1962.

During the 1960s, NASA put out new calls for astronauts every one to two years, changing the selection criteria. For Astronaut Group 3, NASA removed the test pilot requirement, replacing it with fighter jet pilot experience. For Astronaut Groups 4 and 6, the administration sought out scientists rather than pilots—applicants were required to have an M.D. or a Ph.D. in natural sciences or engineering. The last group of this era was Astronaut Group 7, selected in 1969—the year NASA succeeded in landing men on the moon. In all, 77 men became astronauts during the Mercury, Gemini and Apollo missions.

After the Apollo program ended in 1972, there was a decade-long pause in spaceflight as NASA developed the space shuttle. Astronaut Group 8 was recruited in 1978, just before the launch of the first shuttle in 1981. This was NASA’s largest class to date, with 35 astronauts (besting Group 5’s 19), including the first American woman in space, Sally Ride; the first African American in space, Guion Bluford; and the first Asian American in space, Ellison Onizuka, who perished in the Challenger disaster.

“For the shuttle there were two paths—there were the pilots and the mission specialists,” explains Massimino. “The pilots were people with test pilot experience, and the mission specialists were a grab bag—mostly scientists and engineers. The job then was not just a flying job like it was back in the early days. It was also a job that involved doing scientific experiments.”

With the discontinuation of the shuttle program in 2011, there’s currently less of a need to differentiate between pilots and mission specialists. But, notes Massimino, “I don’t see how we could stop taking military test pilots—I think it’s a really important background to have. The skills they have and the knowledge they have is really vital. But I think it’s also the same for the scientists and engineers.” Plus, with the development of new commercial crew vehicles, there will soon be a direct demand for pilots once again.

Since the shuttle era, the NASA astronaut selection process has remained largely the same. Applications for new classes open approximately every two to four years, and while there’s no longer an age maximum, the educational and professional requirements include a bachelor’s degree in engineering, biological science, physical science or mathematics, as well as three years of related professional experience (advanced degrees count towards this criterion) or a minimum of 1,000 hours piloting a jet. Astronaut candidates must also be U.S. citizens, though NASA does train astronauts from other countries who are accepted into their own space programs, like the Canadian Space Agency or the European Space Agency, on separate terms.

NASA still has physical requirements for astronauts. Vision must be correctable to 20/20 in each eye (glasses and corrective surgery are permitted), blood pressure must be at or below 140/90 in a sitting position, and height must be between 62 and 75 inches.

From the time applications are accepted, it takes approximately two years to select the new class of astronauts—once applications are reviewed (often by current astronauts), qualified applicants undergo reference checks and several rounds of interviews and medical exams at Johnson Space Center in Houston. Finalists earn the rank of astronaut candidate (or “ASCAN”), which they will retain for approximately two years during training. If the ASCAN passes training (which includes everything from SCUBA certification to perform underwater spacewalk simulations to learning how to pilot a T-38 aircraft), he or she will officially earn the rank of astronaut.

Despite their varied backgrounds, NASA astronauts share core traits, says Massimino. “The thing that unifies us is a common purpose of exploring space, of doing something that’s important for the world, finding out answers to who we are, where we came from, and how space can benefit our country and our world.”