The term invokes a vision of gliding through the galaxy like a captain navigating uncharted seas. The moniker is adventurous, poetic — and for the men and women who have earned the title, it holds great distinction: Fewer than 600 people have ventured beyond Earth’s atmosphere — and not all of them were called astronauts.

But why American, European, Canadian, and Japanese space travelers are called astronauts is a little mystifying — and a little lost to history. Here’s what we know (and don’t know) about the origin of the word ‘astronaut,’ and what it means to those few who sail through space.

Jennifer Ross-Nazzal, Ph.D., a historian at NASA Johnson Space Center, says ‘astronaut’ dates back to late 1958, when NASA was still in its infancy. The agency used the name ‘astronaut’ in drafts of a job listing that called for applicants to its new astronaut program. Ross-Nazzal says that NASA also considered ‘cosmonaut,’ or universe sailor, before adopting the name astronaut.

Astronaut is a curious choice: While poetic, it’s not apropos of space travel. After all, astronauts don’t actually sail through the stars; they fly past them at thousands of miles per hour. And in fact, NASA’s first astronauts were exclusively pilots. But to distinguish them from U.S. Air Force pilots — and to reinforce their strictly civilian status — these men needed another name.

In the late 1950s, at the time NASA was formed, depictions of space travel were “colorful and kooky,” and on the fringe, filled with “flying saucers” in B-list movies, says David S. F. Portree, a space and science historian. So, he wagers, the name of its spacemen could be anything but.

Perhaps the name ‘astronaut’ was considered, Portree muses, in an effort to be taken seriously.

By 1961 Russia had commandeered ‘cosmonaut’ for its space explorers, and China would later dub its spacemen ‘taikonauts,’ or space sailors, both echoing and strengthening the nautical motif. European, Canadian, and Japanese space organizations also later chose to adopt ‘astronaut.’

While the first NASA astronauts were all military test pilots, today, almost anyone who flies on the agency’s spacecraft is given the title ‘astronaut’ — pilot’s license or not.

In 1965, six scientists were chosen for astronaut training. (Some, including geologist Harrison “Jack” Schmitt, would go on to pilot lunar modules.) And later, payload specialists — men and women who carried out specific experiments for research or commercial organizations during NASA space flights — were dubbed astronauts by the general public, even though they didn’t undergo any of the agency’s formal astronaut selection or training, nor fly NASA spacecraft.

But even with the loosened idea of who can be called an ‘astronaut,’ fewer than 350 Americans have held the official — or unofficial — title.

“Often when people hear the word ‘astronaut,’ they sort of light up,” says Ross-Nazzal. “They think it’s this special word, this special occupation, because so few people have been astronauts.”

With the advent of space tourism, however, the ranks of ‘astronauts’ may grow, however, and the moniker may once again change meaning. 

The Russian Space Agency has flown tourists to space. And companies Virgin Galactic and Blue Origin are developing private space programs that will bring tourists into and out of orbit.

“We’ve seen the meaning of the term ‘astronaut’ change over the decades,” says Portree, who thinks the name should be allowed to embrace even the most casual of space travelers.

“Maybe space tourists should be called ‘recreational astronauts,’” Portree says, adding that “I don’t think that takes anything away from the accomplishments of ‘professional astronauts.’”

Portree likens the comparison of recreational and professional astronauts to that of amateur and professional astronomers. “Amateurs today are armed with stargazing equipment professional astronomers a century ago would have died for,” he says. But, “we don’t see a lot of confusion. Professionals are still professionals; amateurs are still amateurs.” And while the line may blur a bit as technology changes and discoveries are made, the pros “will always know who they are.”