In 1976, Kathy Sullivan was finishing up her Ph.D. in oceanography when an intriguing, if somewhat far-fetched, opportunity presented itself: the chance to become an astronaut. Her expertise was in the geology of the deep-sea floor, a few hundred miles in the exact opposite direction of where a space flight would take her, and joining NASA, she realized, could shut the door forever on her career in oceanography.

“I loved the expeditionary part of oceanography best of all,” she says, “being out at sea, adapting to what odd circumstances came your way, bad weather or broken equipment. I adored that.” But on a visit home during Christmas break, her brother told her that NASA was recruiting astronauts for its new space-shuttle program. It seemed at first to have nothing to do with the path she was on, but once she read up on it, she realized NASA was looking for people to do in space what she loved doing at sea — planning and executing scientific expeditions. She applied and beat out more than 8,700 other applicants, becoming one of the first six women accepted to NASA’s astronaut corps.

Forty-two years and three space-shuttle missions later, including one in which she became the first American woman to do a spacewalk — i.e., float out there in the abyss tethered to the back of a spaceship orbiting at 17,500 miles per hour — she hit another milestone. In oceanography, no less. “As it turned out, the door didn’t end up being completely nailed shut,” she says. “I’ve been able to cross back and forth between the space arena and the ocean arena in a lot of wonderful ways.”

Last June, Sullivan, 69, became the first woman and only the eighth person overall to submerge 36,000 feet into a part of the Pacific known as the Challenger Deep, making her the only human — male or female — to have been in both outer space and the deepest part of the ocean.

“Dr. Sullivan has been a great explorer her whole life,” says Victor Vescovo, the former naval officer and private-equity investor who invited her on the expedition, in which the two of them plunged through the pitch-black ocean — it takes four hours to get to the bottom — in his bespoke 15-foot submersible to take more precise measurements of the depth. (Only 20 percent of the ocean floor has been mapped; we know less about it than we do about the surface of the moon.) “I couldn’t think of anyone better,” Vescovo says, “to be the first woman to the bottom of Challenger Deep.”

Though Sullivan has racked up a hatful of firsts with her adventurous achievements, she was never in it for the headlines or to make the record books. “It’s about the things much greater than me that I have been able to be a part of and contribute to,” she says. Much of her life’s work has been about enabling other scientists’ research rather than pursuing her own. On her first space mission, her team launched a satellite to measure how much energy the sun is adding to the Earth every day, thus gathering years of invaluable data critical for making projections about climate change. In 1990, she was part of the space-shuttle crew that launched the Hubble telescope, which for 30 years has allowed astronomers to peer deeper and deeper into space, finding new galaxies and planets, and to better understand the very origins of the universe.

“I think what it boils down to is I was always just really interested in how things worked,” she says. She didn’t start off pursuing a job or a title so much as a lifestyle. Growing up in Southern California, she devoured stories about Jacques Cousteau and the Apollo astronauts in Life and Look magazines. “ ‘How could my life be like that?’ That was kind of the driving force, which I simplified into: I need to find some kind of work where people buy me airplane tickets,” she says. But her ambitions were strictly terrestrial at first. She had a talent for languages and was fluent in French and German by the time she was done with high school. She was considering a career in the foreign service until she realized as an undergrad that the sciences were a better avenue to the “active, inquisitive life” she wanted to lead.

And while it certainly wasn’t the norm for a girl growing up in the 1950s and 1960s to feel like the sky was the limit, in her family, it was. Her father, an aerospace engineer, taught her how to fly a plane as a teenager. “The ethos in our family was very much ‘Nobody gets to edit what you’re interested in,’ ” she says. “People around you may voice opinions, but they don’t get to choose it for you. That’s the peanut gallery. Ignore them.”

It was a good ethos to have in her pocket walking into NASA in 1978. In the early days, when she took her seat at meetings, Sullivan remembers, “Every now and then I would realize there was a bit of an odd glance being shot around the room that might have been ‘What the hell is that woman doing at the table?’ ” But as soon as she was introduced as one of the astronauts, “that pretty well settled it.” It was the most prestigious title you could have at NASA, and she was accorded the respect it was due. “I mean, I think there were undoubtedly some of the older folks, in particular the older men, that maybe doubted” whether women were up to it, she says.

But any doubts that Kathy Sullivan had the right stuff have long been put to rest. One of her father’s favorite family stories, she says, was when he went to mission control and stood in the viewing room to see her spacewalk. He could see the flight surgeon’s console, which was monitoring her heartbeat. When she stepped out of the hatch it spiked to 78 for a moment before settling back down to 60 as she worked for three and a half hours in equanimity despite the expanse of the universe being at her back. “I was just ready to go,” she says. “I felt completely calm.”

Although outer space and the ocean floor could not be further apart physically, there are some similarities in traveling to them, she says, not least of which is that they are both lethal environments. “You’re in a craft that’s taking you somewhere that you otherwise cannot possibly be and have no business being,” she says. “It’s, of course, not magic, it’s good engineering and design. But the experience has always seemed to me to have a dimension of magic to it. That I can be in this place that is so fascinating, and I’m just me. I feel normal. It’s a little bit of cognitive dissonance to sit in this little five-foot diameter sphere and eat a tuna sandwich and yet be thousands of feet underwater, or to be floating around in a spaceship eating some M&M’s.”

After NASA, Sullivan used her multidisciplinary talents in a number of administrative roles — most notably as head of the National Oceanic and Atmospheric Administration under President Obama. Perhaps less exhilarating than her expeditionary work, it still satisfied her itch to figure out how things work. “It’s just a different kind of jigsaw puzzle,” she says. “I take great satisfaction in being able to develop and curry the talents that can take on great challenges and make an organization hum and do great things.”

There is, of course, a lot left to explore out there, but Sullivan doesn’t have a bucket list of places she is still dying to see. “You’d say, ‘Girl, you’ve been in outer space. You can’t possibly have a list like that,’ ” she says. The sightseeing from the space shuttle would be pretty hard to beat. “I remember looking down at the Earth,” she says. “Below me, it’s early evening [but] we’re still bathed in sunlight, several hundred miles above. It dawned on me, and really sort of stunned me, that there could be some little girl down there pointing up at the sky and saying, ‘Look, Mommy, it’s a satellite,’ and she’s pointing at me.”