Some explorers have focused on alpine heights. Others on polar extremes. Megan McArthur is one of the elite few who can say she's piloted both submarines and spacecraft, exploring expanses from the ocean floor to low Earth orbit, looking down on the planet from 250 miles above. Now McArthur, 54, is retiring from NASA, where she has served for more than two decades as an astronaut and senior ...

From left, European Space Agency astronaut Thomas Pesquet, NASA astronauts Megan McArthur and Shane Kimbrough, and Japanese astronaut Akihiko Hoshide pose for a photo after arriving at Kennedy Space Center, Florida, Friday, April 16, 2021, ahead of the SpaceX Crew-2 launch to the International Space Station.

Joe Burbank/Orlando Sentinel/TNS

Some explorers have focused on alpine heights. Others on polar extremes.

Megan McArthur is one of the elite few who can say she's piloted both submarines and spacecraft, exploring expanses from the ocean floor to low Earth orbit, looking down on the planet from 250 miles above.

Now McArthur, 54, is retiring from NASA, where she has served for more than two decades as an astronaut and senior leader at the agency's Jet Propulsion Laboratory in Houston.

Emily Carney, a space historian, described McArthur as a pioneer, one of the first 100 women to fly in space, and someone with a "magnificent career."

She was the first woman to pilot a SpaceX Dragon spacecraft and the last person to "touch" the Hubble Space Telescope with the space shuttle's robotic arm. She has logged 213 days in orbit — 200 on the International Space Station in 2021, and 13 days aboard the space shuttle Atlantis in 2009, as part of NASA's final flight to carry out repairs and upgrades to Hubble.

"Her contributions have helped shape the future of human space exploration, and we are incredibly grateful for her service," said Steve Koerner, acting Johnson Space Center director.

McArthur also built and piloted a human-powered submarine as an undergraduate at UCLA, where she majored in aerospace engineering. She received a PhD in oceanography from the Scripps Institution of Oceanography at UC San Diego in 2002 — which she completed while training to serve as a NASA astronaut.

"That was a really kind of a crazy time in my life, when I got the phone call to ask me: Did I want to come to work as an astronaut, as an astronaut candidate? Of course I said yes," she said, recalling the phone call she received from NASA in 2000, in an interview from the International Space Station in 2021.

Her thesis advisor, Bill Hodgkiss, professor of applied ocean science at Scripps' Marine Physical Laboratory, said she is the only student he ever had who became an astronaut.

He said she built models to understand the composition of the ocean floor, using sound. Her specialty was ocean floor acoustics. Although there is little obvious overlap between her PhD work and what she has done as an astronaut, the skills she honed have certainly contributed to her successful career.

"Few people get to do for the rest of their life what they did as a graduate student," he said. "What you really learn is how to tackle complicated research problems. And being trained as an engineering-oriented PhD student is a great thing for an astronaut because of the new problems they're going to encounter."

McArthur, who was born in Honolulu, first became enamored of spaceflight as a teenager living in Mountain View, Calif., where her father was stationed at Moffett Field Naval Air Station.

"My dad was a pilot there, and we used to see astronauts come for training, for shuttle landings ... and that's kind of when I first realized this is a real job that real people have," she told a group of Scripps Institute of Oceanography students from the space station.

She said it was her experience building a human-powered submarine as an undergraduate that got her thinking about a career in the oceans.

"Exploring the oceans, is, in a lot of ways, similar to exploring space," she said. There are a lot of similar operational concepts, such as "you have to have all of the equipment that you need to do your work. You have to be able to fix the things that break, and you have to, you know, have a plan and then adapt when things don't work out quite according to that plan."

Carney, the historian, said McArthur joined the astronaut corps at a time when admitting women into the ranks was becoming more common.

"They were flying on the International Space Station, they had done a lot on Hubble, they were an equal partner in spaceflight," she said. "The space shuttle program really helped to democratize spaceflight."

She's worried, however, that the Trump administration, with its cutbacks to the space program and objections to diversity, could reverse the gains women have made, as well as those of Black, Asian and other minority professionals.

She said she is hopeful that commercial space opportunities with companies such as Virgin Galactic will counter that.

McArthur is the chief science officer at Space Center Houston, where she now works to promote spaceflight and STEM (science, technology, engineering and math) research and education to visitors at the center.

"It was an incredible privilege to serve as a NASA astronaut, working with scientists from around the world on cutting-edge research that continues to have a lasting impact here on Earth and prepares humanity for future exploration at the moon and Mars," McArthur said in a statement.

"Seeing our beautiful planet from space makes it so clear how fragile and precious our home is, and how vital it is that we protect it. I am grateful I had the opportunity to contribute to this work, and I'm excited to watch our brilliant engineers and scientists at NASA conquer new challenges and pursue further scientific discoveries for the benefit of all."