HOUSTON—NASA astronaut Frank Rubio is feeling good and so far pleased with how his body is responding to Earth’s gravity after a U.S. record-setting 371 days in space on the International Space Station (ISS).

But he expressed caution during an Oct. 13 NASA news briefing about how his post-Sept. 27 return to Earth might translate down the road as NASA transitions its human spaceflight focus from the seven-person ISS to much longer-duration deep space missions to the Moon and Mars.

“Honestly, it’s such a varied response,” he explained, emphasizing the importance of staying fit through regular exercise while in space and collaborating closely with crewmates and ground control teams in assessing risks and managing the work schedule. “But really, a lot is dictated by genetics, your size and weight,” he said. “So, there is a lot of variability and it’s hard to say with a blanket statement ‘Hey, we will all be good or not.’”

Rubio, a U.S. Army lieutenant colonel, Special Forces helicopter pilot and medical doctor, was selected by NASA for astronaut training in 2017. His first mission to space, initially planned for six months, turned into a U.S. record-setter.

He launched aboard Russia’s Soyuz MS-22 on Sept. 21, 2022, with cosmonauts Sergey Prokopyev and Dmitry Petelin. On Dec. 14, their Soyuz crew transport capsule experienced an external coolant leak while docked to the ISS Russian segment.

The coolant loss, which was traced to a possible micrometeoroid or orbital debris impact, prompted Roscosmos, the Russian space agency, and NASA to deorbit the MS-22 uncrewed on March 28, 2023, after replacing it with the launch of the uncrewed Soyuz MS-23 on Feb. 24, 2023.

The mission by Rubio, Prokopyev and Petelin was extended in response, enabling Rubio to surpass the previous U.S. record of 355 days for the longest U.S. spaceflight. That was set by NASA astronaut Mark Vande Hei on March. 30, 2022, upon his return to Earth from the ISS.

Rubio said that he, Vande Hei and two other current and retired NASA astronauts—Christina Koch and Scott Kelly, who have logged more than 300-day ISS missions—will likely be medically monitored by NASA for the rest of their lives. That will help address the medical challenges to muscle and skeletal strength, cardiac function and changes in vision associated with long space missions.

“I can only speak to my experience, and I’ve been very pleasantly pleased. Honestly, I had a lot of curiosity myself. Just medical curiosity as to how my body was going to respond,” Rubio explained of his return to Earth. “Within a week, I actually felt very normal.”

Rubio initially experienced some pain in his feet and lower back after his lengthy stint in a weightless environment changed to readjusting to Earth’s gravity while reuniting with his wife and their four children.

“Within three days all the vertigo was gone. There was a little bit of vertigo when you first land. For the first couple of days you kind of veer a little to the right and left as you try to walk straight,” he explained. “Your mind is perfectly clear, but your body is not quite responding the way you expect it to. But within 72 hours that all cleared up, and I felt like it was really at a pretty high functional level. But again, that is kind of my response. That is really all I can speak to.”

As to whether he will launch to space again, Rubio said a possible opportunity is likely a couple of years away and will involve a health assessment, a consultation with his family and NASA deciding whether he could contribute to a mission.

“If I don’t fly myself, I hope to be at least helping crewmates, my buddies, get out there and do incredible things in space,” Rubio said.

The current 437-day world’s record for the longest human spaceflight was set by the late cosmonaut Valeri Polyakov aboard Russia’s former Mir space station between Jan. 8, 1994, and March 22, 1995. Rubio is now third on the world’s record list behind Polyakov and cosmonaut Sergei Avdeyev, who logged 380 days in orbit aboard Mir from Aug. 13, 1998, to Aug. 28, 1999.