Experts reveal how long it will take NASA astronauts to recover after nine months stranded in space

U.S. astronauts Suni Williams and Butch Wilmore’s next mission will be readjusting to the Earth’s environment once they return Tuesday evening after being stranded at the International Space Station for nine months.

“There’s a lot of changes inside their body. The immune system is responding to the stress in spaceflight,” Weill Cornell Medicine professor Dr. Christopher Mason told “Fox & Friends” on Tuesday.

Dr. Mason was a principal investigator for NASA’s Twins Study, which studied how astronaut Scott Kelly’s body was impacted by his historic 12-month spaceflight in 2016. Kelly was studied against his identical twin brother, retired astronaut Sen. Mark Kelly, D-Ariz., who remained on Earth.

‘ALIEN’ ENCOUNTER: ISS CREW MEMBER PLAYS JOKE AS SPACEX TEAM ARRIVES

“We saw gene expression change, which is how genes are regulated in their body, and usually telomeres get a little bit longer in space from almost every mission we’ve looked at, so we’d expect some of these changes,” Dr. Mason explained.

Telomeres, according to the National Human Genome Research Institute, are “region[s] of repetitive DNA sequences at the end of a chromosome.”

The astronaut duo has been stranded at the International Space Station in microgravity since June. Their mission was only scheduled to last one week after the launch of Boeing’s first astronaut flight, but they were stuck in space after issues forced NASA to bring the Boeing Starliner back empty.

Spending time in space tends to leave astronauts temporarily taller and leaner, Mason explained.

“All of it is, at least on some measures… all transient. Most of it is a response to space flight, and usually in a few weeks they’re mostly back to normal. It’s the first few days that are the most really dynamic when they get back to Earth,” he added.

STRANDED ASTRONAUTS PREPARE FOR LONG-AWAITED RETURN TO EARTH

Former NASA astronaut Jose M. Hernandez, who once spent 14 days in space, said the duo will need “a lot” of physical therapy to restrengthen their bones and muscles.

“I remember my first two words when I came down was, ‘Gravity sucks,’ because your body starts adapting, and you’ve got to recalibrate your vestibular balance system,” he told “Fox & Friends First” Tuesday.