Chris Hadfield is back safely on Earth after five months on the International Space Station, but it will take several months for the Canadian astronaut's body to return to its pre-flight condition.
Space wreaks its own form of havoc on virtually every aspect of the human body, particularly because gravity isn't around to create the conditions we live with on the ground.
After Hadfield landed on the steppes of Kazakhstan last night, he underwent some medical tests. He flies today to Houston, where NASA medical staff and others will poke, prod, test and otherwise help the 53-year-old regain his physical form.
"He will do more medical testing than most people will ever do in their life, and then he has to build back up his muscle and his bone and his legs," says his son, Evan Hadfield, who acted as his father's social media manager during his time in space.
"Everybody expects he's going to come back and immediately start going crazy doing stuff, but he's got to take some time for himself to get his body back in shape because you [lose] bone, so you need to build that back up. Without gravity, your body does not feel like wasting energy to build muscle and to build bone."
Human bones may feel and look hard, but they are in fact constantly growing.
"Your bones are being continually eaten away and replenished," says Bjarni Tryggvason, one of the first Canadian astronauts who had his own experiences with life in zero gravity during a 12-day flight on the space shuttle Discovery in 1997.
"The replenishment depends on the actual stresses in your bones and it's mainly ... bones in your legs where the stresses are all of a sudden reduced [in space] that you see the major bone loss. There's no significant loss in your arms and other bones."
Astronauts have typically lost from 0.4 to one per cent of their bone density per month in space, the Canadian Space Agency says.
So astronauts on the space station 400 kilometres above Earth work out for a couple of hours each day to try to stave off the atrophy their muscles inevitably undergo. They also take nutritional supplements and medication prescribed to post-menopausal women to try to counter bone loss.
Still, there's work to be done once they're back on Earth.
Bob Thirsk, Canada's first astronaut to do a long-duration flight on the space station, remembers what it took to get his body back to normal after his six-month mission in 2009.
"In the same way that every organ system needs to adapt to weightlessness, every organ system needs to re-adapt after a long period of time back to an Earth environment," Thirsk said in an interview last fall, before Hadfield's launch.
For the first day or two after landing back on Earth, Thirsk's cardiovascular and vestibular balance systems were "pretty wonky," he said.
Movements and activities many earthlings take for granted — walking alone, driving a car — weren't in the cards right away.
"I had trouble maintaining blood pressure to my head and therefore I felt pretty faint and dizzy. In fact, I needed to have a transfusion of normal saline to get my blood pressure up shortly after I got back," said Thirsk, an engineer and physician who resigned as an astronaut in 2012 after nearly 29 years with the Canadian Space Agency.
Thirsk said it took about a day before he felt comfortable walking without someone supporting his elbow. Two weeks later, he was steady enough for his flight surgeon to return his car keys.
Thirsk remembers the "very intensive rehab program" that began as soon as he got back to help build up muscle mass and strength.
"I'd say within six weeks, my muscles were back to their pre-flight status."
Bones, however, take a little bit longer.
"It probably took about a year for my bone calcium level to return to pre-flight levels," says Thirsk. "The rule of thumb is for every month in space it takes two months for the bones to recover."
Most of the effects of weightless are reversible, Thirsk said in an interview Monday, but other potential effects of spaceflight on the human body are less certain.
"We are exposed to higher levels of radiation in space than we are on the surface of the planet so what that means is that potentially in the future we could suffer genetic mutations and cataracts of the lenses in the eye and also of course cancer," Thirsk said.
"We still haven't flown enough people in space to know precisely what are the types of radiation-related illnesses we should be watching out for, but with time we'll eventually be able to monitor that better."
As much as there are the physical effects of long-duration spaceflight, Thirsk also sees a need to consider the mental and emotional well-being of an astronaut after the return to Earth.
"I think it's important from a psychological point of view to try to acclimate back to Earth as quickly as possible.
"In addition to physiological changes in the body, there is psychological changes as well. [Hadfield] has just gone through a mountain-top experience for five months in space and now he has to revert back to functioning as a ground-based astronaut, as a husband, as a father, as a neighbour, and that will take a little bit of time to do."
Thirsk suggests Hadfield take time to relax and "start to establish those human relationships again and that will help the next year."