U.S. astronauts splash down: This is what microgravity does to your body

• Wilmore and Williams splashed down Tuesday evening after undocking from the International Space Station early that morning.

Zoom in: One prominent effect of extended time in microgravity is muscle atrophy or weakening. The longer a space mission is, the more severe such atrophy would be, said Shenhav Shemer, a professor of biology at the Technion — Israel Institute of Technology who researches muscle loss.

• Williams and Wilmore may experience this atrophy differently: Women — who naturally have lower baseline muscle mass and testosterone levels in addition to hormonal and metabolic changes they experience in space — are more susceptible to atrophy and bone loss, Shemer said.

Resistive exercise can help combat that loss, with crew members averaging two hours of exercise per day, according to NASA.

• But even with routine training, Shemer said, astronauts will still experience atrophy in space.

As bones lose their density, the body's mineral content increases elsewhere, which can lead to issues like increased urinary calcium and kidney stones, according to Baylor College of Medicine.

• NASA has found that without Earth's gravity, weight-bearing bones lose on average between 1% and 1.5% of bone mineral density per month.

By the numbers: Wilmore and Williams were initially supposed to be in space for just over a week. Such a short mission would have resulted in temporary, "completely reversible" impacts on muscle mass, Shemer said.

• But with long-duration stays in space, "there are expected to be metabolic and physiological changes that might be irreversible," she said.
• In long space missions, some muscles will be able to return to pre-flight mass within a year — but others could take two to four years, Shemer said. But muscle loss depends largely on individual factors, like physiology and age.

Fun fact: When astronaut Scott Kelly — twin brother to Sen. Mark Kelly (D-Ariz.) — returned to Earth after almost a year aboard the ISS, he was about two inches taller than he was when he blasted off.

• In space, spinal disks are able to expand, causing the spine to temporarily stretch.
• But in less than two days, the (momentarily) taller Kelly shrunk again.

Not-so-fun fact: Microgravity can also impact vision.

• As fluids in the body shift toward the head in space, astronauts can experience pressure on the eyes that can lead to vision problems.
• Over half of crew members experience one or more symptoms of Spaceflight Associated Neuro-ocular Syndrome (SANS), per NASA, which is believed to be caused by the fluid shift toward the head.
• Compression cuffs on the legs can help keep blood lower in the body to counteract shifting fluids, according to NASA.

Zoom out: Returning astronauts will also likely experience difficulty with their coordination of movement related to issues with proprioception, the body's ability to sense its own position and movements, Shemer says.

• After touching down on Earth, astronauts are often put in chairs, as they can have trouble adjusting to the planet's gravity.

Yes, but: There's one change Williams and Wilmore might actually be looking forward to.

• Garrett Reisman, a former NASA astronaut who spent 95 days in space, told CNN earlier this week that the fluid shift in space also dulls smell and taste.
• But upon returning, "it comes back right away," he said.
• Reisman continued, "I remember as soon as we opened the hatch of the space shuttle, I got my first whiff ... it just smelled great."

Go deeper... In photos: NASA astronauts return to Earth after 9 months in orbit

Editor's note: This article has been updated to reflect that the astronauts have returned to Earth.