after nearly a year on the International Space Station.

But when he came back, he was 2 inches taller.

So, what exactly happened up there,

and what does that mean for the future of space travel?

If you're planning a trip to the International

Space Station, be prepared to feel weightless.

The station orbits the planet every 90 minutes,

moving at more than 17,000 miles per hour.

That's 30 times faster than a commercial jet aircraft.

As a result, astronauts on board live in a constant state

of free fall, or weightlessness.

Garrett Reisman: Being up there in microgravity is awesome.

It's, like, the coolest thing,

because it's like you have the power to fly.

Narrator: That's Garrett Reisman,

a former NASA astronaut who's logged 107 days in space.

There are a few immediate side effects, he says,

when you first experience microgravity.

Reisman: So the first thing you really feel is

you feel kinda sick.

You don't feel very good those first couple days.

It's kinda like being airsick or seasick.

We call it space-adaptation sickness.

Your vestibular system, your organs

that provide information to the brain

about your rotation and your acceleration,

they're not working that great without being in gravity.

Narrator: Without gravity working on your body,

your bones and muscles start to break down, too.

In fact, bone density drops by over 1% per month.

By comparison, the rate of bone loss

for elderly men and women is around 1% to 1.5% per year.

And, because it doesn't take much effort

to float through space,

your muscles lose strength and endurance pretty quickly.

Reisman: You have to work out every day.

So, they scheduled two hours a day pretty much every day

while I was on the space station for working out.

What we found was, if you do enough resistive exercise,

you can halt the effects of the bone loss

and the muscle atrophy.

Narrator: Without gravity pulling them down,

fluids pool in the body, tricking it into thinking

it's carrying too much water.

As a result, astronauts have to pee...

a lot.

This makes it easy for them to get dehydrated

and develop kidney stones.

Reisman: So, you have a shift in your fluid.

A lot of the blood volume that normally is down in your legs

ends up up here, and your chest kinda puffs up

and your face puffs up, and you can see it.

If you look at pictures of us on the space station,

it looks like we put on some weight or something

and we're all puffed up.

Narrator: Swelling in the upper body

puts pressure on the eyes as well,

which can cause vision problems.

Reisman: A lot of us, including myself,

had a shift in our vision while we're up in space.

You start out, everything was fine,

and all of a sudden things get blurry.

We could see the effects of it.

We could see swelling in the optic nerve,

we could see folds in the cornea,

but we're still not 100% sure

exactly what's causing it and how to stop it.

Narrator: With all the challenges of space travel,

one benefit is you actually get taller.

Reisman: So, yes, you do get taller when you go to space.

It's the whole reason I signed up for this job.

Your spine is being compressed by gravity.

So, when you go into the microgravity environment

and you no longer have any kind of compressive loads

on the spine at all, it stretches.

I grew about an inch.

Astronaut: Woo-hoo!

Narrator: Without gravity working against it,

the heart doesn't have to work as hard

to pump blood throughout the body.

Over time, this could lead to the heart

actually decreasing in size.

Reisman: There is an effect on the cardiovascular system

about being up in space.

So you do get a reduced aerobic capability.

You can be in great shape,

and after being up in space for a couple days,

you might get on the treadmill, and you might be like,

"Man, I must not have been hitting the gym."

Narrator: The immune system also takes a hit.

Researchers discovered that a lack of gravity

weakens the functions of T cells,

which play a crucial role in fighting off diseases.

Another concern is cosmic radiation.

Astronauts on the station are exposed

to over 10 times the amount of radiation

that we get on Earth.

Reisman: At a couple hundred miles,

we're well above the atmosphere,

but we're still well below the magnetic field of the Earth.

But we still get a large bit of protection

from that magnetic field.

In fact, you could tell, because when you close your eyes,

you see little lightning bolts,

and that's actually a result of some of the radiation

hitting your eyeballs and releasing photons.

Narrator: Artificial shielding on the ISS

only partially protects astronauts from harsh radiation,

leaving them more susceptible to cancer

and other diseases later in life.

Finally, astronauts must also be able to handle

the psychological challenges of confinement and isolation.

Reisman: So, there is a psychological aspect

to being in space, both because of the fact

that you're isolated from the rest of humanity,

it was really strange to be looking out the window

at billions of people down there

that had no way to get to me.

When I was there, I only had two crewmates

at a time on the space station,

so if you don't get along with somebody,

that could be bad, because you don't have

too many choices there in making new friends.

Narrator: And, without a 24-hour sleep cycle,

astronaut circadian rhythm is thrown off,

which can cause more stress and lead to sleep disorders.

Reisman: You're taking jet lag to a whole nother extreme.

Well, the weird thing is that you go around the planet

once every hour and a half.

So every 45 minutes, the sun is either rising or setting.

So you can't, like, tell what time it is

by looking out the window.

Narrator: So, what does all this mean

for the future of space travel?

Well, a trip to Mars would expose astronauts

to even more dangers than those

on the International Space Station.

They would face higher levels of radiation,

shifting gravity fields, and longer travel times,

which would compound all of the negative effects

of space on the human body and mind.

Reisman: I think the biggest issue we gotta deal with

is the radiation.

We don't know precisely what that exact radiation

does to human beings.

But what does gamma rays or what does heavy ions,

what do they do human tissue?

We don't really know.

Narrator: Right now, NASA and other research organizations

are working to develop better technology

that protects astronauts against these hazards,

so maybe one day humans might make it to Mars.