Some of the issues that are important

if you want to have people in space

for long periods of time.

One is that people will tend to lose

bone and muscle mass.

We know this.

If you have to put a cast on your leg,

and you take the cast off after a few weeks,

you'll see that your muscles have shrunk in size.

And if you measured the bone strength,

you'd also see that might have gone down a little bit, too.

And so, it's very interesting that our body has that ability

to adapt to the loads that are put on it,

so that bones and muscles aren't static,

they're always changing.

While we think of bone as being a solid thing

that doesn't change very much,

it changes too.

And it turns out that in weightlessness,

you lose bone.

And then you also cause the muscles

that work against gravity,

what are called the postural muscles,

they'll start to shrink and lose strength.

There are other things in the cardiovascular system,

the heart and blood vessels.

And if you think about it,

standing up in gravity

means you have to work against gravity

in order to keep blood pumping to your head.

So, if you couldn't keep blood pumping in the head,

you'd pass out every time you stood up

because when you're lying down,

you don't have to push against gravity.

But when you stand up,

you got to work against gravity

to keep blood flowing to your head.

And your heart and blood vessels

have a really nicely worked-out system

to make that happen every time.

But that system can also change in weightlessness.

And then the other area that changes

is the system that has to do with balance.

Again, maintaining your balance

is something that you're doing against gravity, right?

If you didn't have gravity present,

you wouldn't have to worry about falling.

But you obviously do have to worry about falling,

and we have a very highly developed sense of balance

to keep us upright

and to prevent us from falling.

And when you see what skaters do,

you realize just how exquisite a system it is.

But when you go into weightlessness,

your balance system changes.

You don't really notice it

while you are in weightlessness,

but when you come back,

you do notice it,

that your balance has changed

and you have a little bit of trouble

maintaining your balance.

And what it shows is that while you're in space,

your brain is trying to allow you

to function in weightlessness.

And so, it re-adapts you to be weightless,

which you don't notice until you come back

and find out that you're now back on Earth

with a balance system that's been adapted to space.

You know, all life developed here on Earth

with gravity being present,

so life evolved under the influence of gravity,

and then we grow up with gravity being present,

so we learn how to walk

and catch a ball

and ice skate

or whatever,

all with gravity being present.

And what if you were to grow up without gravity?

What about the systems that depend on gravity,

like your muscles

or your balance system

or the heart and blood vessels?

Would they develop normally,

or would they be different in some way?

One reason why you might think

that it would go down a different pathway

is from an experiment

that was done some time ago

by two neuroscientists

called Hubel and Wiesel.

And what they did is they had a kitten,

and they put a patch over the eye of the kitten.

And then the kitten grew up to be a cat,

and they removed the patch.

And so, the question is,

can the cat see out of that eye?

Now, there's nothing wrong with the eye, right?

But it just hasn't seen anything,

there hasn't been any light coming in.

And the answer is that the cat can't see out of that eye

because what happens is that the brain

goes down a different pathway when it develops

and the connections that would ordinarily develop

to that eye don't develop.

And that can't be undone,

that's a permanent change.

So, the brain of that cat

is fundamentally different from the brain of a cat

that grew up seeing out of that eye.

That cat grew up with a different brain, in essence.

So, then you wonder,

well, what about gravity?

What if you don't have the forces

that gravity produces?

Is your balance organ going to develop

in the same way,

or will it be different?

If somebody grew up in space,

could they come back to Earth and function,

or would they really be a different person?