Why Are Astronauts Weightless?

[Applause] [Music] Have you wondered what it would be like to be an astronaut floating around in the space station? But why are the astronauts floating? I'm here at the PowerHouse Museum in Sydney to find out if anyone knows the answer.

Why are they floating?

No gravity. No gravity. Gravity? There's no gravity! There's no gravity! Gravity is the stuff that holds us to the planet. When we spin, it doesn't happen up there. Gravity holds you down. So in space, there's nothing there, so you just float. There's no gravity in space. No gravity, there's very little!

But is there really no gravity in space? After all, the moon orbits the earth due to gravitational attraction. So where would the space station fit in this picture?

Can you make me a roughly to scale model of the Earth and the moon? And then we're going to position the space station in there. Do everybody like that? Yeah? About this close up? Maybe about out there? Are you ready to see the real distance? M, what do you reckon?

Okay, it's actually about here. Maybe a little further? I think we were wrong. Now, if the moon is here and the Earth is there, whereabouts would you put the space station? Somewhere around here? Midway? About halfway? Yeah, right about here. So that's maybe like a quarter of the distance to the Moon.

Let me show you the real distance. Are you ready for the real distance? The space station is right about there. That close? Okay, close! Wow, we're not good at this game!

The space station is actually only about 400 km away. So if you're in Sydney, it's a little further than the drive to CRA. But does it make sense that the Earth exerts a big gravitational force on you but nothing on the astronauts just a short distance away?

You reckon there'd be no gravity there? It's not much; they float. We've seen them. Well clearly they're not. They're floating around. I know they float, and the question is why?

No idea. I'm really confused. Well, in truth, the force on the astronauts is almost as much as the force on you. So then why are they floating while you're stuck here?

The answer is the astronauts aren't floating; they're falling. And not only that, but the space station they're in is falling as well. So why doesn't the space station come crashing into the Earth?

Well, the reason is the space station and astronauts inside have this huge sideways velocity of about 28,000 kilometers per hour. So even though they're falling towards the Earth, they're going so fast that as they fall towards the Earth's surface, it curves away from them.

If the Earth didn't put a gravitational force on them, they would fly out into space. Sure, yeah, right! So it's due to that gravitational pull that they can actually keep orbiting the Earth. If it was to stop moving, it would be plummeting!

Exactly! But would they?

They would still feel weightless until you smash into something. So the space station and the astronauts inside are constantly accelerating towards the Earth's core, but they never get any closer.

And because they're accelerating at the same rate, the astronauts feel weightless. They have this amazing sensation of [Applause] [Music] floating.