The problem with zero gravity

By Danit Brown

Posted May 25, 2022

Media Player Error
Update your browser or Flash plugin

Transcript

D: Yaël. Let's check with NASA and see if we can do A Moment of Science in zero gravity!

Y: There's one big problem.

D: What's that--money?

Y: Well yes, but the big one is that there's no such thing as zero gravity.

D: Come on. You see astronauts floating around weightless all the time.

Y: Yes, but weightlessness and zero gravity are two different things. Think about it. The earth's gravity keeps the moon in orbit. And astronauts are generally much closer to earth than the moon is, which means that the earth's pull on them has to be much stronger.

D: Good point. So what's really going on?

Y: Well, while we're on Earth, we feel our weight because, as the Earth's gravity pulls us into its center, the ground pushes back against our feet. When astronauts orbit the earth, they're still subject to gravity, but they're moving sideways so quickly that even though they're being pulled towards the earth, they're not getting any closer to the center of the planet. In other words, they're basically in a state of constant freefall, and that's why they're weightless.

D: But that doesn't mean zero gravity can't exist. Isn't it just a matter of moving beyond the Earth's pull?

Y: Well, gravity never disappears entirely. It just gets weaker. And every object with mass, including you and me, has what's known as gravitational attraction.

D: I've been told that I'm fairly attractive.

Y: ANYWAY, although you're subject to all those forces, the Earth's gravity masks their pull because the Earth is so large and so close. So moving away won't free you of the Earth's gravity. All that would happen is that the Earth's pull would be masked by the pull of some other object, like Mars.

An astronaut floats with a tether in space with Earth seen partially in the background

Even astronauts in space are still subject to gravity. (NASA's Marshall Space Flight Center / flickr)

There's just one problem when it comes to zero gravity. It doesn't actually exist.

You see astronauts floating around weightless all the time, but weightlessness and zero gravity are two different things. Think about it.

The earth's gravity keeps the moon in orbit. And astronauts are generally much closer to earth than the moon is, which means that the earth's pull on them has to be much stronger.

So what's really going on?

Well, while we're on Earth, we feel our weight because, as the Earth's gravity pulls us into its center, the ground pushes back against our feet. When astronauts orbit the earth, they're still subject to gravity, but they're moving sideways so quickly that even though they're being pulled towards the Earth, they're not getting any closer to the center of the planet. In other words, they're basically in a state of constant freefall, and that's why they're weightless.

So wouldn't that mean zero gravity is just a matter of moving beyond the Earth's pull?

Well, gravity never disappears entirely. It just gets weaker. And every object with mass, including you and me, has what's known as gravitational attraction.

Although you're subject to all those forces, the Earth's gravity masks their pull because the Earth is so large and so close. So moving away won't free you of the Earth's gravity. All that would happen is that the Earth's pull would be masked by the pull of some other object, like Mars.

Read more

Sources