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.  

To hear this program click here

A Moment of Science®