At what height above Earth is zero-gravity?
At what height does zero gravity take effect? Would this vary from different parts of the Earth, and if so, why would this be the case?
Chris Smith put this astronomical question to the Open University's David Rothery...
David - I think there might be a misconception here that, as you get further and further away from the Earth, you’re feeling less of a gravitational pull and at some point you stop feeling the pull of the Earth’s gravity. That’s not what happens!
If you double your distance from the centre of the Earth, the gravity would decrease to a quarter of what it is at the surface, but that’s not zero G.
Zero G in space is because you’re in a space capsule accelerating under gravity, orbiting the Earth, or going from one body to another and you’re not accelerating [relative to the spacecraft]: the person within the spaceship is accelerating at the same rate as the vessel. So it’s like the lift dropping down the lift shaft. So zero G is not distance away from the Earth.
Chris - So people who are on the International Space Station, the reason that they are in orbit around the Earth is because gravity is hanging onto them and keeping them in orbit? But they’re just free-falling around the Earth all the time so that they’re weightless, but that’s not the same as zero G?
David - Correct. They’re in freefall. They’re still experiencing the Earth’s gravity but so is the spaceship that they’re in. So, between them and the space ship, there’s no acceleration.
Chris - Let’s take it to it’s logical conclusion; the reason that Pluto is 6 billion kilometres from where we are here on Earth and it’s still orbiting the Sun is because the Sun’s gravity is hanging onto Pluto, even though it’s that far away. And gravity does get weaker with distance, but there’s still enough to hold onto Pluto and things beyond it.
David - Absolutely!