Desperately Seeking Zero Gravity

You are an astronaut on the International Space Station (ISS) floating in your tin can moving at 7.7 kilometres per second at a distance of 400 kilometres above the Earth enjoying the freedom of zero g.

Well no. The astronauts in the ISS are not at zero g, in fact they are experiencing plenty of gravity it is just that they are falling at the same time.

The force of gravity here on the surface of the Earth is about 9.8 metres per second per second (m/s²). That is, if you drop an object, in one second it will have a velocity of 9.8 m/s. In the following second it will add on another 9.8 m/s to its velocity and will have a new velocity of 19.6 m/s. This will continue on until it inevitably hits the ground.

The ISS is only another 400 km above the surface of the Earth and the force of gravity up there is only slightly reduced to about 8.7 m/s².

So if it weren't for the extreme forward velocity of the ISS the astronauts would fall and hit the Earth in about 5 minutes. Instead they hurtle forward, still falling but the surface of the Earth curves away at exactly the same rate so nothing bad happens.

Also, the gravitational field of the Earth is not constant. It varies very slightly from location to location with strong spots near Hudson's Bay Canada and the tip of India and weak spots near Iceland and Papua New Guinea.

To complicate things further, the Moon is also pulling on the ISS and it is in orbit around the Earth. This means that the gravitational pull of the Moon on the ISS and its astronauts is always changing very slightly.

The result is that the ISS is constantly experiencing slight accelerations due to an ever changing gravity field (from its perspective).

_{Image credit: Nickshanks link CC BY-SA 2.5 license}

So Just Where Is Zero G?

Let's go on a hunt for a spot with perfect zero g.

The Earth has a certain mass and it pulls on whatever objects are near it. The Moon also has a certain mass and it too pulls on whatever objects are near it. There must therefore be a place between them where the two forces cancel out.

Well, there is and it is about 340,000 km above the surface of the Earth which is about 40,000 km above the Moon (the Earth-Moon distance varies a bit so there is actually a range of answers for this).

Let's go there and see if we experience zero g.

We plunk down our 90 million dollars and hop into our new SpaceX Dragon 2 capsule. The Falcon Heavy blasts us up into low Earth orbit and then the second stage puts us into a Moon trajectory. We get there in two or three days to find a spot with nearly zero gravity.

What went wrong? Why is there still gravity there?

This balance point will still experience a gravitational pull from the planets and the Sun. The planets are in orbit around the Sun, their distances are always changes so the pull from them is always changing. It is changing very slightly but it is still changing nonetheless.

To make it even more complicated, the Earth-Moon system itself is in orbit around the Sun and this orbit is not a perfect circle. Therefore the distance of this balance point to the Sun is also always changing very slightly and so is the gravitational pull.

All of this means that is very unlikely that you will ever be able to find a spot where all of the different forces from all of the different solar system bodies balance out perfectly.

The Centre of the Earth

If you dig down into the Earth the force of gravity will actually decrease. This is because the matter above your head pulls you upwards and this will reduce the pull of the matter still below your feet. This effect is known as the shell theorem in physics.

If you keep digging deeper and deeper until you eventually reach the centre of the Earth you will find yourself in zero gravity. All of the matter of the Earth is above your head and it is pulling you in all directions equally.

Even here you will not experience perfect zero g for the same reasons described previously. The pull of gravity from the Sun, Moon and planets are all varying by small amounts as they move in their orbits and change their respective distances from this spot.

No joy yet.

One Last Try - Let's Imagine If We Find The Perfect Balance Point

Okay, let's make one last try at finding zero g. Imagine we do find that perfect balance point somewhere in the solar system even if it is for only a brief few seconds. Is this a spot of perfect zero g?

Possibly not because gravitational waves are still arriving in our solar system all the time from objects such as binary neutron stars or even binary black holes and a whole other assortment of astronomical configurations.

Gravitational waves distort the very fabric of space-time causing space to contract in one direction and expand in another direction in a cruciform pattern. The effect is extremely small; the length of the 4 km long arm of the LIGO detector changes by only one-ten thousandth of the width of a proton. Small but still there.

Closing Words

So it turns out that actual zero gravity is very hard to find. You can go into orbit and experience reduced gravity but it will be exceedingly difficult to find actual zero gravity.

This is why NASA and other space agencies tend to refer to the ISS as a micro-gravity environment. The forces of gravity are not zero, they are just very small.

Thank you for reading my post.

Post Sources

https://denali.gsfc.nasa.gov/research/earth_grav/
https://www.popsci.com/science/article/2010-06/esa-satellite-maps-earths-gravity-3-d
https://en.wikipedia.org/wiki/Shell_theorem
https://en.wikipedia.org/wiki/Gravitational_wave
https://en.wikipedia.org/wiki/Micro-g_environment
https://en.wikipedia.org/wiki/Weightlessness