It always fascinates me to see videos and photographs from the International Space Station (ISS), how simple day to day activities are so altered because of the altered Physics of constant free fall in orbit around the Earth. Many of the videos are showing what people call the ‘effects of zero gravity.’ This is actually not the case. There is gravity in space. Let’s take a closer look at the situation of an ISS astronaut.

If we do the math (I do realize the term ‘do the math’ turns away a lot of people, so I will keep it short), the force of gravity is F = G*m*M / r^{2}, where G is the gravitational constant, m is a person’s mass, M is the Earth’s mass, and r is the distance between the objects. We always treat the centre of the Earth as the place where Earth’s gravity originates. This is because it can be shown mathematically that for a spherical body of uniform density, the force of gravity it exerts is the same as if all of its mass was at its centre. This is an approximation because the Earth is not a perfect sphere or a uniformly dense object, but it works for this example.

So a person on the surface of Earth is a distance of 12,800 Km from the centre, and a person on the ISS is 400 Km above that, or 13,200 Km from the Earth’s centre. All the other parameters in the equation are the same, so the ratio,

F_{ISS} / F_{E} = (G*m*M / r_{ISS}^{2}) / (G*m*M / r_{E}^{2}) = r_{E}^{2} / r_{ISS}^{2}

Throw in the numbers and we get F_{ISS} / F_{E} = 0.94.

This means that the amount of gravity the Earth exerts at the ISS orbit altitude is 94% of the gravity exerted at the Earth’s surface. So why do astronauts feel weightless? Why do they feel as if there is no gravity? The answer is that they are in free fall!

Let’s imagine throwing a ball. If you throw a ball it goes in a big arc before hitting the ground. The amazing thing is that the ball is actually *orbiting the Earth, *the only problem is that the Earth gets in the way. Let’s imagine we could throw a ball so fast and so high that it would actually miss the Earth. The result is illustrated in Newton’s Cannon Experiment.

This is what is happening to the ISS astronauts. They are falling! They keep falling toward the Earth, but they keep missing it, as if they have terrible aim! Gravity keeps on pulling them in, but their horizontal velocity keeps them at a constant distance from the Earth. The Moon is also falling toward the Earth, but it keeps missing it too (luckily). They feel no weight under their constant acceleration, in exactly the same way a skydiver appears to float as they fall.

So there actually is gravity in space, but since the astronauts are in free fall, they just don’t feel it. Instead, they get to live their lives with no concept of up and down, nothing to keep their bones and muscles strong, and they find new challenges in simple daily activities we all take for granted on the ground, like drinking from a glass.

Go take a skydiving lesson, and you’ll get 5 minutes of astronaut experience!