WHAT IS GRAVITY, AND WHY ISN’T THERE ANY IN SPACE?

Dr Nicholas J. M. Patrick

NASA astronaut

Gravity is a pulling force that every object in the universe exerts on every other object. And there’s plenty of it in space!

The bigger and closer an object is, the harder its gravity pulls. The Earth is very big and very close to you, so it exerts a lot of gravitational force on you, holding you down to the ground and preventing you from drifting off into space. We call this force your weight. Everything else exerts a little gravitational force on you as well: the Moon pulls on you, for example, although not enough for you to notice. The Moon also pulls on the Earth’s oceans, causing the tides.

But gravity doesn’t just exist here on Earth – it fills space. Within our solar system, our enormous Sun’s gravity reaches out and holds the Earth and the other planets in their orbits around it, just as Earth’s gravity holds the Moon in its orbit.

So, if Earth’s gravity can reach out to the Moon and beyond, why don’t astronauts feel it when they’re orbiting the Earth in a spacecraft? Why do you feel ‘weightless’ in orbit?

The answer is – somewhat surprisingly – that when you are in orbit, you are actually falling towards the Earth because of the pull of Earth’s gravity. And since you are falling, you’re not standing on anything, so you don’t feel your weight on your feet and legs. The reason you never hit the ground when you’re in orbit in a spacecraft is that you are falling around the Earth. You’re travelling forwards at 17,500 miles per hour, so quickly that the curved Earth falls out from under you as quickly as you fall towards it.

As an astronaut, I’ve experienced weightlessness for weeks on end, living aboard the space shuttles Discovery and Endeavour and the International Space Station. When we’re not working, we enjoy the view, and practise our floating technique. With a little practice, you can float motionless in the middle of the station for several minutes, until the slight breeze from the air-conditioning fans blows you gently towards a vent!