This question was asked by Joseph from Plymouth, UK.
Although gravity has always existed on Earth, it wasn’t until the 17th century that we began to understand exactly what it is. A famous physicist called Isaac Newton realised that there must be a reason why objects always fall downwards. The legend tells that he realised this after an apple fell onto his head while he was sitting under a tree; although this is probably untrue it’s a funny story. Isaac Newton worked out that this reason is a force called gravity.
Any object that has mass (it takes up space and has density) has gravity, which means that it pulls other objects towards itself. This gravitational pull becomes stronger the closer to the object you are, and the bigger the object is. So although people have gravity it’s so slight that we don’t notice it. The stars, planets and moons have a much more noticeable effect on each other. Stars have a lot of gravity, so they pull planets towards them; at the same time the planets’ own gravity is pulling the star towards them, just not as strongly. This is why planets orbit stars, and moons orbit planets.
How much an object weighs depends how much gravity is acting upon it, so you would weigh different amounts on different planets and moons. If you weighed 100kg on Earth you would only weigh 38kg on Mars and 16.5kg on the Moon. You’d weigh 236kg on Jupiter! This is also why astronauts bounce on the Moon instead of walking or floating; there is gravity but it’s a lot less than we have on Earth.
Because gravity causes every object to pull other objects towards it, there’s gravity in space as well. But because there’s a lot of distance between the objects in space, unless you’re close to one of them the gravity is very weak. Scientists call this ‘microgravity’. You might think that this is why orbiting astronauts float, but it isn’t. Astronauts aboard the International Space Station (ISS) are only 400km above the Earth’s surface; at that height gravity is still about 90% as strong as it is on the surface.
Astronauts and objects on the ISS float because they’re actually falling towards the Earth’s surface, pulled downwards by gravity. They orbit instead of plummeting towards the surface because of the speed the ISS is going. Isaac Newton worked this out too. He explained that when you fire a cannon, the ball travels horizontally for a short way before falling to the ground; if you fire it at a faster speed the ball travels further before falling. Newton reasoned that because the Earth is spherical, there must be a speed at which the ball would keep travelling horizontally and falling towards the surface but never actually hit it. He was right – the ISS is constantly falling towards the Earth but because it’s also travelling around it at 27,600 kph, it never hits.