How does gravity work in space for kids?

Gravity is what any object with mass does to the space around it: it pulls. The more mass, the stronger the pull; the farther away you are, the weaker it gets. But it never reaches zero. Earth's gravity still holds the Moon across hundreds of thousands of kilometres, and the Sun holds the outermost planets in their orbits from an enormous distance. So a spacecraft a few hundred kilometres up is nowhere near leaving gravity behind.

At the height where the International Space Station flies, about 400 km, Earth's pull is only slightly weaker than in your kitchen. If the station could be stopped dead and held there, everything inside would fall straight down, exactly like a dropped spoon.

What makes the difference is speed. The station races around the planet at about 28 000 km/h and completes a lap in roughly 90 minutes. Think of throwing a ball: it curves and lands. Throw it harder and it lands farther away. Throw it hard enough and the ground curves away beneath it as fast as the ball falls, so it never arrives. That is an orbit: falling forever, and missing.

Inside, the walls, the water, the laptops and the crew are all falling at the same rate. Nothing pushes up on anyone's feet, so there is no feeling of weight, and a tomato left in mid-air stays where it is. Engineers prefer the word microgravity to zero gravity, because small forces remain: faint air drag, the pull of the station's own mass, the nudge of a thruster.

You can sample a version of it on Earth. A lift that starts down quickly, or the crest of a rollercoaster, gives a second of the same sensation, and research aircraft fly parabolic arcs to stretch that moment for the benefit of experiments. Astronauts simply get it for months on end.