The ISS is not just the largest international engineering project of all time. It is a laboratory in the sky – an orbiting workshop that allows scientists to study a whole range of subjects in a very special environment: an environment where everything is weightless.

On Earth, everything we do is strongly influenced by gravity. We are so accustomed to gravity that we take it for granted: it is perfectly natural for something to fall when you drop it, after all. Our bodies – like the bodies of every other living thing on Earth – have evolved to resist gravity and to take advantage of it. We have strong skeletons to support us, and a powerful heart to pump blood "uphill" against the force of gravity.

A candle flame on Earth

Some very odd things happen in conditions of weightlessness. A good example might be something as simple as a candle flame. On Earth, the flame rises into its familiar shape because the hot gases created by burning are lighter than the cool air that surrounds them.

A candle flame in weightlessness

This cool air is drawn into the base of the candle wick, and provides oxygen to keep the candle wax burning. But in weightlessness, the hot gas is no lighter than the cool air: nothing rises. The result is a small, almost invisible globe-shaped flame – an almost perfect sphere. The actual burning takes place only on the surface of the sphere, where fuel from the candle wax can mix with the oxygen in the air outside.

By studying the way things burn in weightlessness, scientists can learn a lot about combustion. For instance, the way gases diffuse together can be examined. This delicate reaction is very hard to observe on Earth, where gases are “swamped” by the much larger effects caused by gravity.

New knowledge gained in space has practical uses on Earth

New knowledge gained in space has practical uses on Earth. If you know exactly how things burn, for example, you can design engines for cars that use less fuel and create less pollution.

Another interesting research area involves mixing liquids – and again, it has strong practical applications. On Earth, many of the metals in everyday use are alloys: that is, mixtures of two or more metals, combined when molten. Most aircraft, for instance, are largely made from an alloy called duralumin, which is mainly aluminium with a dash of copper and a few other metals. On Earth, gravity strongly affects the way the different metals mix together. Experiments in weightlessness can teach us a lot about the nature of "mixing" – and we can use the new information to make better alloys back on Earth.

Two aluminium alloy samples solidified in space and on Earth under the same freezing conditions