Living in space

About 400 km above your head there is a house. It is 109 metres from end to end, about the length of a football pitch, and it weighs about 420 tonnes. It was carried up in pieces and bolted together in orbit. Seven people usually live in it. Since November 2000 there has not been one moment when it was empty, so for more than 25 years some of the human race has always been off the planet. It is the International Space Station.

Everything on board floats: people, pencils, drops of water. That is not because gravity has gone. At that height it is still about nine tenths as strong as on the ground. The station floats because it is falling. It is falling round the Earth at 8 km a second, and everything inside is falling with it, so nothing presses down on anything else. The crew live in a lift that never reaches the bottom.

Floating is fun, but the body was built for the ground. With no weight to carry, the bones that hold you up lose between 1 and 1.5 per cent of their mineral every month, and muscles shrink. So every astronaut exercises for at least two hours a day. This is Shannon Walker running on the station’s treadmill in 2010. Look at the harness: without it, her first step would send her drifting off.

The station goes round the Earth about once every 90 minutes. That means the Sun rises and sets 16 times in every 24 hours, usually with a little under an hour of daylight and then about half an hour of night. Nobody can sleep to that rhythm, so the crew ignore it. They keep to one clock, sleep in small cabins with the lights off, and work an ordinary day.

Nothing is wasted up there, because everything has to be brought by rocket. The air is cleaned and used again. So is the water, including the crew’s sweat and breath. In return they get this: a bay window with seven panes that looks straight down on the Earth. This is the Canadian astronaut David Saint-Jacques photographing the ground going by in 2019.

Sometimes the work is outside. A spacesuit is a small spacecraft shaped like a person, with its own air, cooling and radio. This is Bruce McCandless on 7 February 1984, the first person to fly free with no line tying him to his ship. The longest anyone has stayed in space in one go is 437 days. A journey to Mars and back would take longer still, and nobody yet knows how well people would stand it.