Living in space

Some 400 km above your head, people are living. The International Space Station, ISS for short, has been continuously inhabited for over 25 years and races around the Earth once every 90 minutes or so. It is above all a laboratory: researchers there study what happens to flames, crystals, plants and the human body when weight disappears. But why does everything float up there in the first place?

Weightless does not mean gravity free

Many people think there is no gravity on the ISS. Wrong: at 400 km altitude Earth's pull is still almost 90 percent as strong as on the ground. The secret is free fall. The station falls around the Earth without pause, and the astronauts fall with it in exactly the same way. So why does it never hit the ground? Because at the same time it races sideways at almost 8 kilometres per second: it does keep falling, but it moves so far to the side that the curved Earth drops away beneath it and the ground never gets any closer. When everything falls together, nothing presses on anything: no pressure on the soles of your feet, no scale showing a number. Astronauts experience this shared falling as weightlessness. You know the feeling for a moment when you jump off a diving board or ride over the crest of a roller coaster.

The body winds down

For the body, weightlessness is a permanent holiday, and that is the problem. On Earth, muscles and bones constantly work against their own weight, and in space that work disappears. The result: muscles shrink, and without training bones lose around one percent of their mass per month. The heart also has less to do, and body fluids shift upwards, puffing up the head and sometimes pressing on the eyes. That is why about two hours of sport are on the daily schedule: a treadmill with straps, an exercise bike and a pulling machine that stands in for weight training.

Everyday life without a down

Without weight, everyday life becomes a puzzle. Food often comes from pouches and cans, because crumbs float away and can end up in equipment or lungs. Drinks are sipped from bags through straws, and salt and pepper only exist as liquids. Astronauts sleep in sleeping bags strapped to the wall, otherwise they would drift through the station at night. And since the ISS circles the Earth in 90 minutes, the crew experiences 16 sunrises and 16 sunsets per day. Bedtime therefore follows the clock strictly, not the window.

Wasting nothing

Every litre of water and every breath of oxygen has to be carried up expensively by rocket. So the ISS is a master of recycling: there is no shower cubicle on board, but systems collect the moisture from breath and sweat, and even the urine, and turn around 90 percent of it back into drinking water. Part of the water is split into fresh oxygen using electricity from the solar arrays. These loops are more than thrift: anyone who one day wants to fly to Mars for months cannot have supplies delivered along the way.