What Happens to the Body in Space: Weightlessness

Imagine you float

Imagine you are inside the International Space Station and you let go of a pen: it stays in the air. Many people think there is no gravity up there. In fact, at 400 kilometres gravity is still about 90 per cent of its strength on the ground. The station and everyone in it are falling around the Earth, and they never land because they also move sideways at about 7.66 kilometres per second. Falling together feels like weightlessness. Open the sandbox under the picture and change the orbit: a lower station must move faster.

Speed, period and sunrises

The speed of a circular orbit is v = √(GM/r), where GM is 3.986 × 10^14 for the Earth and r is the distance from its centre. For the ISS, r = 6,371 + 400 = 6,771 km, which gives v of about 7.7 km/s, or roughly 27,600 km/h. One lap is 2π r / v, about 92 minutes. A day has 1,440 minutes, so the crew sees 1,440 / 92, or about 16 sunrises and 16 sunsets every day. Set the slider to 2,000 km and the lap grows to about 127 minutes.

Fluids, spine and bones

Without weight the body changes in days. Blood and other fluids shift towards the head, which gives astronauts a puffy face and thin legs. The spine is no longer pressed down and lengthens by a few centimetres; it shortens again after the return. Bones and muscles do not need to carry anything, so without exercise the bones lose roughly 1 to 1.5 per cent of their mass every month. That is why every astronaut trains about two hours a day on a treadmill, a bicycle and a resistance machine.

A tiny spaceship you wear

Outside the station there is no air at all, so there is no pressure to keep the body working. A spacesuit is therefore a personal spaceship. It holds air at a safe pressure, gives oxygen, removes the carbon dioxide and protects against tiny fast grains of dust. It also has to deal with temperature: in the open, a surface in sunlight heats to about +120 degrees Celsius, and in shadow it falls to about −150. The suit has layers of insulation and cooling water running through a tight undergarment.

More radiation

On the ground we are shielded by a blanket of air. Its weight above each square centimetre equals that of 10 metres of water, the same 1 atmosphere you met in the chapter about the sea. The Earth's magnetic field helps too. In orbit the air lies below the station, so astronauts receive much more radiation than people at home. Space agencies measure each person's dose and set career limits. For flights to the Moon and Mars, radiation is one of the main problems to solve.

The fire of re-entry

To return, a capsule must drop out of orbit and meet the air at about 7.7 kilometres per second. The air in front of it is compressed so strongly that it glows. The heat shield, made to burn away slowly, can reach about 1,600 degrees Celsius on its outer face while the crew inside stays comfortable. Parachutes slow the capsule, and small engines soften the final touch. After months in orbit, a crew member lands with legs that feel heavy and an inner ear that has to learn up and down again.

In orbit