Science Puzzle
How Do Astronauts Weigh Themselves in Space?
On the International Space Station, doctors keep a close eye on astronauts' bodies, because muscle and bone waste away in orbit. Body mass is one of the simplest things to check. But bathroom scales up there read zero: you, the scales and the whole station are falling round Earth together, so nothing presses down on anything.
How can an astronaut find out their mass in space?
The Answer
By measuring how hard they are to speed up. A known force pushes or pulls the astronaut, and the faster they accelerate, the less mass they have.
Weight and mass aren't the same thing. Weight is the pull of gravity on you, and it's what bathroom scales really measure. Mass is how much of you there is, and it's the same everywhere. One way to find mass without gravity is through inertia: the more mass something has, the harder it is to get moving. A full shopping trolley needs a bigger shove than an empty one, and that would still be true on the space station.
The device on the station today is called SLAMMD. The astronaut sits on an arm and holds on tight. Two springs pull the arm along a track about a metre long with a steady, known force. Sensors time how quickly the astronaut speeds up, and a computer uses Newton's second law, force = mass × acceleration, to work out their mass to within about 0.2 kilograms.
Gravity hasn't gone away up there, by the way. At the station's height it's still about 90 per cent as strong as on the ground. Astronauts float because they're falling round Earth all the time, along with everything around them.
The principle: Mass and inertia. Mass can be measured by how hard something is to accelerate, using force = mass × acceleration, which works without gravity; weight is gravity's pull, and scales can't read it when everything is falling together.