How do you weigh an object in space?

Since you can’t actually weigh an object in a weightlessness environment, astronauts use body mass measurements to determine a difference in weight. This isn’t as trivial as it seems however.

These measurements are made with an inertial balance – which is basically a spring-like device and a timer. By measuring the time between oscillations (or the back and forth or up and down movement) the device makes a correlation between body mass and the actual weight.

On the space shuttle, astronauts use a chair-like device in which the astronaut sits in the chair as it moves back and forth. The speed of the motion which is determined by the mass in the chair is measured and correlated to a weight.

Sources: a good description of an inertial balance at Cornell University’s astronomy page. Also the New York Times’ Book of Science Questions and Answers.