How do astronauts weigh when floating in space?

On the International Space Station (ISS), astronauts use specialized Russian and American equipment to measure body mass instead of weight.

The microgravity of low Earth orbit is a harsh environment for the human body. The effects of this environment are still being studied, and astronauts must ensure they maintain optimal physical condition while floating in the International Space Station (ISS).

European Space Agency (ESA) astronaut André Kuipers uses SLAMMD to measure body mass. (Photo: NASA/ESA).

On Earth, exercise often depends on weight. But how do astronauts measure their weight when they are in near-weightlessness in space? There are actually two devices on the ISS that help them do this: NASA's Space Linear Accelerated Mass Measuring Device (SLAMMD) and Russia's Body Mass Measuring Device (BMMD). Both use spring action to measure the astronaut's body mass instead of weight.

A person's weight is their mass multiplied by the acceleration caused by the gravitational field around them. For example, the gravity on the Moon is less than on Earth, so the Apollo astronauts felt "lighter" walking on the Moon's surface. However, their mass would have remained the same. That's why astronauts on the ISS measure their mass, not their weight.

Mass, like weight, is usually measured in kilograms. This can be confusing and disconcerting to many people. Mass is the amount of matter a body contains, regardless of gravity.

SLAMMD applies Isaac Newton's second law of motion: The magnitude of a force is equal to the mass on which it acts multiplied by the acceleration of that mass (F = ma). Located in the European-built Columbus laboratory on the ISS, SLAMMD is part of the Human Research Facility Rack — a set of drawers that house a variety of integrated instruments. SLAMMD protrudes on a "guide arm."

To use SLAMMD, the astronaut wraps his legs around the leg rest, places his stomach on the belly pad, and rests his chin on the headrest. Two springs in a drawer are released, and their force pushes the guide arm against the astronaut, causing him to be pushed backwards.

The astronaut knew the force exerted by the spring because it was built to certain specifications. The astronaut's acceleration as it was pushed back by the spring was measured by an optical instrument that tracked the motion of the guide arm and how fast it moved. The acceleration was calculated by dividing the change in velocity over that distance (about a meter) by the time it took. A computer connected to SLAMMD then performed a simple calculation F = ma to determine the astronaut's body mass to an accuracy of 0.2 kg.

NASA astronaut Tom Marshburn uses the Body Mass Measurement Device (BMMD) in the Zvezda service module on the ISS. (Photo: NASA).

The Russian BMMD is housed in the Zvezda module of the ISS . Like the SLAMMD, it also uses springs. However, instead of pushing once, an astronaut squats on the device and moves it up and down like a pogo stick. The speed at which the device oscillates depends on the mass of the astronaut: When no one is sitting on it, the BMMD oscillates much faster. Therefore, timing it helps estimate the astronaut's body mass.

Measuring body mass is important in space , to ensure astronauts don't lose too much mass. This is because humans lose an average of 1% of their bone density every month in this harsh environment. Muscles also shrink and the heart weakens because it doesn't have to work as hard to pump blood around the body.

To combat the effects of microgravity, astronauts on the ISS exercise for two hours a day in the station's gym to maintain muscle mass, bone density and cardiovascular health. Body mass measurements help check whether they are healthy and fit.