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    "headline": "ISS astronauts are weighed by a spring-loaded arm; NASA’s device reads body mass within half a pound",
    "description": "With no gravity to press on a scale, station crews use Newton's second law instead, while a Russian machine on board times them bouncing up and down like riders on a pogo stick.",
    "datePublished": "2026-11-09T11:05:00+00:00",
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    "keywords": "astronauts, International Space Station, NASA, physics",
    "articleBody": "A bathroom scale is useless on the International Space Station, so NASA replaced it with two springs and a stopwatch-style calculation: a known force shoves a crew member along an arm, and the acceleration reveals body mass to within about 0.2 kilograms, or half a pound, according to a published explainer on the hardware. The device has the kind of name only an agency could love: the Space Linear Acceleration Mass Measurement Device, or SLAMMD. It lives in the station’s Columbus laboratory module, where NASA astronaut Matthew Dominick was photographed servicing it on May 22, 2024. A second machine on the Russian side takes a stranger approach. Crew members crouch on a spring-mounted platform and bounce, pogo-stick style, while the device times each cycle. The popular version of this story calls it a powered pogo stick. Springs do the work, not motors, in both machines as they are described in the explainer we read. How the springs do the weighing Floating in orbit, your body presses on nothing. A scale reads zero because a scale measures weight, the pull of gravity on a body held still. Mass is a separate quantity, a measure of how stubbornly an object resists being moved, and it stays put when gravity seems to vanish. So the station’s hardware ignores weight and tests that stubbornness directly. NASA’s daily station report for December 30, 2013 lays out the recipe: two springs generate a known force against a crew member riding an extension arm, and the resulting acceleration yields the mass. It is Newton’s second law, force equals mass times acceleration, turned around and solved for mass. In practice, the astronaut wraps their legs around a support, presses their stomach to a pad, and rests their chin or head on a brace. The springs release, the arm travels about a meter, and an optical sensor tracks how quickly it moves. A laptop finishes the arithmetic. Here is a worked example with round, made-up numbers, not NASA’s specifications. Suppose the springs push with 200 newtons and the sensor records 2.5 meters per second squared. Divide 200 by 2.5 and the answer is 80 kilograms, or about 176 pounds. A heavier astronaut under the same push accelerates more slowly, and the sum returns a larger number. Why a pogo stick works too The Russian machine, the Body Mass Measurement Device in the Zvezda service module, relies on a different signal: rhythm. Empty, it oscillates quickly. Loaded with a person, it slows down, and the length of each bounce gives away the rider’s mass. Standard spring physics explains the payoff. The period of a mass on a spring grows with the square root of the mass, so mass grows with the square of the period. A bounce that takes twice as long means four times the mass, which is why even small timing differences register clearly. Why NASA bothers Body mass on the station is research data as well as a routine check. That same 2013 NASA report shows three crew members, Hopkins, Mastracchio and Wakata, taking measurements in support of two named studies, Pro K and Body Measures. The next entry in the log covers a separate exercise study aimed at limiting losses of muscle, bone and cardiovascular function on long missions. That context explains the fuss over half a pound. A crew member drifting down in mass over months would be invisible on any instrument that depends on gravity, and the change could be the first sign that diet or exercise needs adjusting. Limits of the trick Neither machine is as simple as stepping on a scale. SLAMMD has to be set up and stowed again, and that 2013 task list records both steps. It also needs a body that holds still against a pad, because an arm or leg flopping around during the push would muddy the reading. Our description of the Russian device rests on that single explainer, since the NASA pages we opened cover only SLAMMD. And the 0.2-kilogram accuracy figure comes from the same explainer rather than a NASA specification sheet. The physics, at least, needs no verification from orbit. In May 2024, Dominick was still maintaining the hardware in Columbus, and the station’s crews were still settling a very earthly question with two springs and F = ma. Further reading Wikipedia: Mass versus weight NASA: ISS Daily Summary Report, December 30, 2013 Image: NASA Johnson Space Center, Public domain via Wikimedia Commons This article was produced with AI assistance and reviewed by our Editorial Desk. How we work.",
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