How Do Astronauts Weigh Themselves in Space?

High-tech scales for the zero-G traveler.

Astronaut Karen Nyberg using the Space Linear Acceleration Mass Measurement Device (SLAMMD) on board the space station. (NASA)
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It’s a morning ritual most of us take for granted. Still dripping from the shower, you step on the scales to assess the damage caused by eating most of Aunt Leslie’s pecan pie during an ill-thought-out midnight snack. You grimace and then start the day.

For astronauts floating high above Earth, determining if last night’s meal of thermostabilized turkey and freeze-dried cornbread dressing has caused a bit of a belly is far from simple. The first obstacle is that in the microgravity of space, their weight is essentially zero, all the time. But astronauts still have mass, which they can measure to determine what their weight on Earth would be, and to let medical researchers track changes in body mass due to deconditioning in orbit.

Currently, crews on board the space station use two devices to calculate their weight. The first is NASA’s Space Linear Acceleration Mass Measurement Device (SLAMMD), which relies on Newton’s Second Law of Motion. As astronauts Michael Barrett and Koichi Wakata explain in this video, a force from two springs mounted on an extension arm is generated against the astronaut. Because the force is a known quantity, the resulting acceleration is used (remember F=ma?) to determine the crewmember’s mass. The SLAMMD is accurate to 0.5 pounds.

The other device is Russia’s Body Mass Measurement Device (BMMD), seen being demonstrated by Reid Wiseman in this Vine he posted last October.

The astronaut rides the BMMD somewhat like a pogo stick, and the oscillation of a spring is used to measure mass.

Both devices work, but both have the disadvantage of taking up room in an already cramped space station. They also require a lot of power, another scarce resource on the station.

That has led a group of European researchers to propose a mass-measuring system based on the Kinect technology familiar to gamers. The name “Kinect” actually refers to a type of motion sensing input device, not just a vehicle for awkward dancing to One Direction songs on “Just Dance 4” after a few too many adult beverages.  The Kinect system uses camera inputs to create a 3D model of the astronaut. The model is then cross-referenced with a statistical model of almost 30,000 people to predict the astronaut’s weight. In addition to being 97 percent accurate, the proposed device is small, lightweight and economical in its energy requirements.

Unlike SLAMMD and BMMD, which measure mass, the Kinect system would measure body volume, and one potential problem is water shifting inside an astronaut’s body in microgravity, which could affect the accuracy of the measurements. Paolo Ariano, a researcher at Fondazione Istituto Italiano di Tecnologia’s Center for Space/Human Robotics, says the team is working on a new prototype of their Kinect-based device and is looking for additional funding so it can be tested in space.

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About Sarah LeClaire

Sarah LeClaire is an intern at Air and Space Magazine and works as an Explainer at the National Air and Space Museum’s Steven F. Udvar-Hazy Center.

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