How do you weigh yourself in space?
Weighing yourself in space sounds like a paradox. In microgravity, objects don’t pull toward the Earth the same way they do on the surface. Yet the International Space Station (ISS) and its crew still need to understand mass and how it affects everything from exercise routines to docking maneuvers. This week, during Expedition 73’s stay on the ISS, a NASA astronaut demonstrated the practical steps and the science behind weighing in orbit, shedding light on how space travelers monitor their body mass and how the station as a whole stays properly calibrated.
The principles: mass vs. weight in orbit
On Earth, weight is a measure of the force gravity exerts on a body, while mass remains constant. In the ISS, gravity is still present, but everything is in free fall around the planet. That microgravity environment makes traditional scales unreliable for measuring weight. Instead, astronauts rely on a combination of methods to assess mass and body composition, including inertial measurement devices, body mass estimations, and specialized scale systems designed for zero-G conditions.
What the astronaut demonstrated
During the week of November 17–21, 2025, the station crew highlighted how they track body mass and how the ISS itself is weighed to ensure accurate measurements of the entire complex. The demonstration included:
- Using inertial sensors to estimate mass changes caused by muscle atrophy or fluid shifts, which are common in microgravity.
- Employing calibration routines for equipment that indirectly reflects mass, such as the station momentum and docking simulators that rely on precise mass models for maneuvers.
- Explaining the role of body composition assessments, including muscle and fat measurements, to guide exercise prescriptions on orbit.
Why mass matters on the ISS
Mass accounting is more than a numerical curiosity. The ISS relies on a delicate balance of propulsion, control moment gyros, and docking systems to maintain stable operations. Accurate mass data helps engineers predict how the station will respond to thruster firings and attitude adjustments. For the crew, understanding their own mass changes informs nutrition plans, exercise routines, and health monitoring—critical factors for long-duration missions and future deep-space exploration.
How crew members maintain their health in microgravity
Exercise is a central part of life aboard the ISS. Astronauts perform resistance and cardio workouts to counteract muscle and bone loss in microgravity. Regular body measurements, including mass estimation when appropriate, help scientists assess the effectiveness of exercise regimens and dietary plans. The week’s discussions underscored the ongoing collaboration between flight surgeons, researchers, and pilots to keep the crew fit and mission-ready.
Looking ahead: implications for future missions
Understanding mass in space isn’t just about current operations. As space agencies plan longer missions to the Moon, Mars, and beyond, precise mass management will be essential for life support planning, habitat design, and propulsion calculations. The ISS continues to serve as a living laboratory where astronauts turn theoretical physics into practical procedures, ensuring that future explorers carry robust health maintenance routines and reliable mass measurement techniques with them into deep space.
Bottom line
Even in weightless conditions, the concept of mass remains central to life on the ISS. The week of November 17–21, 2025, showcased how astronauts weigh themselves in space, why it matters for individual health, and how accurate mass data supports safe and efficient operations of the orbiting laboratory and its future missions.
