You may have heard people say that aboard the International Space Station (ISS) there’s “zero gravity,” but in fact, gravity is still very much present. The station orbits Earth at an altitude of about 250 miles — a height at which it might not matter to astronauts that gravity is constantly shaping Earth’s surface — but the pull of Earth’s gravity is only slightly weaker than at the surface. In fact, it’s roughly 90% of surface gravity. So why do astronauts and objects aboard the ISS appear to float? Well, to start with, a better term to describe the situation aboard the ISS would be microgravity instead of zero gravity.
Using the term “zero gravity” can give a misleading impression that there is no gravity at all on the ISS, which isn’t true. After all, gravity just vanishing would have some pretty strange effects — hence why that’s not what’s going on. Rather, microgravity describes a state in which gravitational acceleration is dramatically reduced in its effect, which is the case on the ISS. Objects are essentially in free-fall together, so they float relative to their surroundings. Understanding this distinction helps clarify how spacecraft work and what makes the environment of the ISS so unique. Studying microgravity also reveals how life and matter behave without the constant pull of Earth. From how flames burn to how the human body adapts, the microgravity environment provides insight that reshapes engineering, medicine, and our understanding of physics. All this knowledge is vital for future deep-space missions.
How does microgravity work on the ISS?
Microgravity refers to the condition in which objects are subject to the same acceleration relative to their surroundings. Essentially, these objects are falling together with their environment. On the ISS, the effect of weightlessness arises because the station and everything inside it are in continuous free fall around Earth. Imagine throwing a baseball horizontally from a very tall tower. It will begin to fall toward Earth almost immediately, but because of its forward velocity, it moves sideways as well. However, if thrown fast enough, it would circle Earth, constantly in free fall but never hitting the ground. A similar principle applies to the ISS. The station is traveling at around 17,500 mph in orbit, so while gravity pulls it towards Earth, the station’s forward motion causes it to fall around Earth rather than straight down.
As the station falls, so do the astronauts and all the equipment and air inside it. Because they’re all accelerating at nearly the same rate, objects appear to float relative to the station’s interior. But gravity is still acting on everything. The reason why the entire ISS doesn’t drift off into deep space is that Earth’s gravity keeps it bound in orbit. The same principle applies to our moon. It’s in constant free fall around Earth, thus it’s orbiting our planet.
The term microgravity captures the fact that the apparent gravity is extremely small, more accurately 1 x 10^-6 g (on the order of the millionths of Earth’s surface gravity), even though the underlying gravitational field remains strong. In short, on the ISS, there is gravity pulling it towards Earth, but because the entire structure is falling around the planet, the internal environment feels weightless. This weightless feeling allows unique scientific, engineering, and human physiological phenomena to be studied in ways impossible on Earth.
Why microgravity matters
NASA and its international partners maintain the ISS as a microgravity laboratory precisely because the weightless environment opens up scientific and technological opportunities unavailable under normal Earth gravity. On Earth, many physical processes, such as sedimentation, buoyancy, and convection, are dominated by gravitational effects. Gravity can mask subtler behaviors (interestingly, gravity can even affect time). In microgravity, those influences are suppressed or removed, enabling researchers to investigate fluids, materials, biological systems, and fundamental physical phenomena under truly novel conditions.
In terms of human health and systems for long-duration spaceflight, it’s crucial for astronauts to take microgravity into consideration. Astronauts living and working in orbit experience bone loss, muscle atrophy, changes in circulation, and other adaptations. Studying these helps NASA develop counter-measures (forms of exercise, protocols, nutrition, spacecraft design) necessary for longer missions. Also, equipment and materials can be tested in microgravity to verify robustness for a deep-space journey. In this way, the ISS serves as a stepping stone for humanity’s ambitions beyond low Earth orbit. With the eyes of the space agencies turned toward missions to the moon and Mars, understanding how humans and hardware behave in microgravity is a necessary step. The knowledge gained from the ISS influences spacecraft design, life-support systems, mission planning, and health safeguards, all in support of future deep-space exploration.
