Why is the ISS a weight-free zone in the sea of gravity?

The International Space Station (ISS) (Fig. 1) is a laboratory the size of a mansion, orbiting Earth at an altitude of 250 miles (400 km). It was assembled in space using modules sent into orbit in separate launches, because no rocket is powerful enough to launch such a large structure in one piece. The station features habitable areas with breathable air for astronauts, as well as airless sections housing machinery operated by robotic arms, which are accessible only via spacewalks. The laboratory is used for conducting scientific experiments in the unique microgravity environment, where people and objects feel weightless. But why is the ISS a weight-free zone if gravity at that altitude is still almost as strong as on Earth?

Fig. 1. The International Space Station (ISS) is depicted against the backdrop of Earth and the Sun. In space, above the Earth's atmosphere, the Sun looks white.

Why are astronauts weightless in orbit? At the ISS's altitude, gravity is only 10% weaker than on Earth's surface. If we built a tower reaching the ISS, our weight at the top would still be 90% of what it is on the ground. Astronauts feel weightless not because there is no gravity, but because there is no ground support exerting pressure on their bodies.

On Earth, when support is withdrawn, we also experience weightlessness as we enter free fall. During this state, a scale shows zero weight since it moves at the same rate, applying no pressure on the body. Astronauts aboard the ISS travel at the same speed as the station and everything around them. In this environment, a scale would apply zero force and register zero weight, which is why a satellite in orbit is considered to be in a state of free fall. But how can it be falling if its distance from Earth remains unchanged?

Inertial frame of reference.  The key lies in the chosen frame of reference. An observer in the inertial frame perceives objective reality because they travel in a straight line and at a constant speed, unaffected by external forces. This frame does not necessitate the use of fictitious forces, such as centrifugal and Coriolis forces, to explain what happens around. In contrast, an observer in a non-inertial frame, such as those on the Earth, experiences a subjective reality because their perspective is influenced by forces acting upon them.

The ISS is falling around the Earth. In Fig. 2, the inertial frame is aligned with the vector of the station's orbital velocity. In the absence of gravity, the station would follow this straight line, receding from Earth. Gravity curves the straight path into a circular trajectory, continuously pulling the satellite towards Earth. Observers in a non-inertial frame, such as we on the Earth's surface, would see the station as maintaining the same distance from the Earth. Whereas observers in the inertial frame, aligned with the vector of the orbital velocity, would perceive the satellite as moving away from them, falling towards Earth, but evading a crash due to its sideways motion.

Fig. 2. In the absence of gravity, the ISS would continue along the vector of its tangential velocity.

The ISS maintains a stable orbit because its velocity tries to drag it away from Earth with the same impetus that gravity tries to pull it toward it. At 7.67 km/s (4.75 miles per second), this velocity perfectly counteracts the force of gravity at the orbital altitude, which is 90% of that on the ground. As everything onboard travels at the same velocity and is subjected to the same gravitational force, no pressure is exerted by the station's components on their neighbours, thus creating a microgravity environment. Even without the station, astronauts and objects would continue to orbit Earth together but independently from each other. This unique weight-free zone provides a perfect opportunity for developing innovative medical treatments and advanced space technologies.

Enjoy this breathtaking footage of Earth rotating beneath the International Space Station. Over 24 hours, the ISS completes 16 orbits around Earth, witnessing 16 sunrises and sunsets.