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 piece by piece, because no rocket is powerful enough to launch such a large structure in one go. 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 experience weightlessness. But why do they weigh nothing on the ISS when gravity at that altitude is still nearly 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 altitude of the ISS, gravity decreases by just 10%. If we could build a tower reaching the ISS, our weight at the top would still be 90% of our weight on the ground. Astronauts experience weightlessness not because of the absence of gravity, but because there is no ground to support them.

On Earth, when support is removed, we also feel weightless as we enter free fall. During free fall, a scale shows zero weight because it falls at the same rate as we do, exerting no pressure on our feet. Astronauts on the ISS, along with everything surrounding them, move at the same speed as the station, causing a scale to also show zero weight. This explains why a satellite in orbit is considered to be in a state of free fall. But how can it be falling if its altitude remains unchanged?

Inertial frame of reference. The answer lies in the frame of reference we select as observers. Observers in an inertial frame get an objective view of reality because they move in a straight line at a constant speed, unaffected by external forces. This frame doesn't require the use of fictitious forces, like centrifugal and Coriolis forces, to explain what happens. Conversely, observers in a non-inertial frame, such as those on Earth, gain a distorted, subjective view of events, affected by the 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 continue along this line, moving away from Earth. Gravity curves this straight trajectory into a circular path by continuously pulling the satellite towards Earth. Observers in the non-inertial frame, who associate themselves with Earth, would say that the station maintains a constant altitude above the Earth's surface. Observers in the inertial frame, who align with the straight path dictated by the orbital velocity, would describe the satellite as falling towards Earth but evading it due to its sideways movement.

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

The ISS maintains its altitude because its speed counteracts Earth's gravitational pull, ensuring a stable orbit. The station moves at 7.67 km/s (4.75 miles per second), effectively offsetting the gravitational acceleration of 0.9 g. Since everything onboard shares the same speed and gravity, astronauts exert no pressure on objects, and vice versa, creating a weightless environment. Even without the station, astronauts and objects would continue orbiting Earth on their own. This weight-free environment provides a unique opportunity for developing innovative medical treatments and advanced 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.