The idea that there is no gravity in space is one of the most pervasive misconceptions in physics and space travel. The visual evidence of astronauts floating inside the International Space Station (ISS) suggests a complete absence of the gravitational force that holds us to Earth. However, the phenomenon of weightlessness experienced in orbit is not due to a lack of gravity, but is instead a consequence of how objects move within a gravitational field. Understanding this distinction provides insight into the mechanics of spaceflight and the universal force of attraction.
The Truth About Gravity’s Reach
Gravity does not suddenly vanish at the boundary of Earth’s atmosphere; rather, it is a force that extends infinitely throughout the universe. The force of gravity between two objects is directly related to their masses and inversely related to the square of the distance between them. While gravity weakens as distance increases, it never actually reaches zero unless the distance is infinite.
The International Space Station (ISS) orbits Earth at an altitude of approximately 400 kilometers (250 miles). At this height, the gravitational acceleration from Earth is still remarkably strong. Astronauts and the ISS are subject to roughly 87% to 90% of the gravitational pull that people experience while standing on the planet’s surface.
This massive gravitational force at the ISS altitude is what keeps the station locked in orbit around Earth. If there were truly no gravity, the station would fly off in a straight line. The force is very much present and at work.
The Real Cause of Weightlessness
The appearance of floating in the ISS is not the result of zero gravity, but of being in a constant state called freefall. Orbit is essentially a continuous fall around a celestial body. The ISS moves horizontally at tremendous speed, approximately 28,000 kilometers per hour (17,500 mph), which is necessary to achieve orbit.
As the station is pulled down by Earth’s gravity, its high lateral velocity causes it to constantly miss the planet’s surface. This continuous falling motion means the spacecraft, the astronauts, and all objects inside are accelerating toward the Earth at the exact same rate.
When everything accelerates together, no object is being supported by another, creating the sensation of weightlessness. This is the same brief feeling experienced when an elevator cable snaps or during the peak drop of a roller coaster ride. Inside the ISS, this condition is continuous, resulting in what is technically known as microgravity, or very small gravity effects.
Gravity Versus Perceived Weight
A clear distinction exists between the concept of gravity and the concept of weight. Gravity is the fundamental force of attraction between masses, which is an inherent property of the universe. Weight, however, is a measurement of the force of gravity acting on an object’s mass when that object is being resisted by a supporting force.
An astronaut maintains the same mass in space as they do on Earth, as mass is simply the amount of matter an object contains. Their weight becomes negligible, or zero, because there is no ground or scale pushing back to resist the gravitational force. A scale would read zero because the astronaut, the scale, and the ISS are all falling together, meaning no relative force is exerted between them.
The sensation of weight we feel on Earth is the result of the ground pushing up against our feet, counteracting gravity. In orbit, the only force acting on the astronaut is the gravitational pull. Since they are falling with it, they experience no internal resistance, leading to apparent weightlessness.
How Far Does Earth’s Gravity Extend
The gravitational force of Earth, while infinite in range, decreases rapidly with distance according to the inverse square law. This law states that the force is inversely proportional to the square of the distance from the center of the mass. If the distance from Earth’s center is doubled, the gravitational force drops to one-fourth of its original strength.
This rapid decay means the practical influence of Earth’s gravity drops off quickly compared to the massive distances in space. For instance, the gravitational acceleration from Earth is reduced to only 1% of its surface value at an altitude of approximately 57,339 kilometers (35,629 miles).
True “zero gravity,” where the force is entirely absent, is a theoretical concept that would require an object to be infinitely far from any mass. In the real universe, every object with mass exerts a gravitational pull. Objects in space are always under the influence of some gravitational field, whether it is from Earth, the Sun, or a distant galaxy.