A satellite is a man-made object placed into a repeating path, or orbit, around a larger celestial body like Earth. A common misunderstanding is that these spacecraft are floating in a region where Earth’s gravity no longer pulls. If gravity is still present, why do satellites not fall back to the ground? The answer lies not in escaping gravity, but in a precise, continuous engagement with it.
The Critical Balance of Speed and Gravity
The idea that satellites exist in a gravity-free environment is incorrect; gravity is the force that keeps them in orbit. Even at the altitude of the International Space Station (ISS), the gravitational pull remains strong, about 90% as powerful as it is on the ground. To counteract this constant downward pull, a satellite must possess immense horizontal speed. This sideways speed prevents the downward gravitational force from causing the object to crash.
For a stable Low Earth Orbit, a satellite must maintain a speed of 7 to 8 kilometers per second (about 17,500 miles per hour). This speed creates a delicate equilibrium with gravity. If the velocity were too slow, gravity would dominate, causing the satellite to curve down into the atmosphere. If the speed were too fast, the satellite would escape Earth’s gravity.
The Definition of Orbital Freefall
The key to understanding a stable orbit is recognizing that a satellite is continuously falling toward Earth. An object in orbit is in a state of perpetual “orbital freefall,” influenced only by gravity. The satellite’s extreme horizontal speed ensures that as it falls, it constantly misses the planet, tracing a curved path around the Earth instead of impacting the surface. This continuous falling motion defines an orbit.
This state of freefall also explains the weightlessness experienced by astronauts. Since the satellite and everything inside are accelerated by gravity at the same rate, there is no contact force pushing up against them. The resulting sensation is not due to a lack of gravity, but rather to the entire system falling together, a condition described as microgravity.
Why Orbits Are Not Permanent
Despite the precise balance achieved, no orbit in Low Earth Orbit is truly permanent, primarily because of atmospheric drag. Even at high altitudes, the upper atmosphere is not a perfect vacuum; trace amounts of gas molecules still exist. A satellite traveling at thousands of miles per hour collides with these molecules, creating friction that exerts a continuous drag force. This drag constantly saps energy from the satellite’s velocity, causing it to slow down.
As the speed decreases, the critical balance with gravity is disturbed, and the planet’s pull begins to win. The loss of velocity causes the satellite’s orbit to decay, slowly spiraling inward toward the denser atmosphere. This process accelerates until the satellite finally re-enters and typically burns up due to intense heat. To prevent this, many satellites, including the ISS, must periodically use small thrusters to boost themselves back to a higher, stable altitude.