The Principle of Continuous Freefall
Satellites stay in orbit by achieving a delicate balance between two opposing forces: Earth's gravitational pull and their own horizontal motion (inertia). Instead of falling directly to Earth, a satellite moves forward so rapidly that as gravity pulls it downwards, the Earth's surface curves away beneath it, resulting in a continuous 'fall around' the planet.
Achieving Orbital Velocity
To maintain a stable orbit, a satellite must reach a specific orbital velocity. This velocity is fast enough to ensure its forward momentum continuously carries it around the Earth, preventing it from falling back into the atmosphere, yet not so fast that it escapes Earth's gravitational field entirely. For a low Earth orbit, this speed is approximately 28,000 kilometers per hour (17,500 mph).
The Role of Centripetal Force
Earth's gravity provides the necessary centripetal force that constantly pulls the satellite towards the planet's center, keeping it on a curved path. Without this inward force, the satellite's inertia would cause it to fly off into space in a straight line, as dictated by Newton's First Law of Motion.
No Air Resistance in Space
Once a satellite reaches a stable orbit high above most of Earth's atmosphere, it encounters negligible air resistance. This allows it to maintain its orbital velocity with very little external propulsion. Fuel is primarily used for the initial launch, orbital adjustments, and maintaining its position over time, rather than for continuous forward thrust.