How Low Earth orbit Works

To remain in low Earth orbit, a satellite must travel sideways at approximately 28,000 kilometers per hour (about 17,500 miles per hour). This isn't arbitrary—it's the precise speed needed at LEO altitudes to balance Earth's gravitational pull with the satellite's tendency to fly off into space. Any slower and gravity wins, pulling the object back to Earth; any faster and the object escapes to a higher orbit or leaves Earth's influence entirely.

This tremendous velocity means a LEO satellite completes one full orbit around Earth every 90 to 120 minutes, depending on its exact altitude. The International Space Station, orbiting at roughly 400 kilometers, circles our planet about 16 times per day. Astronauts aboard experience a sunrise and sunset every 45 minutes, a vivid reminder of their incredible speed.

Achieving this velocity requires powerful rockets that must overcome both gravity and atmospheric resistance during launch. Once in the vacuum of space, however, no fuel is needed to maintain this speed—Newton's first law ensures the satellite continues moving at 28,000 km/h indefinitely, barring external forces like atmospheric drag.

A satellite in LEO isn't floating motionlessly—it's actually falling toward Earth continuously. However, it's also moving sideways so fast that as it falls, Earth's curved surface falls away beneath it at exactly the same rate. This creates what appears to be a stable circular path, but is really perpetual freefall around the planet.

The balance point occurs when gravitational force pulling inward equals the centrifugal effect of the satellite's circular motion pushing outward. At LEO altitudes, Earth's gravity is still about 90% as strong as at the surface—strong enough to provide the centripetal acceleration needed to bend the satellite's straight-line trajectory into a curve. The common notion that there's "no gravity" in orbit is a misconception; astronauts feel weightless because they and their spacecraft are falling together.

This delicate equilibrium varies with altitude. Lower orbits require faster speeds because gravity is stronger closer to Earth, while higher orbits within the LEO range can maintain stability at slightly slower velocities. A satellite at 200 kilometers altitude needs different parameters than one at 1,500 kilometers, though both occupy the low Earth orbit zone.

While we often describe orbits as "circular," virtually all LEO trajectories are actually ellipses—stretched circles with Earth's center at one focal point. A perfectly circular orbit is a special case of an ellipse where both focal points coincide. Most satellites follow slightly elliptical paths, with a lowest point (perigee) and highest point (apogee) that differ by tens or hundreds of kilometers.

The satellite moves faster when closer to Earth at perigee and slower at apogee, following Kepler's laws of planetary motion. This variation occurs because gravitational force is stronger at lower altitudes, accelerating the satellite as it descends and decelerating it as it climbs. The total energy of the orbit remains constant, converting between kinetic energy (speed) and gravitational potential energy (altitude).

The orbital plane—the flat geometric surface containing the elliptical path—remains fixed in space relative to distant stars, even as Earth rotates beneath it. This means a LEO satellite might pass over different parts of Earth's surface on each orbit. Satellites in polar orbits eventually cover the entire planet as Earth rotates, while equatorial orbits repeatedly trace the same latitudinal band.

Even at LEO altitudes, Earth's atmosphere hasn't completely disappeared—it's merely extraordinarily thin, millions of times less dense than at sea level. These residual atmospheric molecules still collide with satellites, creating friction that gradually saps orbital energy. The effect is small but persistent, like a gentle headwind that never stops blowing.

Atmospheric density decreases exponentially with altitude, so drag varies dramatically across the LEO range. Satellites at 160 kilometers experience significant drag and would fall back to Earth within days without propulsion, while those at 600 kilometers might remain aloft for decades. The International Space Station at 400 kilometers requires periodic "reboosts"—rocket firings that restore lost altitude—to counteract this drag and prevent atmospheric reentry.

Solar activity amplifies this effect unpredictably. When the Sun is active, it heats Earth's upper atmosphere, causing it to expand outward and increase density at LEO altitudes. During solar maximum periods, satellites experience two to three times more drag than during solar minimum, forcing mission planners to budget extra fuel for station-keeping maneuvers.

Orbital decay is the inevitable consequence of atmospheric drag in low Earth orbit. Each atmospheric molecule that collides with a satellite steals a tiny amount of kinetic energy, converting the satellite's motion into heat. This energy loss doesn't immediately cause the satellite to fall—instead, it causes a gradual transition to a lower, slower orbit. Paradoxically, as the orbit lowers into denser atmosphere, the satellite must speed up to maintain orbital balance, which increases drag further.

The decay process accelerates as satellites descend. A satellite might lose only a few kilometers of altitude per year at 800 kilometers, but at 300 kilometers it could lose several kilometers per month. This creates a runaway effect: lower orbit means more drag, which causes faster orbital decay, which means even more drag. Eventually the satellite reaches altitudes where atmospheric heating becomes severe, and the object burns up during reentry.

Mission operators must either accept this limited lifespan or carry fuel for periodic orbit-raising maneuvers. The International Space Station uses Russian Progress cargo vehicles and other spacecraft to perform regular boosts that maintain its altitude. Inactive satellites without propulsion systems become space debris, gradually descending until they harmlessly disintegrate in the upper atmosphere—a natural cleaning mechanism that prevents permanent accumulation of objects in LEO.