A satellite is a human-made object intentionally placed into orbit around the Earth for functions ranging from communications to Earth observation. The idea that satellites orbit within the atmosphere is a common misunderstanding. Satellites must operate far above the dense layers of air because atmospheric friction would quickly cause them to fall back to Earth. To maintain a stable path, a satellite must achieve a specific speed at an altitude where air density is negligible, effectively placing it in the vacuum of space. Satellites operate in distinct orbital zones, each serving a particular purpose.
Setting the Stage: The Boundary Between Air and Space
Satellites cannot orbit within dense air due to atmospheric drag, which is the resistance caused by air molecules. Even at extreme altitudes, residual friction slows an orbiting object. For a satellite to maintain orbit, its speed must perfectly balance the Earth’s gravitational pull. If the atmosphere is too thick, drag slows the satellite, causing it to lose altitude and re-enter.
The conventional boundary separating the Earth’s atmosphere from outer space is the Kármán line, set at 100 kilometers (62 miles) above mean sea level. Below this line, aerodynamic forces are dominant, requiring aircraft to use wings. Above this altitude, an object must rely on orbital velocity to stay aloft, marking the transition to astrodynamics. At this height, air density drops enough for a satellite to achieve a sustained orbit.
Low Earth Orbit: The Closest Satellites
Low Earth Orbit (LEO) is the closest orbital regime to the planet, spanning 160 kilometers up to 2,000 kilometers. This zone is the most densely populated because it requires the least energy compared to higher orbits. LEO satellites travel at high speeds, about 7.8 kilometers per second, completing a revolution in roughly 90 minutes. This rapid movement allows them to circle the globe multiple times daily.
The International Space Station (ISS) operates within LEO, typically between 330 and 435 kilometers, as do remote sensing and Earth observation satellites. Proximity to the surface provides the advantage of high-resolution imaging and lower latency in communication signals. Large constellations, such as Starlink, utilize LEO for global broadband internet service. However, LEO satellites cover a small geographic area at any time, requiring a vast network to achieve continuous global coverage.
Medium Earth and Geostationary Orbits: The Higher Zones
Medium Earth Orbit (MEO)
Medium Earth Orbit (MEO) occupies the region between the outer edge of LEO at 2,000 kilometers and the Geostationary Orbit altitude of 35,786 kilometers. MEO satellites have slower orbital periods than LEO and offer a wider field of view, meaning fewer satellites are needed for coverage. MEO is primarily used for global navigation satellite systems, including the United States’ Global Positioning System (GPS) and Europe’s Galileo system. These systems rely on MEO altitude to provide precise positioning, navigation, and timing services across the globe.
Geostationary Orbit (GEO)
Geostationary Orbit (GEO) is a single, specific orbital path located exactly 35,786 kilometers above the Earth’s equator. Satellites in this orbit travel at the same rotational speed as the Earth, taking precisely one sidereal day to complete an orbit. This synchronization makes the satellite appear stationary when viewed from the ground. GEO is the preferred location for large communications satellites, television broadcasters, and weather monitoring spacecraft. The stationary position allows ground antennae to remain fixed on the satellite without tracking its movement, ensuring constant communication over vast regions.
The End of Orbit: How Atmospheric Drag Causes Decay
Even in the sparse environment of LEO, the residual atmosphere exerts a continuous force leading to orbital decay. Atmospheric drag acts like a brake, slowing the satellite and causing it to lose the velocity needed to maintain altitude. As the satellite slows, its orbit shrinks, pulling it into slightly denser air, which increases the drag in a reinforcing cycle.
This process continues until the satellite descends low enough to begin its final, uncontrolled re-entry. Most small LEO objects heat up and disintegrate due to intense friction between 72 and 84 kilometers, burning up safely before reaching the surface. Larger structures, such as the ISS or heavy satellites, require controlled, propulsive maneuvers to direct their re-entry over uninhabited areas, preventing debris from posing a risk to the ground.