The altitude at which satellites orbit the Earth is deliberately chosen to suit the spacecraft’s specific mission. Satellites occupy distinct zones in space, each with unique characteristics that influence their speed, coverage area, and communication properties. These orbital bands range from a few hundred kilometers to tens of thousands of kilometers above the surface. They are classified based on their distance from Earth, establishing three primary orbital regimes.
Low Earth Orbit
Low Earth Orbit (LEO) is the closest operational orbital zone, stretching from approximately 160 kilometers up to 2,000 kilometers above the Earth’s surface. This region is favored for missions requiring high-resolution imaging and low communication latency. Satellites in LEO travel at very high speeds, completing a full orbit in about 90 to 120 minutes. The International Space Station (ISS) and the Hubble Space Telescope operate within this band, at altitudes around 400 to 540 kilometers respectively.
LEO is becoming increasingly crowded with large constellations of communication satellites, such as Starlink, which utilize the low altitude for fast data transmission. The primary drawback of LEO is the presence of residual atmospheric drag, which is high enough to cause orbital decay over time. Satellites in this region, like the ISS, must perform periodic boosts to maintain their altitude and counteract this constant drag. This lower orbit means that a single satellite can only view a small portion of the Earth at any moment, necessitating large constellations for continuous global coverage.
Medium Earth Orbit
The next layer is Medium Earth Orbit (MEO), extending from 2,000 kilometers up to the geosynchronous altitude. This intermediate zone is crucial for global navigation systems. The most widely known MEO occupants are the satellites forming the Global Positioning System (GPS), which orbit at approximately 20,200 kilometers. Other global navigation systems, such as Galileo and GLONASS, also utilize this altitude band.
Satellites in MEO have an orbital period of around two to twelve hours, moving slower than LEO satellites and appearing to drift across the sky from a ground perspective. This higher altitude provides a significantly wider field of view, meaning fewer satellites are needed for continuous global coverage for positioning and timing services. The increased distance results in slightly higher communication latency compared to LEO, but the reduced atmospheric drag offers a longer orbital lifetime with less need for frequent orbital corrections. Certain communication systems use MEO, offering a compromise between the low latency of LEO and the wide coverage of the highest orbits.
Geosynchronous and Geostationary Orbit
The highest common operational orbits are the Geosynchronous Orbit (GSO) and its special subset, the Geostationary Orbit (GEO), found at a precise altitude of 35,786 kilometers above the Earth’s equator. Satellites at this specific height have an orbital period that exactly matches the Earth’s rotational period of one sidereal day. A satellite in a GSO returns to the same position in the sky at the same time each day, tracing a figure-eight pattern relative to the ground observer if its orbit is inclined.
The Geostationary Orbit (GEO) is a circular GSO directly above the equator, giving the satellite the unique property of appearing fixed in the sky from an observer on the ground. This fixed position is highly beneficial for long-haul telecommunications, television broadcasting, and continuous weather monitoring, as ground-based antennas do not need to track the satellite. Only three GEO satellites, spaced evenly, are theoretically required to cover nearly the entire globe, excluding the extreme polar regions. The immense distance to GEO introduces a noticeable signal delay, or latency, which can be a drawback for interactive applications.
Why Altitude Matters
The choice of a satellite’s orbital altitude is determined by a trade-off between coverage, latency, and the physical forces at play. Lower orbits require less energy to reach but are subject to greater atmospheric drag, which slowly pulls the satellite toward the Earth. This drag necessitates carrying extra fuel for periodic propulsion burns to maintain altitude, as seen with the Hubble Space Telescope.
Orbital speed is inversely related to altitude; the lower the orbit, the faster the satellite must travel to counteract Earth’s gravitational pull. LEO satellites travel at speeds around 7.8 kilometers per second, while GEO satellites move much slower at approximately 3.07 kilometers per second. Higher orbits require much more energy to launch and have higher signal latency, but they provide a dramatically larger coverage area, allowing a single satellite to service an entire continent. Mission goals, such as the need for detailed ground observation or the requirement for a fixed point of communication, ultimately dictate where a satellite is placed in the layered structure of Earth’s orbits.