The time it takes for a satellite to orbit the Earth is known as its orbital period, representing the duration required to complete one full revolution around the planet. The period is not a single, fixed number because it depends almost entirely on the satellite’s specific path. Determining the exact time requires calculating the orbital path, which is defined by the satellite’s speed and its distance from Earth. Orbital periods range from about 90 minutes for the closest satellites to exactly 24 hours for those much farther out.
Altitude Is the Primary Factor Determining Orbital Speed and Time
The fundamental principle governing a satellite’s orbital period is the balance between its forward velocity and Earth’s gravitational pull. A satellite is perpetually falling toward Earth, but its horizontal speed is so great that the planet’s surface curves away beneath it. This continuous fall defines an orbit.
The strength of gravity diminishes predictably with increasing distance from the Earth’s center. A satellite in a lower orbit experiences a stronger gravitational force, requiring it to travel at a higher velocity to maintain its trajectory. This higher speed, combined with a smaller circumference, results in a short orbital period.
Conversely, a satellite placed at a higher altitude is subject to a weaker gravitational tug and maintains a stable orbit while traveling at a slower speed. This slower velocity and the larger circumference translate into a longer time required to complete one circuit. The overall time for a revolution is primarily dictated by the distance from the center of the Earth.
Standard Orbital Zones and Their Corresponding Periods
Space agencies categorize orbits into distinct zones, each associated with a predictable range of orbital periods. These zones are defined by altitude and correspond to the practical applications for which the satellites are designed. The three primary zones are Low Earth Orbit (LEO), Medium Earth Orbit (MEO), and Geostationary Earth Orbit (GEO).
Low Earth Orbit (LEO)
LEO is the closest zone, spanning altitudes from approximately 160 km up to 2,000 km above the surface. Satellites in LEO, such as the International Space Station, complete an orbit quickly. Their orbital periods typically fall between 90 and 120 minutes. This rapid motion means the satellite’s ground track shifts constantly, allowing a single satellite to cover the entire globe.
Medium Earth Orbit (MEO)
MEO occupies the region between the LEO and GEO zones, from 2,000 km up to 35,786 km. Satellites in this mid-range area have orbital periods that vary significantly, generally from about 2 hours up to 24 hours. The most well-known MEO occupants are navigation systems, including the Global Positioning System (GPS) constellation. GPS satellites orbit at about 20,200 km and have a precise orbital period of about 12 hours.
Geostationary Earth Orbit (GEO)
GEO is the farthest major zone, existing at a unique altitude of exactly 35,786 km above the equator. Satellites placed here have an orbital period of precisely 24 hours, which perfectly matches the time it takes for the Earth to rotate once. This synchronization causes the satellite to appear stationary over a fixed point on the Earth’s surface, making it ideal for continuous communication and weather monitoring.
Why Orbital Periods Are Not Always Fixed
While orbital mechanics can calculate a precise period for a specific altitude, a satellite’s actual time of revolution is not static over its lifespan. External forces, collectively known as perturbations, constantly alter the trajectory and, consequently, the orbital period.
Atmospheric Drag and Orbital Decay
Atmospheric drag is the most significant perturbing force for satellites in Low Earth Orbit. Even thin atmospheric particles create friction, slowing the satellite down over time. This loss of energy causes the satellite to gradually lose altitude in a process called orbital decay, which reduces the orbital period.
Gravitational Perturbations
Gravitational perturbations from other celestial bodies, particularly the Moon and the Sun, also affect a satellite’s orbit. These distant gravitational fields introduce continuous tugs that subtly change the path and period. The Earth itself is not a perfect sphere, and its uneven mass distribution creates a non-uniform gravitational field.
Maintaining the Orbit
To counteract orbital decay and maintain the intended period, many satellites, especially those in LEO, must periodically fire thrusters. This “re-boosting” process pushes the spacecraft back into its proper, higher altitude. Without these maneuvers, the orbital period shortens until the satellite descends low enough to burn up in the atmosphere.