Orbit is the continuous path an object in space follows around a central body, such as a planet or star. This path is a curved trajectory that keeps the orbiting object suspended above the central mass. Satellites and spacecraft achieve this perpetual motion by constantly moving sideways at high speed.
The Essential Balance of Gravity and Velocity
The fundamental principle of establishing an orbit relies on a precise balance between two forces: the downward pull of gravity and the object’s forward momentum. Gravity is the force that constantly attracts the orbiting object toward the center of the Earth. To counteract this pull, the object must be given a sufficient horizontal velocity, also known as tangential motion.
The concept is often illustrated using Isaac Newton’s thought experiment of a cannon placed on a very tall mountain. If a cannonball is fired with a low velocity, it will travel a short distance and fall to Earth in a curved path. If the cannonball is fired fast enough, it will travel forward at a speed that matches the rate at which the Earth curves away beneath it.
This results in a state of continuous freefall, where the object is constantly falling toward the Earth but never actually hitting it. To maintain a stable orbit, the object’s velocity must be perfectly perpendicular to the gravitational pull at any given moment. If the speed is too low, the downward pull overcomes the sideways motion, and the object spirals inward. If the speed is too high, the object will escape the central body’s gravitational influence entirely.
Understanding Orbital Paths
The geometry of an orbital path is determined by the specific balance of velocity and gravity, resulting in shapes defined by conic sections. The two most common paths are the circular orbit and the elliptical orbit. A perfectly circular orbit is rare and requires the orbiting object to maintain a precise, constant speed at a fixed distance from the central body.
An ellipse occurs when the initial velocity is slightly too fast or too slow for a perfect circular path. In an elliptical orbit, the distance between the orbiting object and the central body is constantly changing. The point closest to the Earth is called the perigee, and the point farthest away is called the apogee.
The degree to which an orbit deviates from a perfect circle is measured by its eccentricity. Orbits with low eccentricity are nearly circular, while those with high eccentricity are highly elongated. Comets, for example, often have highly eccentric orbits around the sun.
Altitude, Speed, and Orbital Decay
The altitude of an orbit is inversely related to the speed required to maintain it. Satellites in lower orbits must travel significantly faster because they are closer to the Earth’s center of mass, where the gravitational pull is stronger. For instance, objects in Low Earth Orbit (LEO) travel at approximately 7.8 kilometers per second and complete an orbit in about 90 minutes.
Conversely, a satellite in a much higher orbit, like the Geostationary Orbit (GEO) at 35,786 kilometers, moves much slower, at around 3.07 kilometers per second. To stay in orbit, a lower object must move faster to generate the necessary tangential motion to avoid falling.
In the lowest orbits, the slight presence of the Earth’s atmosphere creates a phenomenon known as orbital decay. Although the atmosphere is incredibly thin at LEO altitudes, the residual drag slowly steals energy from the satellite’s forward motion. To prevent decay, spacecraft must periodically use onboard thrusters to execute an orbital boost and regain lost altitude.
Common Types of Orbits and Their Purpose
Different orbital altitudes and geometries are chosen based on the specific function of the satellite.
Low Earth Orbit (LEO)
LEO is defined as an altitude below 2,000 kilometers and is used for applications requiring close proximity to the ground. This orbit is favored for high-resolution Earth observation, remote sensing, and large communication constellations. The reduced distance allows for lower communication latency.
Medium Earth Orbit (MEO)
MEO sits between LEO and the highest orbits, ranging from about 5,000 to 20,000 kilometers above the surface. This region is used by Global Navigation Satellite Systems, such as the U.S. GPS network. MEO offers a balance between the low latency of LEO and the wide coverage of higher orbits, meaning fewer satellites are needed for continuous global coverage.
Geostationary Orbit (GEO)
The highest and most specialized orbit is GEO, located precisely at 35,786 kilometers above the equator. Satellites placed here have an orbital period that exactly matches the Earth’s rotational period, which is one sidereal day. This means the satellite appears to remain fixed over a single spot on the Earth’s surface, making it useful for continuous broadcast, television, and weather monitoring.