Orbits. Orbital motion and types of orbits

Orbital motion

We call orbital motion the continuous displacement of an object in space around another more massive object, guided by the influence of gravity. This motion is the result of a delicate balance between the gravitational force that pulls the object toward the central body and the tangential velocity of the object, which tends to push it away. This balance generates orbital trajectories that can vary in shape from perfect circles to ellipses and open trajectories such as parabolas and hyperbolas. It is a dynamic phenomenon observed in natural planets and satellites, asteroids and spacecraft, all following Kepler’s laws, which describe the relationship between the shape, velocity and position of the object in its orbit.

In addition to the gravity of the massive object around which the orbital path is established, the orbital motion can be influenced by by the gravity of other nearby objects. This is known as gravitational perturbation and can make orbits more complex. For example, the Moon is influenced by both Earth’s gravity and the Sun’s gravity, resulting in a slightly elliptical orbit around the Earth.

Types of orbits by geometric shape

The trajectory followed by an object in space is determined by its velocity and the gravitational pull of the central body. According to the laws of orbital mechanics, these paths always take the form of a conic section.

Elliptical orbits

An elliptical orbit is the most common trajectory in the universe and features an oval or ellipse shape. In this type of orbit, the distance between the orbiting object and the central body changes constantly throughout its path. This means there is a point of closest approach known as perigee (or periapsis) and a point of maximum distance called apogee (or apoapsis). The object’s speed is not uniform: it moves much faster when it is close to the central body and slows down as it moves away. Most planets, natural satellites, and spacecraft follow elliptical trajectories.

Circular orbits

A circular orbit is a special, theoretical case of an elliptical orbit where the eccentricity is exactly zero. In this type of trajectory, the object maintains a completely constant distance from the center of the celestial body at every single point along its path, which means its orbital speed also remains unchanging. Although finding a perfect circular orbit in nature is practically impossible due to the gravitational perturbations of other bodies, aerospace engineers aim to approximate this shape when launching certain satellites to ensure their instruments always operate at the same altitude.

Parabolic orbits

A parabolic orbit is an open trajectory that marks the exact boundary between objects that remain trapped by gravity and those that manage to escape. It occurs when a body travels at the exact escape velocity of the system. The object approaches the central body, makes a single sharp turn around it, and then moves away permanently into deep space, slowing down progressively but never coming to a complete stop. It is a single-pass trajectory.

Hyperbolic orbits

A hyperbolic orbit is another open trajectory, but it differs from a parabolic one because the object travels at a speed clearly exceeding the escape velocity. The central celestial body deflects the object’s path due to gravity, but it lacks the necessary force to retain it. The object retains a considerable amount of speed even when it reaches an infinite distance from the central body. This is the type of orbit described by comets originating from outside the solar system or by spacecraft when performing a gravitational assist maneuver to gain speed and propel themselves toward other planets.

Types of orbits by space mission requirements

When artificial satellites are designed and launched, their orbit is meticulously selected based on altitude, inclination relative to the equator, and the specific needs of the mission on Earth.

Low Earth orbit

Known by its acronym LEO, this orbit is located at an altitude of between 160 and 2000 kilometers above the Earth’s surface. Being so close to Earth, satellites travel at high speeds (about 27,500 kilometers per hour) and complete a full loop around the planet in approximately ninety minutes. It is the ideal region for weather observation, mapping, and spy satellites, as it allows for very high-resolution imaging, and it is home to the International Space Station.

Medium Earth orbit

Known as MEO, this orbit encompasses the region of space located between 2000 and 35,786 kilometers in altitude. Satellites placed in this zone take anywhere from a few hours to a full day to complete a single orbit. Because it offers a perfect balance between data resolution and wide coverage of the Earth’s surface, it is used almost exclusively for global navigation and positioning satellite constellations, such as the American GPS system or the European Galileo system.

High Earth orbit

Known as HEO, this orbit comprises any orbit whose altitude sits above 35,786 kilometers from the Earth’s surface. At these immense distances, Earth’s gravitational pull is much weaker, causing satellites to move very slowly and take more than twenty-four hours to complete a single revolution. It is used for scientific satellites observing deep space and for globally monitoring our planet’s magnetic environment.

Polar orbits

A polar orbit is one that features an inclination close to ninety degrees relative to the Earth’s equator, meaning the satellite passes directly over or very close to the North and South Poles on every revolution. While the satellite moves vertically from pole to pole, the Earth rotates horizontally beneath it. This combination allows the satellite, after a set number of revolutions, to progressively scan and photograph every strip of the entire planet’s surface.

Sun-synchronous orbits

A sun-synchronous orbit is a special type of polar orbit whose altitude (usually between 600 and 800 kilometers) and mathematical inclination are calculated so that the orbital plane shifts subtly by about one degree per day. In this way, the orbit synchronizes with Earth’s annual journey around the Sun, ensuring that the satellite passes over a specific point on the surface at exactly the same local solar time every day. This is crucial for scientific and remote sensing missions, as all photos of a given area maintain the same light angle and shadows.

Geosynchronous orbits

A geosynchronous orbit is one that has an orbital period exactly equal to Earth’s rotation period on its own axis, which is twenty-four hours. This means the satellite takes the same time to circle the planet as the Earth takes to spin once. If the orbit has some inclination, the satellite will not look static from the ground; instead, it will appear to trace a geometric figure shaped like a figure-eight in the sky throughout the day, crossing the sky at the exact same times.

Geostationary orbit

The geostationary orbit (GEO) is a specific and highly important case within geosynchronous orbits. To achieve it, the satellite must be placed exactly at 35,786 kilometers in altitude with an inclination of zero degrees, meaning it is perfectly aligned over Earth’s equator. At this precise distance and position, the satellite moves identically to the Earth’s rotation, causing it to appear completely motionless at a fixed point in the sky when viewed from the ground. This allows fixed satellite dishes to point directly at it without needing tracking systems, serving as the foundation for satellite television and global telecommunications.

Molniya high-eccentricity orbits

The Molniya orbit is a markedly elliptical trajectory design with a sharp inclination of about 63.4 degrees, originally developed to bypass the issue that geostationary satellite signals do not reach northern polar regions effectively. In this orbit, the satellite passes very quickly at low altitude through the Southern Hemisphere (perigee) but travels extremely slowly at high altitude over the Northern Hemisphere (apogeo). Thanks to this, the satellite spends about eight hours of its twelve-hour orbit “hanging” almost motionless over the northern part of the Earth, providing stable communications to high-latitude zones.

Importance of orbits

Understanding orbits is crucial for space navigation and space mission planning. Scientists and space engineers use calculations and mathematical models to predict and control spacecraft orbits, ensuring that they stay on safe and efficient trajectories.

In addition to natural orbits, humans have succeeded in placing satellites in orbit around the Earth, which are used for various applications, such as communication, Earth observation, navigation, and scientific research.

The online orbit simulations on this page are an excellent way to delve deeper into orbital motion and the most important types of orbits. Give them a try!