An Orbit represents the consistent, repetitive trajectory that an object in space follows around another celestial body. Objects in orbit, termed satellites, can be either natural, such as Earth or the moon, or artificial, like the International Space Station. Many planets are orbited by natural satellites known as moons. Within the solar system, planets, comets, asteroids, and other celestial bodies revolve around the sun. The majority of these orbiting objects move along or near an imaginary flat surface called the ecliptic plane.

Types

Upon launch, a satellite or spacecraft is typically placed in one of several specific orbits around Earth. Alternatively, it may embark on an interplanetary journey, orbiting the Sun until reaching its final destination, such as Mars or Jupiter. Several factors influence the choice of orbit for a satellite, depending on its intended objectives.

• Geostationary (GEO)
• Low Earth (LEO)
• Medium Earth (MEO)
• Polar and Sun-synchronous (SSO)
• Transfer and geostationary transfer (GTO)
• Lagrange points (L-points)

Shape

Orbits come in various shapes, with all of them being elliptical, resembling an ellipse or oval. Planetary orbits typically have a close-to-circular shape, while comet orbits are highly eccentric or “squashed“, resembling thin ellipses rather than circles. Satellites orbiting Earth, including the moon, undergo changes in their distance from Earth over time. The point in a satellite’s orbit closest to Earth is termed its perigee, while the farthest point is known as the apogee.

Similarly, for planets, the point closest to the sun in their orbit is referred to as perihelion, with the farthest point being aphelion. Earth reaches aphelion during the summer in the Northern Hemisphere. The period required for a satellite to complete one full orbit is referred to as its orbital period. For instance, Earth has an orbital period of one year. Additionally, the inclination of an orbit represents the angle the orbital plane makes when compared with Earth’s equator.

How Do Objects Stay in Orbit?

Newton’s first law of motion states that an object in motion will remain in motion unless acted upon by an external force. In the absence of gravity, an Earth-orbiting satellite would continue along its path in a straight line. However, gravity pulls the satellite back toward Earth, resulting in a constant tug-of-war between its momentum and gravitational force. For an object to enter orbit, its momentum must be balanced with the force of gravity. If the object’s forward momentum is excessive, it will pass by without entering orbit.

Conversely, if momentum is inadequate, gravity will pull the object down, causing a collision with the planet. When these forces reach equilibrium, the object continually moves towards the planet while maintaining sideways motion, preventing collision. The speed needed to sustain orbit, known as orbital velocity, is approximately 17,000 miles per hour at an altitude of 150 miles (242 kilometers) above Earth. Satellites at higher orbits experience slower orbital velocities.

Read also: Formation of Planets