Orbit
An orbit is a path taken by an object in space such as a planet, moon, asteroid, or satellite around another object due to gravity. Gravity causes objects with mass to be attracted to nearby objects. Smaller objects in space orbit around larger objects, while objects having similar mass orbit each other, with neither object at the center.
Types of Orbit
There are different types of orbits where the satellites or spacecraft are placed around the Earth once they are launched. Certain satellites or spacecraft are sent on an interplanetary journey orbiting the Sun until their destination. Depending on the purpose of the satellite or spacecraft, they are placed in one of the orbits.
The different types of orbits are:
Geosynchronous Equatorial Orbit (GEO)
Low Earth Orbit (LEO)
Medium Earth Orbit (MEO)
Polar orbit and Sun-synchronous orbit (SSO)
Transfer orbits and geostationary transfer orbit (GTO)
High Elliptical Orbit (HEO)
Figure 1. Different types of orbits.
Geosynchronous Equatorial Orbit (GEO)
Satellites in geosynchronous equatorial orbit are also known as geostationary satellites since they circle the Earth above the equator at the same velocity as the Earth and in a direction similar to Earth's rotation.
The satellite orbits at an altitude of 35,000 km, and it takes 23 hours 56 minutes, and 4 seconds to rotate the Earth, traveling at the same rate similar to the Earth. This makes the GEO satellites appear to be stationary at a fixed position in the sky for the ground observers.
Figure 2. Geostationary Satellite Orbit
Geostationary satellites are far from the Earth than any other satellites in an orbit, providing a large coverage range. Therefore, as few as three geostationary satellites can offer coverage globally. So only fewer satellites are needed in GEO than in LEO, but their coverage is limited to specific geographic regions.
Certain satellites need to stay constantly above one particular area over the Earth, such as telecommunication satellites. These satellites use geostationary orbits, which allow fixed communication links to be established from the ground to the satellite. Weather monitoring satellites are also placed in GEO to observe weather over a large coverage area.
Low Earth Orbit (LEO)
A low Earth orbit (LEO), as the name implies, is an orbit that is relatively close to Earth's surface. LEO satellites are placed at the lowest altitude among all satellite types, from an altitude of less than 1,600 km to as low as 160 km above Earth.
The coverage area of LEO satellites is small as they are close to the Earth's surface. Many LEO satellites are required in order to provide coverage globally, known as LEO constellations. Environmental conditions in GEO are less challenging compared to LEO satellites, which operate in harsh environments such as radiation and atmospheric drag.
LEO has become one of the most commonly used orbits by satellites due to various reasons. As LEO satellites are closer to the Earth, it is used in producing high-resolution images of the Earth's surface. International Space Station (ISS) is placed in LEO as it is easier to access space and for any space-related activities.
LEO satellites travel at a faster speed, which takes approximately 90 minutes to circle the Earth. They are also typically smaller and less expensive than GEO satellites. Though LEO is mostly used nowadays, individual LEO satellites are less efficient for certain applications as they move faster across the sky.
Sources:
The European Space Agency, Types of Orbits, 2020.
Gary Quinsac, Space Project
Society of Physics Students @ Duke