- An orbit is the curved path that a spacecraft, planet, moon, star, or other object travels while being pulled by another object’s gravity. The attraction between mass-containing objects in space is caused by gravity. Sometimes they can start to orbit each other if this attraction pulls them together with enough force.
- A satellite or spacecraft is often launched into one of several specific orbits around Earth, or it may be sent on an interplanetary mission, in which case it no longer orbits Earth but instead travels in an orbit around the Sun until it reaches its target, such as Mars or Jupiter.
- Depending on what a satellite is intended to accomplish, the number of considerations determine which orbit is optimal for it. There are different types of orbits which are mentioned below.
- Geostationary Orbit (GEO)
- Low Earth Orbit (LEO)
- Medium Earth Orbit (MEO)
- Polar orbit and Sun-synchronous Orbit (SSO)
- Transfer orbits and geostationary transfer Orbit (GTO)
- Lagrange points (L-points)
1. Geostationary Orbit (GEO)
- Geostationary orbit (GEO) satellites move at precisely the same speed as Earth to complete one full rotation of the planet above the equator in 23 hours 56 minutes and 4 seconds. This gives GEO satellites the appearance of being “stationary” over a specific location. The speed of GEO satellites should be around 3 km per second at an altitude of 35 786 km to properly match the rotation of the Earth. Compared to many satellites, this is far further away from the surface of the planet.
- Satellites that must continuously orbit a specific location on Earth, like telecommunication satellites, employ GEO. In this manner, an antenna on Earth can be fixed and fixedly aimed at that satellite. It can also be utilized by weather monitoring satellites, which can keep an eye on a particular region to see if any weather patterns develop there.
- Three satellites positioned equally apart can give nearly worldwide coverage because GEO satellites have a wide area of coverage on Earth. This is so that it can cover enormous areas at once when a satellite is this far from Earth. This is similar to being able to see more of a map when you are further away from it than when you are closer by a centimeter. Therefore, fewer satellites are required in GEO than at a lower height to view the entire Earth simultaneously.
2. Low Earth Orbit (LEO)
- As the name implies, a low Earth orbit (LEO) is an orbit that is quite close to the surface of the Earth. It is often less than 1000 km above Earth, although it can be as low as 160 km, which is low relative to other orbits but still very high above the planet’s surface.
- Even the lowest LEO is more than ten times higher than that because most commercial aircraft do not fly at altitudes much higher than 14 km.
- LEO satellites do not always require to follow a specific course around Earth in the same way since their plane can be inclined, unlike satellites in GEO that must always circle near the equator. As a result, there are more options for satellite paths in LEO, which is one of the reasons it is such a popular orbit.
- LEO is useful for several reasons because of its proximity to Earth. It is the orbit that satellites most frequently utilize for imaging since being close to the surface enables them to capture photos with a higher resolution. It is also the orbit in which the International Space Station (ISS) is located since astronauts can more easily and more quickly fly to and from it. The speed of satellites in this orbit is around 7.8 km/s; at this speed, a satellite completes one orbit of the Earth in about 90 minutes; as a result, the ISS completes 16 orbits of the Earth each day.
- However, because they move so quickly across the sky and are difficult for ground stations to detect, individual LEO satellites are less valuable for operations like telephony.
- Instead, to provide continual coverage, LEO communications satellites frequently operate as a huge combination or constellation of several spacecraft. These constellations, which include multiple of the same or similar satellites, are occasionally launched together to form a “net” encircling Earth to maximize coverage. This enables them to simultaneously cover a big portion of Earth by cooperating.
3. Medium Earth Orbit (MEO)
- A medium Earth orbit (MEO) is an Earth-centered orbit with an altitude between 2,000 and 35,786 km (1,243 and 22,236 mi) above sea level, which is higher than a low Earth orbit (LEO) but lower than a high Earth orbit (HEO).
- The boundary between MEO and LEO is an arbitrary altitude determined by accepted convention, whereas the boundary between MEO and HEO is the specific altitude of a geosynchronous orbit, in which a satellite circles the Earth in 24 hours, the same period as the Earth’s rotation. The orbital period of all MEO satellites is less than 24 hours, with the minimum period (for a circular orbit at the lowest MEO altitude) being about 2 hours.
- Solar radiation pressure, the dominant non-gravitational perturbing force, perturbs satellites in MEO orbits. Other perturbing forces include the albedo of the Earth, the thrust of navigation antennas, and thermal effects related to heat re-radiation.
- The MEO region includes the Van Allen radiation belts, two zones of energetic charged particles above the equator that can damage satellite electronic systems without special shielding. A medium Earth orbit is also known as a mid-Earth orbit or an intermediate circular orbit (ICO).
4. Polar Orbit and Sun-Synchronous Orbit (SSO)
- Sun-synchronous orbit (SSO) is a type of polar orbit. SSO satellites traveling over polar regions are “synchronous” with the Sun. This means that they are always in the same ‘fixed’ position relative to the Sun. This means that when it crosses the equator, local time remains constant (or any other latitude). It should be noted that this does not imply that it passes through a specific location every day.
- Having a consistent local time of passing over the earth allows researchers to obtain consistent data – for example, if images were collected over several weeks at times ranging from 1:30 pm to 1:30 pm, the land would look very different – and many features would be hidden in the dark.
- Because satellites typically collect reflected light from the earth’s surface that originates from the sun, the conditions must remain consistent throughout each orbit we are looking for changes and comparing data from different orbits. By using clever gravitational techniques, the Synchronous Orbit records this. This orbit has a high inclination (the plane is angled about the equator) and allows the earth to rotate beneath the satellite while keeping the solar illumination angle constant.
5. Transfer Orbits and Geostationary Transfer Orbit (GTO)
- Transfer orbits are a type of orbit that is used to transition from one orbit to another. Satellites are not always placed directly in their final orbit when launched from Earth and carried to space by launch vehicles. Often, satellites are placed in a transfer orbit, which is an orbit in which the satellite or spacecraft can move from one orbit to another using relatively little energy from built-in motors.
- This allows a satellite to reach a high-altitude orbit, such as GEO, without requiring the launch vehicle to travel to this altitude, which would require more effort – it’s like taking a shortcut. This method of reaching GEO is an example of one of the most common transfer orbits, known as the geostationary transfer orbit (GTO).
- Depending on where the satellite is in its orbit, a highly eccentric orbit like this one can quickly take it from very far away to very close to the Earth’s surface. In transfer orbits, the payload uses engines to move from one eccentric orbit to another, putting it on track to higher or lower orbits.
- The Following liftoff, a launch vehicle travels to space along the path depicted in the figure by the yellow line. When the rocket arrives at its destination, it releases the payload, which sends it into an elliptical orbit along the blue line, further away from Earth. The apogee is the point on the blue elliptical orbit that is farthest away from Earth, and the perigee is the closest point.
- When the payload reaches the apogee at 35 786 km GEO altitude, it fires its engines in such a way that it enters and stays in the circular GEO orbit, as shown by the red line in the diagram. The GTO is thus the blue path from the yellow orbit to the red orbit.
6. Lagrange Points (L-points)
- When two massive bodies orbit each other, gravitational forces balance at five points around these bodies. And it is only at these gravitational sweet spots, known as Lagrange points, that a smaller object can maintain equilibrium. As a result, in the Earth-Sun system, a spacecraft or natural object can orbit the Sun while maintaining a position relative to the Sun and the Earth by ‘hovering’ at these Lagrange points.
- When we talk about the Earth-Sun system or the Jupiter-Sun system, we’re referring to how the two bodies interact (i.e., how one orbits the other) and the Lagrange points that go with it.
- L4 and L5 are the only two stable Lagrange points among the five. This means that if a small object at L4 or L5 is nudged, there will be a strong restoring force and it will return to this location. If a small object is nudged at one of the three unstable Lagrange points, L1, L2, or L3, it will break orbit and drift into interplanetary space.
- L1, unstable L1, is located between the Earth and the Sun. It is approximately 1.5 million kilometers (1 million miles) from Earth and provides unobstructed views of the Sun.
- Lagrange point L2, unstable L2 is also 1.5 million kilometers from Earth, but on the opposite side to L1, making it an ideal location for deep space research.
- Lagrange point L3, unstable L3, is located beyond the Sun and is the farthest away from Earth of the Lagrange points.
- L4, stable L4, is 60° ahead of the Earth, i.e., Earth-leading. In two months, Earth will be in L4.
- L5, stable Lagrange point L5 is 60 degrees behind Earth or Earth-trailing. Earth was at L5 two months ago.