Satellites and spacecraft, whether orbiting Earth at altitudes of a few hundred miles or exploring Saturn 886 million miles away, will always enter an orbit around a celestial body at some point during the rendezvous.
In astronautics and astronomy, an orbit refers to the repeating path an object follows around a larger celestial body in Space due to its gravity. In our solar system, the object orbiting a larger body usually is a type of artificial or natural satellite.
Examples of natural satellites include our Moon orbiting the Earth, while artificial satellites are created by humans and are launched into Space and placed in orbit around a planet or other celestial body by launch vehicles like orbital rockets.
Currently (as on January 2022), more than 8 000 artificial satellites, of which approximately 4 852 are active, are orbiting the Earth. Numerous other spacecraft are orbiting neighboring planets in our solar system.
The specific focus of this article is on the different types of orbits that artificial satellites and spacecraft created by humans follow around the Earth and other celestial bodies in our solar system.
Before examining the different orbits, one first needs to get a clearer understanding of what an orbit is, how it is achieved and maintained, and the different characteristics that define a specific orbit.
How A Spacecraft Reaches & Stays In Orbit
As previously mentioned, an orbit refers to the repeating path an object follows around a larger celestial body in Space due to its gravity. Depending on the type of orbit, the object will travel at different altitudes and speeds and also follow different paths around a planet.
To reach orbit, a satellite or any other object is launched on top of an orbital rocket capable of accelerating through our thick atmosphere and gaining enough velocity and altitude to reach Space and place the satellite in orbit above the planet’s surface.
To stay in orbit, a satellite must travel parallel to the planet at a high enough speed not to fall back to the surface but also be close enough to the planet to continue experiencing and be influenced by its gravity. The illustration below demonstrates how this works in practice:
In the above illustration, the image on the left shows how a satellite reaches orbit. The rocket launches vertically to allow the vehicle to travel as quickly as possible through the thickest part of the atmosphere, where it experiences the most air resistance.
Shortly after launch, the rocket starts to turn horizontally and perform a gravity turn to gain speed while still ascending, a process that the vehicle’s second/upper stage continues to perform after the first stage is spent and separated from the rest of the craft.
(Learn more about what a gravity turn is and why orbital rockets need to perform them to reach orbit in this in-depth article.)
The upper stage continues to accelerate and turn until it travels parallel to the Earth’s surface. Upon reaching orbital velocity, the spacecraft/satellite is released into Low Earth Orbit, where it stays or continues to another orbit or travels further into the solar system.
To stay in orbit, though, as illustrated in the image on the right, a satellite must travel both fast enough and still be under the influence of the planet’s gravity. Essentially, a satellite “falls” around the Earth, with its forward speed preventing it from falling to the surface.
If it travels too fast at a certain altitude, it will break free from the gravitational pull and fly off into Space. However, if the planet’s gravitational force is too strong and the vehicle’s speed is too low, the satellite will reenter the atmosphere and burn up or impact the surface.
The speed at which a satellite must travel to stay in orbit is primarily determined by the altitude at which a satellite orbits a planet and its eccentricity (which will be explained in the next section). In summary, the type of orbit determines the speed of a satellite.
Characteristics Of An Orbit
Each of the different orbits that satellites/spacecraft follow around a planet or other celestial bodies have their unique characteristics that define it. Some of the most notable characteristics that define a specific orbit include:
- Speed
- Altitude
- Inclination
- Eccentricity
1) Speed
To stay in orbit, a spacecraft needs to travel at high velocities parallel to the Earth. The speed at which it needs to travel is primarily determined by its altitude above a planet’s surface. Generally, the closer the orbit is to the surface, the faster it needs to travel.
For example, the International Space Station (ISS), at an altitude of 400 km (250 miles), needs to orbit the Earth at 28 000 km/h (17 500 mph) to stay in orbit. The Moon, at a distance of 384 000 km (238 855 miles), orbits the Earth at 3 683 km/h (2 288 mph).
2) Altitude
As the upcoming section will illustrate, the altitude at which a spacecraft orbits a planet is used to classify the different orbit types. The altitude above the planet’s surface is critical, as different heights above the surface have distinct advantages and disadvantages.
A satellite in Low Earth Orbit can observe and study detailed information about smaller regions and cross multiple areas several times a day, while a satellite in High Earth Orbit provides a much broader but less detailed view of the planet’s surface and atmosphere.
3) Inclination
Inclination refers to the specific direction a satellite follows around a planet and is measured in degrees as it deviates from a planet’s equator. (Most satellites follow the same eastward direction a planet rotates as it orbits the Sun to benefit from the planet’s rotational speed).
For example, a satellite flying eastward directly above the equator (known as a prograde orbit) has an inclination of zero. A satellite flying westward directly above the equator (known as a retrograde orbit) has an inclination of 180 degrees.
Similarly, a satellite following a polar orbit (orbiting between the north and south poles of a planet) will have an inclination of 90 degrees since it travels at a 90° angle to the equator.
Like altitude, a satellite’s inclination also defines the type of orbit it follows, which primarily depends on the specific purpose/mission of the spacecraft.
4) Eccentricity
The orbit of a spacecraft around a planet also has a specific shape known as its eccentricity. It varies between 0 and 1, with zero indicating a perfectly circular orbit. The higher eccentric the orbit becomes, the closer to 1 the value becomes.
In an orbit that is not perfectly circular (does not have an eccentricity of 0), the closest point to the planet’s surface that a satellite will reach is known as its periapsis, while the furthest point away from the surface is known as the apoapsis.
Types Of Orbits Around Earth
Although the planets and moons in our solar system have very different characteristics, they have a similar influence on objects orbiting them. As a result, using the paths satellites follow around the Earth is ideal for illustrating the different types of orbits spacecraft follow.
NASA primarily classify the different orbits satellites, or spacecraft, follow around the Earth into three categories according to their height above the planet’s surface or altitude. All orbits fall within one of these categories:
- Low Earth Orbit (LEO)
- Polar Orbit
- Sun-Synchronous Orbit
- Medium Earth Orbit (MEO)
- Semi-Synchronous Orbit
- Molniya Orbit
- High Earth Orbit
- Geosynchronous Orbit
- Geostationary Orbit
- Lagrange Points
1) Low Earth Orbit (LEO)
The vast majority of satellites and spacecraft launched, more than 72%, are put into a Low Earth Orbit. The orbit’s close proximity to the planet’s surface makes it possible to make detailed observations of the planet’s atmosphere, surface, and oceans.
A spacecraft’s orbit is typically classified as a Low Earth Orbit when it travels at an altitude of 180 – 2 000 km (112 – 1 243 miles) above the Earth’s surface. To achieve and stay in Low Earth Orbit, it has to maintain a speed of approximately 28 000 km/h (17 500 mph).
Some satellites in Low Earth Orbit follow the same eastward rotation of the planet they orbit but are not restricted to this path like a satellite in geostationary orbit, for example. They orbit a planet in several different directions, so their inclination can vary significantly.
For example, the International Space Station (ISS), at an altitude of 420 km (261 miles), orbits the planet at an inclination of 51.6°. This direction is to accommodate both the United States and Russia, who are the primary partners responsible for operating the station.
(The most dangerous part of a rocket’s flight is its accent, where it can fail and fall on populated regions downrange. This inclination allows Russian rockets launched from Baikonur Cosmodrome to safely reach Space before crossing into populated China.)
Polar Orbit
Another good example of an orbit that deviates substantially from the standard eastward direction many satellites travel in is the polar orbit. A spacecraft in this orbit has an inclination of 90°, which allows it to orbit around the north and south poles of the planet.
Since it can’t make use of the Earth’s rotation for a “speed assist” in an eastward direction, a launch vehicle must produce all the energy by itself to put its payload into a polar orbit, which also requires more fuel.
In this orbit, a satellite orbits the planet every 99 minutes, with one-half spent in daylight and the other half in darkness as the craft crosses a pole and moves into the part of the planet not illuminated by the Sun.
As a satellite follows a polar orbit around the Earth, the planet slowly rotates underneath it. As a result, after a 24-hour period, a satellite would have covered most of the planet’s surface twice, once in daylight and once on the side not illuminated.
These characteristics of a polar orbit make it ideal for surveillance satellites, as well as scientific spacecraft mapping the surface, which need constantly updated and detailed views of multiple regions throughout the day.
Sun-Synchronous Orbit
A sun-synchronous orbit is a specific type of polar orbit. A satellite still orbits both poles of the planet, but its orbit is synchronized in such a way that it remains in a “fixed” location relative to the Sun.
It simply means that a satellite will pass over the same location on the equator every day at exactly the same local time. The process can be better understood by using the illustration below (courtesy of NASA/Robert Simmon).
The image above illustrates a sun-synchronous orbit. As demonstrated, a satellite passes over the same location on the equator at 1:30 pm every day.
Approximately 99 minutes after the first pass, the satellite passes over the equator again but “synchronized to the sun’s location,” it passes slightly to the west over a location where the local time is also 1:30 pm. The same process repeats on the third pass.
As a result, no matter where it crosses the equator, the local time will always be 1:30 pm as the orbit moves in sync with the Sun.
This type of orbit is especially beneficial for observing and monitoring long-term changes to a specific region by passing over them at the same time every day.
2) Medium Earth Orbit (MEO)
The region between low and High Earth Orbit is known as Medium Earth Orbit and provides a slightly broader coverage of the planet’s surface than spacecraft in Low Earth Orbit. It also travels at lower speeds, extending the time it takes to complete an orbit.
A spacecraft’s orbit is typically classified as a Medium Earth Orbit when it travels at an altitude of 2 000 – 35 780 km (1 243 – 22 233 miles) above the Earth’s surface.
This orbit makes it ideal for certain types of spacecraft like Global Positioning System (GPS) satellites, but also communication satellites (specifically in the Northern Hemisphere). The two primary orbits at this altitude are the semi-synchronous and Molniya orbits.
Semi-Synchronous Orbit
A semi-synchronous orbit is a very stable and predictable orbit where satellites travel around the planet at an altitude of approximately 22 200 km (13 794 miles) in a close-to-circular orbit. It takes 12 hours to complete an orbit, allowing it to orbit the same location twice daily.
This stable orbit makes it ideal for navigational satellites like the Galileo Constellation, which consists of 24 satellites and is operated by the European Union Agency For The Space Programme (EUSPA). It serves as an alternative to the American GPS navigational system.
Molniya Orbit
The Molniya Orbit is a highly eccentric orbit primarily used by Russian satellites to provide communication capabilities over high latitudes. Geostationary satellites orbit the equator, providing little coverage for countries at these high latitudes in the Northern Hemisphere.
This orbit has an inclination of 63.4° and, like the semi-synchronous orbit, it takes 12 hours to complete one orbit, allowing satellites to pass over the same location twice daily.
Its highly elliptical shape allows spacecraft to spend two-thirds of the time over the Northern Hemisphere since it speeds up as it passes over the Southern Hemisphere at its closest approach to the planet and slows down as it passes back over the Northern Hemisphere.
3) High Earth Orbit
At an altitude of 35 000 km (21 748 miles) or greater, satellites get a broad few of the whole planet & also orbit at greatly reduced speeds or stay in a fixed position relative to a specific location on Earth. This provides unique advantages in performing specific tasks.
A spacecraft’s orbit is typically classified as a High Earth Orbit when it travels at an altitude of 35 780 km (22 233 miles) or higher above the Earth’s surface. Also, as the altitude of an object orbiting above the Earth’s surface increases, its speed decreases.
For example, at an altitude of 35 780 km (22 233 miles), a satellite in a geostationary orbit travels at a speed of approximately 11 000 km/h (6 835 mph). The Moon, at a much higher altitude of 384 400 km (238 855 miles), orbits the Earth at roughly 3 700 km/h (2 300 mph).
The most notable High Earth Orbits are geosynchronous and geostationary orbits, as well as Lagrange Points.
Geosynchronous Orbit
At an altitude of 35 786 km (22 236 miles), a satellite in geosynchronous orbit takes 23 hours, 56 minutes, and 4 seconds to orbit the Earth, which matches the planet’s rotation. As a result, it remains in a fixed position when viewed from any location on the planet.
Although its longitude remains constant throughout the day, a spacecraft on this orbit may meander to the north or south but will always return to the same location above the planet’s surface at the same time of day.
Geostationary Orbit
A special kind of geosynchronous orbit is known as a geostationary orbit. It orbits the planet at the same altitude and speed as a “standard” geosynchronous orbit, but unlike the former, it remains in exactly the same location relative to the Earth’s surface throughout the day.
It orbits Earth directly above the Equator with an inclination of 0°. A spacecraft needs to make the occasional orbital adjustment (known as stationkeeping), typically by firing its Reaction Control System (RCS) thrusters to maintain this precise path around the planet.
(Learn more about Reaction Reaction Control Systems, what they are and how spacecraft use them for small orbital adjustments and performing other maneuvers in this article.)
It also follows a perfectly circular orbit around the planet, meaning both its inclination and eccentricity of 0, which gives this type of orbit certain advantages.
This position is ideal for communication satellites since stations on the planet’s surface only need to point their antennas (satellite dishes) to one location and are able to keep in constant communication with the satellite without making any directional adjustments.
This position is also ideal for weather satellites that need a constant broad view of the planet’s surface to monitor and track any changes in atmospheric conditions like the development and movement of hurricanes and other large-scale weather phenomena.
As a result, more than 20% of all active satellites orbiting the Earth can be found in a geostationary orbit. It also explains why a High Earth Orbit is commonly referred to as a geostationary orbit.
Lagrange Points
More than a million kilometers (621 371 miles) away from Earth are five unique locations in Space where Earth’s gravity cancels out the gravity of the Sun, allowing spacecraft to revolve around the Sun with the Earth at these points. They are known as Lagrange points.
The illustration below will help to provide a better understanding of how each of the five Lagrange points works.
As the illustration above shows, the five Lagrange points stay in a fixed position relative to the Earth as it orbits the Sun. Although the 5 points do not directly orbit the Earth, they orbit the Sun while remaining in “the same position” when viewed from any location on Earth.
The first three points (L1, L2, L3) are not considered stable. Like a ball balanced on top of a pyramid, the smallest disturbance will cause it to roll down one side. As a result, the occasional orbital correction needs to be performed to keep a spacecraft in place.
The last two points (L4, L5) are considered stable. Like a bolder in a valley between two steep hills, a disturbance may cause it to move up against a hill, but it will always roll back to the bottom of the valley. As a result, a spacecraft will always return to the same position.
Depending on the type of observation required or the specific purpose of a spacecraft, a specific Lagrange point can be utilized. One exception is Lagrange point 3 (L3), which lies directly opposite the Sun, making communication with Earth stations impossible.
Lagrange point 1 (L1) is ideal for observing solar activity since it lies between the Earth and the Sun. This is why the joint NASA/ESA Solar And Heliospheric Observatory satellite is placed in this orbit to observe and monitor the Sun’s behavior.
Lagrange point 2 (L2) is ideal for studying the universe since it is behind the Earth, and the Earth & Sun are in line with each other. It means spacecraft must only be shielded on one side from both celestial bodies. The James Webb Space Telescope makes use of this orbit.
Conclusion
At any given time, Earth is surrounded by thousands of artificial (human-made) satellites. These spacecraft orbit the Earth at different altitudes, directions (inclinations) & speeds and also have different shapes (eccentricities).
As illustrated in this article, each orbit has its unique characteristics, providing certain advantages and drawbacks. Depending on the purpose or mission, a spacecraft can be placed in a specific orbit around a celestial body that allows it to perform its task optimally.
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