Space Terms 2: LEO, MEO, GEO, and HEO
Austin Morris, Director of Engineering
6 minute read
Space is big. As Douglas Adams put it so eloquently in The Hitchhiker’s Guide to the Galaxy, space is really, really big. So let’s not talk about all of space right now, let’s just talk about the orbit of the Earth, which is still big. Let’s break it down and try to understand the different sections of Earth orbit and how they all play into the way that a satellite orbits.
One of the important things to keep in mind when thinking about orbit and orbital paths is altitude. Altitude is something that a lot of people are familiar with, especially those who have any experience with flying, amateur rocketry, or an acute fear of heights. All three, in my case. To put it simply, altitude is essentially the height of something relative to a planetary reference plane, such as the ground directly beneath it. So you might say that someone balancing on the very top of the Eiffel Tower is at an altitude of 1,063 feet. Now, most people wouldn’t say that, as it’s unnecessarily specific, but it is otherwise not terribly inaccurate. It is important to remember that most measurements, especially heights, are relative to something, as mentioned before. Frequently when someone mentions altitude they are referring to the altitude of something above sea level, rather than the ground. This is how Denver can be the Mile High City, despite the fact that it isn’t floating thousands of feet above the ground. So when we talk about altitude in this piece, let’s assume that we are talking relative to sea level.
The reason that it’s important for us to define altitude at this point is to talk about the different orbital classifications for geocentric orbits, which are orbits around the Earth. The first is Low Earth Orbit, or LEO, that covers altitudes below 2,000 kilometers (1,240 miles). LEO is where the International Space Station (ISS) orbits, around 400km (250mi). This is a comfortable height that allows other objects, like natural satellites (think meteorites and meteoroids) and man-made orbital debris to naturally decay from their orbits, while the ISS boosts itself up every now and then with onboard thrusters to maintain its orbit. The reason for this is that it provides a safer environment for the ISS and the astronauts aboard, knowing that other objects at the same altitude will not stay at the same altitude and will continue to fall to Earth, posing less risk than if they were to continue passing by the ISS repeatedly.
Not all objects in LEO are in decaying orbits. In other words, past a certain altitude, the effects of the atmosphere are no longer felt by objects to a noteworthy degree, and thus they remain in orbit for effectively forever. The thing is, the atmosphere doesn’t just suddenly stop, like stepping through a door. The atmosphere fades out, becoming less and less dense as altitude increases, eventually reaching a point where it has become virtually nonexistent. For this reason, orbiting objects at lower altitudes tend to lose speed due to air friction and eventually descend back into the atmosphere, most often burning up and disintegrating before ever being a concern on the ground. Objects less than 200km in altitude tend to fall within a matter of days, while objects above 200km and below 400km may decay in a matter of weeks or months, even up to a year. As the altitude increases, so does the time to decay. Closer to 800km is where objects may stay in orbit for decades, and above 800km the effect of the atmosphere becomes typically negligible. As such, the remainder of LEO, up to 2,000km, is oftentimes used for objects that the operators are not interested in frequently boosting back up.
As we continue to increase in altitude, the next stop is Medium Earth Orbit, or MEO. MEO begins where LEO ends, at 2,000km in altitude, and extends quite a ways further, up to nearly 36,000km. MEO is used for many communications and navigation satellites, including things like the Global Positioning System (GPS), Global Navigation Satellite System (GLONASS), and the Galileo navigation system. Many of these systems exist in the form of satellite constellations, which is a term that is becoming increasingly well-known with the advent of smallsat constellations like Starlink (but that’s a story for another time). The most commonly used altitude within MEO is around 20,200km, because that particular orbit allows for an orbital period of 12 hours. This means that an object at this altitude will complete a full revolution around the Earth and return to its starting point once every 12 hours. This can be convenient in several ways, and is one of the reasons that the GPS constellation has orbited at this altitude for over 40 years.
We reach the actual end of MEO a bit before 36,000km, at precisely 35,786km, at which point we have now entered Geosynchronous Orbit, also called GSO. GSO begins at this specific altitude for a specific reason: at 35,786km, objects have an orbital period of 24 hours, meaning that they will return to the same point in the sky at the same time every day. It is important to mention at this point that there is also such a thing as Geostationary Orbit, or GEO, and that this term is frequently used to refer to both Geosynchronous and Geostationary, although they are not exactly one and the same. Geostationary orbit refers to a special kind of geosynchronous orbit in which an object orbits circularly around the Earth on the same plane as the equator. As such, an object in a true geostationary orbit will appear, to an observer on the surface of the Earth, to remain permanently at the same point in the sky.
In practice, of course, nothing is quite truly perfect. Satellites are no exception to this, which means that objects in geostationary orbit are better considered to be only approximately geostationary, as they have to make occasional adjustments to their orbit in order to account for slight variations and drift in their orbit. Nevertheless, maintaining a geostationary orbit provides many advantages, including the fact that ground stations need not track across the sky to stay in contact with this satellite and instead can just continue pointing at the same dot in the sky all the time.
The parting gift I give you today is the esoteric and infrequently-discussed High Earth Orbit, or HEO. HEO refers to a geocentric orbit at an altitude higher than that of the Geosynchronous range, thus having an orbital period greater than 24 hours. The most well-known object in HEO is our Moon, which takes 27 days to orbit the Earth.
The thing I find most interesting about objects in HEO is that, because they travel at a rate slower than the rotational speed of the Earth, they lie to us a little bit in telling us which direction they are going. A satellite in HEO may be orbiting the Earth in the same direction as the Earth’s rotation, moving eastward across the globe, but because it does so slower than the rotation, the point on the surface directly beneath it instead seems to move westward. I think we have all, at times, felt like an object orbiting at this altitude, working very hard and moving very fast, and yet seeming to be making only backward progress. At those times, never lose sight of the hard work and effort that it has taken to get you to where you are at that moment.
Recommended column to read next: Space Terms: A Beginner's Guide to Jargon