On the basis of the distance from Earth, the types of orbits are classified into low earth orbit, medium earth orbit, geostationary orbit, and high earth orbit. Each of these orbits serves specific applications concerning coverage area, cost, and purpose. We will take a look at all of the orbits mentioned above and understand the purpose they serve due to their unique vantage points.
A lot of reference materials and even our own featured image for this post show LEO, MEO orbits to be circular. That can be a bit misleading because all of these orbits, except geostationary, can be elliptical too. But in their defense, it is easier to represent diagrammatically and avoids clutter. The prime objective is to show the variation in altitude.
An elliptical orbit is particularly useful when service providers wish to relay their services over a particular region for a longer period. From Kepler’s second law, the elliptical orbit would mean that the satellite spends a longer time at its apogee (furthest point from Earth) in contrast to its perigee. So for example, if some satellite owners have a large user group in Russia, they would make sure that the apogee of their satellite’s orbit is somewhere over Russia.
Low Earth Orbit
The low earth orbit is the most populous and most accessible realm of all. More than 800 satellites are currently in orbit in the Low-Earth region. The most popular of these is the International Space Station and the Iridium network of communication satellites.
Altitude range of the Low earth orbit
The low earth orbit extends from 160km above Earth and ends at 2000km. It is not possible to achieve an orbit below 160km without artificial thrusters due to the atmospheric drag at that altitude. The mean orbital velocity of any satellite that needs to reach an LEO should be 7.5km/s (27,000km/h). This value decreases as the altitude increases. The relatively low altitude of satellites in this orbit offers some attractive perks for satellite manufacturers and hobbyists.
Orbital Period of the Low earth orbit
Satellites in the low earth orbit have an orbital period in the range of 90-120 minutes.
Advantages and disadvantages of the Low earth orbit
The low altitude of the Low earth orbit is apt for communication and imaging satellites. Due to the altitude, communication signals require less power and time to travel between the Earth stations and the satellites. Also, imaging satellites can capture very detailed pictures. Satellites for the Low-Earth orbit are easier to build and can be less expensive than their counterparts in higher orbits. It is a very popular orbital choice for hobbyists who wish to launch CubeSats. CubeSats are incredibly tiny satellites used for small amounts of data collection and other experiments.
However, the popularity and ease of launching a satellite into this orbit have inadvertently contributed to the issue of space debris. In fact, the International Space Station uses layers of shields to protect itself from space debris. Satellites in this orbit have to deal with atmospheric drag. They usually have a shorter lifespan than geostationary satellites.
Medium Earth Orbit
The Medium Earth orbit is also known as the Intermediate Circular Orbit.
All orbits above low earth orbit and below geostationary orbit are said to be in medium-earth orbit. To be precise, MEO extends from 2000km and ends right below 35,786km.
Satellites in this region have an orbital period ranging from 2-24 hours.
Advantages and disadvantages
The medium earth orbit is very useful in providing connectivity and navigation to the polar regions. An orbital period of 12 hours can be achieved by satellites in this region. That is a unique period that allows these satellites to orbit Earth twice in one day. The purpose of most of the satellites in the MEO are communication, navigation, and to provide a gravity-less environment for scientific experiments.
Nevertheless, this orbit presents its fair share of disadvantages too. With the increase in altitude compared to LEO, propagation delay will begin to creep into the transmission of signals. The power required to transmit these signals will also increase. Thus, satellites in this region are a bit expensive as compared to satellites in the low-earth orbit. Similar to the LEO, satellites in this orbit have to consider atmospheric drag too. The interference from the upper layers of the atmosphere reduces the lifespan of these satellites in comparison with satellites in the GEO. This orbit also put satellites in proximity to the Van Allen radiation belts. These belts are regions of highly charged particles maintained by Earth’s magnetic fields. The charged particles in this region can hurt the performance of satellites.
As we’ve seen earlier, the geostationary orbit is a unique area in the space around our planet. It is located at a very precise altitude and has a circular orbit. Additionally, this orbit is always circular. Satellites in this orbit appear stationary from Earth. Clarke’s Orbit is another name for the GEO.
All satellites in this orbit have an orbital period equal to Earth’s rotational period i.e. 24 hours.
Advantages and disadvantages
A major benefit of this orbit is the fact that Earth stations can be stationary. They don’t have to track satellites continuously. Moreover, the coverage area from this altitude is pretty good.
The cost of launching a satellite into GEO is high. Due to the increase in distance from Earth stations, there is a considerable lag in communication. To traverse the large gap, signals require more power. This in turn further increases the cost of a satellite.
High Earth Orbit
Any orbit beyond the geostationary orbit is known as high earth orbit. High earth orbit is loosely attributed to any orbit beyond 35,786km. High Earth Orbit has the same abbreviation as Highly Elliptical Orbit. These two orbits may not necessarily be the same. A highly elliptical orbit may be considered a high earth orbit at its apogee.
Naturally, satellites in this orbit have an orbital period longer than twenty-four hours. Due to this, all satellites in this orbit appear to be retrograde, even if they have prograde orbits. Recall that retrograde means moving backward. Since Earth rotates at a faster speed, satellites in HEO appear to be heading in the opposite direction.
Advantages and disadvantages
Satellites in the HEO are useful to study our planet’s magnetosphere and for other astronomical observations. These satellites face less exposure to atmospheric drag as compared to satellites in the LE/ME orbits. Communication delays and high costs of manufacturing and launching a satellite into this orbit are some of the areas of concern.
To summarize, each of this classification is purely on the basis of altitude from the surface of Earth. In the next post, we will take a look at some unique types of orbits (like the Molniya orbit) from some of the categories mentioned above.