Early Greek astronomers pictured the universe resembling an onion. At the center was Earth. Surrounding it, like onion layers, were successive layers of the universe, extending out past the planets to the outermost layer that contained the distant stars.
Copernicus and Galileo helped to replace this early “geocentric” model of the universe with a better one in which planets orbited the sun — the heliocentric model. People today scoff at the old onion-layer universe, but this visualization has made a comeback when it comes to satellites and other space-bound tech. The types of orbits that our satellites are launched into form successive layers surrounding Earth.
Layers of the Sky
The number of possible types of orbits that spacecraft can be launched into is unlimited, since even tiny differences in speed, direction, and angle of launch will carry a spacecraft into different orbits. And all these possible orbits have distinct properties that determine what missions a spacecraft in that orbit can perform — and how difficult it is to launch a vehicle into that orbit.
For example, most space launches are “eastbound” — they use the Earth’s rotation to provide an extra little boost of speed on their way to orbit. But some satellites are launched on a nearly north-south trajectory, allowing them to pass over the polar regions and to survey different parts of the Earth on every orbit.
But the most important distinction among types of orbits is how high (or distant) an orbit is, because this determines how fast the spacecraft must go to enter that orbit and, thus, how much rocket power it will need to get there.
For this purpose, NASA classifies spacecraft and satellite orbits into three successive layers: low Earth orbit (often shortened to LEO), medium Earth orbit and high Earth orbit. Other international space agencies, such as the European Space Agency use this same simple terminology.
Low Earth orbit, according to CSIS, includes orbits extending from just above the atmosphere, at an altitude of about 100 miles or 160 kilometers, out to about 2,000 kilometers. Satellites and spacecraft in LEO take about 90 minutes to two hours to make one full orbit. These orbits “only” call for reaching a speed of some 8 kilometers per second (around 5 miles a second, or 18,000 mph).
Low Earth orbit offers several advantages for a spacecraft. If you want to look back at Earth, you have a closer view from low orbit. Earth’s Van Allen belts also protect you here from deep-space radiation. In part, because of this protection factor, all human spaceflights since the Apollo missions of the 1960s and 1970s have stayed in low Earth orbit, and this is where the International Space Station orbits. But the biggest advantage of LEO is that it is easier to reach than other types of orbits. Thus, more than half of all active satellites are in low orbits.
The View From on High
At the other end of the range of orbit types is high Earth orbit— orbits at or beyond what NASA calls the “sweet spot” of 42,162 kilometers from the center of the Earth, or about 23,000 miles above ground level.
At this level, as Space.com explains, a satellite orbiting directly above the equator is in a geosynchronous Earth orbit (often abbreviated to GEO). It takes a satellite in GEO about 24 hours to orbit the Earth once. Since this is the length of one day, a satellite in this orbit appears to hang in the sky directly above one spot on the equator.
This is extremely convenient for some technologies, such as satellite dishes. They don’t have to slew around to follow a satellite; they can just point to one spot in the sky where the satellite will always be.
Orbits that are variations on the geosynchronous theme can also be very handy. If a satellite is placed in an inclined orbit at this same altitude, observers on the ground will see it drift north and south in the sky each day, following what looks like a figure-eight pattern, but it will never drift east or west far enough to drop below the horizon.
GEO and its variations are so useful that high Earth orbit accounts for about a third of all satellites, and the great majority of satellites not in low Earth orbit. Some research satellites are in even more distant high Earth orbits, often to minimize interference from Earth-based sources of radio noise.
Caught in the Middle?
Between LEO and high Earth orbit (including GEO and its cousins) is the domain called medium Earth orbit. While these orbits are less frequently used than either low Earth orbits or high Earth orbits (which between them account for more than 90% of all satellites), these intermediate orbits are useful for some applications.
For example, satellites in a circular orbit at 26,560 kilometers from the center of Earth have an orbital period of exactly 12 hours, making them easy to track and locate. Global Positioning System (GPS) satellites, which are used to determine precise locations on Earth, use orbits of this type. Another medium orbit, called by the Russian name Molniya, is particularly well suited for communicating with locations at high latitudes on Earth, which GEO satellites above the equator may be unable to reach.
Today these types of orbits are mainly of interest to specialists, while human spaceflight is — for now — confined to low Earth orbit, or suborbital missions that pop up briefly into space but never reach orbital speed. But in the decades to come, these onion-layer levels of orbits around Earth will become far more familiar — perhaps the equivalent of postal codes, identifying regions that people visit, or even their home addresses.
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