Orbital altitudes, are some better than others and why?

Question

The geostationary orbit of about 35,786 km above mean sea-level allows satellites to rotate at the same speed as the Earth on its axis, making them seem as if they don't move relative to the ground.

Do any other orbital altitudes have particular advantages, for example, why was the orbital height of 431 km chosen for the International Space Station? Why are satellites placed in certain orbits? To limit the scope of this question, I am not really talking about inclination (e.g. polar, retrograde, prograde, etc.).

If any particular orbital altitudes have technical names, that would be a bonus.

Answer 1

First, lets talk about simple (near to) circular orbits. Usually, you can divide them into Low, Medium and High.

All geocentric orbits with an altitude up to 2,000km (1,240mi) are classified as a Low Earth Orbit (LEO). Any object that is supposed to stay in space for a longer period of time (such as satellites or the ISS) usually has an altitude of >300km, since orbits below that altitude would be impractical due to atmospheric drag in the Thermosphere, causing orbital decay.

Although the altitude of the ISS is higher than that, it still loses ~50-100m a day and has to do orbital boost burns every so often.

LEOs are better than higher-altitude orbits because they require less Delta-V to reach, so it is cheaper to build space stations there. They are worse because you can see objects in LEO only from a small portion of the Earth's surface.

(Geocentric) orbits above 2,000km to just below geosynchronous orbits (35,786km) are classified as Medium Earth Orbits (MEO), "most commonly at 20,200 km or 20,650, with an orbital period of 12 hours", as used by the GPS satellites.

MEOs are good because with an orbital period of 12 hours, you can easily calculate when the satellite is going to pass over you and you still have a reasonable viewing angle.

(Geocentric) orbits above 35,786km are classified as High Earth Orbits (not HEO!). Weather and communications satellites tend to be in High Earth Orbits because they can "see" a large portion of the earths surface from this altitude.

Some examples for interesting elliptic orbits are the Tundra and the Molniya orbit.

Geosynchronous orbits obviously only work when placed above the equator. If you want to have a satellite stationary over very high latitudes, they don't work.

So instead you use one of the two orbits. They are highly elliptical and have an inclination of ~63.4°. The high eccentricity means that a satellite has a long apogee dwell, so it remains around a certain point on the surface for most of its orbit, then quickly swoops around the earth and comes into vision again after a short time.

The Tundra orbit has a perigee of 1,000km and an apogee of 70,580, giving it an orbital period of 24 hours. The only current user of Tundra orbits is the Sirius Satellite Radio. It uses a constellation of three satellites, with which it can provide all of North America with its stream for 24 hours a day.

The Molniya orbit has a perigee of 1,000km and an apogee of ~40,000, giving it an orbital period of 12 hours. They were primarily used by the Molniya communications satellites and some spy satellites during the Cold War.

Another point to consider is that orbits intended for manned spacecraft shouldn't cross the Van Allen radiation belt (although that will be hardly avoidable when going to the moon), so that excludes orbits of around 1,000-60,000km from the list for manned craft.

Answer 2

Different orbits allow for different benefits. For example lower orbital altitudes have a lower space debris population (or at least guarantee debris will de-orbit faster), also specific inclination and altitude combinations allow for sun-synchronisation, so you can perform Earth observations and guarantee the sun is is the same relative position with every pass.

If you want to get a general idea of which altitudes have specific benefits you could look at space debris populations or just look at where things are right now in the satcat.

The ISS as you specifically asked about was placed in the orbit because it minimises risk from space debris without having such significant drag that the Fuel requirements for station keeping are too high!

Answer 3

So let's talk about geostationary first.

As you pointed out, geostationary altitudes let the satellites 'hover' over a certain area of the Earth, this is useful for weather monitoring, communications (a single satellite can be dedicated to an entire continent, permanently), and probably other things I haven't thought of. One disadvantage though, is signal latency. Some satellite internet providers have satellites at this altitude, and while they provide good enough coverage for doing something like reading StackExchange, forget about trying to play multiplayer games.

So there's two considerations of altitude, coverage and lag. The higher your altitude, the higher your coverage (meaning less total satellites needed to cover the globe, if that's your intent), but it comes at the cost of increased lag.

GPS satellites are a good example of high-altitude satellites that aren't geostationary. Their high altitude (20000 km) allows for a reduced number of satellites, but they're also low enough that signal lag is acceptable. If they were to orbit much lower, signal latency would be improved, but more satellites would be required, which would be more expensive.

As you get lower, photo resolution would improve, which is obviously going to be very important for spy satellites and other imaging satellites. One example is the KH-11 series of satellites, which orbited in 300x1000km orbits.

As you get even lower, different factors come into play. As FraserOfSmeg noted, the lower altitude orbits (i.e. a couple hundred kilometers) will decay faster. Sometimes that's good. If you want to launch a satellite that only requires a few weeks of testing, if you launch it into a low orbit you'll have time to test it, and then it will fall back into the atmosphere, reducing the problem of space debris.

Another important consideration of orbital altitude is if you want to go somewhere multiple times, like Hubble or ISS. The higher the orbit, the harder it is to get there, and the less stuff (science, food, equipment, etc.) you can take with you. Again, at the lower orbits you also have to balance the cost of re-boosting the orbit when it decays too much. The ISS isn't set at 431km like you said in your question, the orbit is constantly decaying and getting reboosts.

Geostationary altitude is the only one that I'm aware of that has a special name, but beyond that orbits are generally divided into Low Earth Orbit (LEO, generally anything below 1000km, but that definition varies depending on who you ask), Medium Earth Orbit (MEO, above LEO but below geostationary) and High Earth Orbit (HEO, above geostationary.)

Answer 4

The ideal orbit of a space station should be high enough that atmospheric friction is low and reboosts don't need to happen too often, but also low enough that supply flights don't become too expensive. The ISS orbital height is a compromise between these two requirements. Its inclination is a compromise between the ideal inclinations for the two launch sites Baikonur and Kennedy Space Center. It was chosen to be slightly better for Baikonur. A higher inclination also allows the ISS to observe a larger part of the Earth surface.

Another interesting orbit is the heliosynchrous orbit. This is a highly inclined orbit which always passes the equator at the exact same local time. Or in other words: when it passes over the equator, the sun is at exactly the same angle. Such satellites are used for sun observation and for meteorological purposes. There are multiple such orbits with different inclinations and different heights (which also means a different number of orbits per earth-day).

When you want the opposite of a geostationary satellite - one which moves over the ground as fast as possible - you would put it into a low retrograde orbit. The difference between a retrograde and a usual (prograde) orbit is that the satellite orbits the earth in the opposite direction the earth is rotating. Instead of eastwards, the satellite orbits westwards. This kind of satellite is quite rare in practice because it requires more energy to launch (the rotational speed of the ground which usually assists rocket launches now works against it). Also, very few launch sites allow westward launches. The Kennedy Space Center, for example, isn't allowed to perform such launches for safety reasons: A launch failure would endanger the Orlando metropole region.