Before humanity managed to send man-made objects into space seventy-five years ago, the field of orbital mechanics didn’t really exist. Only the movements of other celestial bodies mattered. Things have really changed since then.
Some history
The Nazis in World War II were the first to successfully launch a V2 rocket into what is technically outer space.
The Cold War caused a race between Russia and the US to put people in space first, which resulted in the Russians managing to send satellite Sputnik 1 into orbit in 1957. From there the Russians turned to dogs (Laika, the first animal in space), and then people (Yuri Gagarin). The US like to emphasize their follow-up of sending the first person to the moon.
Nowadays, we have an international space station, and roughly 3600 satellites in orbit around our planet. So how does all of that work?
Orbital basics
In the twelve tasks of Asterix, Obelix throws a Javelin so far it circles the globe and returns to its starting point. Although this is a cartoon/comic, the principle is actually valid.
To achieve a stable orbit, an object must travel so fast around the Earth that the centripetal force of gravity is exactly so strong as to keep the object moving in a circle around the globe. Basically, the object falls around the Earth.
A stable orbit depends on the distance from the Earth and the speed of the object. The further the distance from Earth (higher orbits), the slower the speed to make an orbit stable.
Mass has little to do with it, unless the object has significant enough mass relative to Earth – like the moon, for example.
Elliptical orbits
Orbits are always elliptical. If you look at the planets orbiting the sun, they all have an elliptical orbit. Below is a diagram of Earth’s orbit.
As you can see, the distance from the Earth to the sun varies over the course of a year. A perfectly circular orbit is that special case of the elliptical orbit where the distance to the orbited object is always equal.
Note that the speed of an object in an elliptical orbit is not constant. It will go slower at its apoapsis and faster at its periapsis (see diagram).
Geostationary orbit
Back to satellites in orbit around Earth. Remember how the speed of an object decreases, the higher its orbit is? At a specific distance from Earth, the speed of an object will be exactly equal to the rotational speed of the Earth (1670 kilometers per hour at the equator). If you put a satellite in orbit at the right speed, going the right way, it will stay above the exact same place on Earth. We call this a geostationary orbit.
Satellites
We mostly use orbital mechanics to put satellites in orbit around earth. As stated above, there are 3600 satellites in orbit. They perform all kinds of functions like monitoring weather, providing GPS, and showing us satellite overlays on Google Maps.
A GPS receiver works by finding a set of GPS satellites in the sky above. The GPS satellites send out a signal containing the precise time. The receiver uses the known position of the satellites and the time signal to determine the exact direction and distance of the GPS satellite relative to the receiver. With this information from several satellites, the position of the receiver on Earth can be determined. Contrary to popular belief (and mine, actually), GPS does not require geostationary orbits.
Geostationary orbits are useful for weather and communication satellites. There’s actually a list of them on Wikipedia. They are easier to receive signals from on Earth, because you can point your antenna at them directly, and don’t have to keep the antenna moving to match satellite movement.
Conclusion
Since we started putting things in space, orbital mechanics have become increasingly important. If you read or write science fiction, or are interested in science, it’s good to know some of the basics.
