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InSat-4, active communications satellites serving India. They're in geosynchronous orbit don't you know? (Google/Analytical Graphics)
What is Geosynchronous orbit? It is an orbit that completes one revolution in the same amount of time it takes for the Earth to rotate once on its polar axis. A person on Earth, for example, could view a satellite in this type of orbit at the time and place at least once a day. Geosynchronous orbit is a subtype of geostationary orbit; a special orbit that occurs only near the Earth’s Equator.
The most well known and used type of artificial satellites, telecommunication satellites, use a geostationary orbit because the speed of telecom satellites match the speed of the Earth’s rotation. They seem fixed in one point in the sky. This makes it easy to point your satellite dish in a fixed direction, and the satellite can point its telecommunications equipment at a fixed point on the ground.
However there are other type of orbits that are geosynchronous but not geostationary. Good examples are the polar orbiting weather satellites used by the National Weather service. While not good for continuously transmitting a signal they are great for gathering data which can be regularly submitted to a designated station on the ground.
Geosynchronous orbit makes possible many of the satellite applications we take for granted today, from our television shows to weather forecasts, or transmissions from the Hubble Telescope. All rely on some form of geosynchronous orbit to do their jobs.
So what keeps any satellite in orbit around the earth? The answer is simply speed. Everything in the Universe is under the influence of gravity in one way or another. The only thing that anchors massive stars and planets is the speed of their orbits. When any satellite, natural or artificial, reaches a certain speed it is able to diminish the strength of center of gravity so that in the end it at best only able to pull that object into a circular path around it. For artificial satellites they don’t have the advantage of natural satellites which are normally massive enough to stay in orbit indefinitely. They experience decay of orbit as the Earth’s gravity slowly reasserts its hold on them until they eventually slow down and crash back to Earth. So these satellites use the speed of the Earth’s rotation as a benchmark to maintain their orbits.
If you liked this article there are others on Universe Today you might enjoy. First there is a great article on using to Google Earth to track satellites. There is another article about some of the dangers facing the International Space Station.
You can also find great and interesting resources on the internet. The centennial of flight site has a great definition of the difference between geostationary and geosynchronous orbit. Another great site is the NOAA website which has good article about the environmental satellites the agency uses.
Another excellent resource to take advantage of is Astronomy Cast. They have episode on a lot of space related information. One relevant to this article is the 3/12/09 Questions Episode.
Sources:
Wikipedia
Space Daily
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