Satellites come in many shapes and sizes and have many uses. Select one of the five areas below to see how we use these orbiting machines. A satellite's orbit depends on its task, speed, and distance from Earth. There are four types of Satellite orbits.
LEO Low Earth OrbitWhen a satellite circles close to Earth we say it's in Low Earth Orbit (LEO). Satellites in LEO are just 200 - 500 miles (320 - 800 kilometers) high. Because they orbit so close to Earth, they must travel very fast so gravity won't pulled them back into the atmosphere. Satellites in LEO speed along at 17,000 miles per hour (27,359 kilometers per hour)! They can circle Earth in about 90 minutes.
A Low Earth Orbit is useful because its nearness to Earth gives it spectacular views. The crew in a Space Shuttle traveling in low earth orbit took this picture. Satellites that observe our planet, like Remote Sensing and Weather satellites, often travel in LEOs because from this height they can capture very detailed images of Earth's surface.
The LEO environment is getting very crowded. The United States Space Command keeps track of the number of satellites in orbit. This is a graphic display of the objects in low earth orbit. According to the USSC, there are more than 8,000 objects larger than a softball now circling the globe.
Some people worry about the number of items now in low earth orbit. Not all of these things are working satellites. There are pieces of metal from old rockets, broken satellites, even frozen sewage. At 17,000 mph, even a small bolt can hit a space shuttle with the impact of a hand grenade. Which is exactly why the US Space Command keeps track of these things!
Polar orbits are useful for viewing the planet's surface. As a satellite orbits in a north-south direction, Earth spins beneath it in an east-west direction. As a result, a satellite in polar orbit can eventually scan the the entire surface. Its like pealing an orange in one piece. Around and around, one strip at a time, and finally you've got it all. For this reason, satellites that monitor the global environment, like remote sensing satellite and certain weather satellite, are almost always in polar orbit. No other orbit gives such thorough coverage of Earth.
Geosychonous Equatorial OrbitA satellite in geosychonous equatorial orbit (GEO) is located directly above the equator, exactly 22,300 miles out in space. At that distance, it takes the satellite a full 24 hours to circle the planet. Since it takes Earth 24 hours to spin on in its axis, the satellite and Earth move together. So, a satellite in GEO always stays directly over the same spot on Earth. (A geosynchronous orbit can also be called a GeoSTATIONARY Orbit.)
Because they're so far away, GEO satellites have a very broad view of Earth. For instance, the footprint of one EchoStar broadcast satellite covers almost all of North America. And, since they stay over the same spot on Earth, we always know where GEO satellites are. If our antenna points in the right direction, we'll always have direct contact with the satellite. Many communications satellites travel in geosynchronous orbits, including those that relay TV signals into our homes.
A satellite in elliptical orbit follows an oval-shaped path. One part of the orbit is closest to the center of Earth (perigee) and the other part is farthesWhile most communications satellites are in geosychonous orbit, the footprints of GEO satellites do not cover the polar regions of Earth.
So communications satellites in elliptical orbits cover the areas in the high northern and southern hemispheres that are not covered by GEO satellites. t away (apogee). A satellite in this orbit takes about 12 hours to circle the planet. Like polar orbits, elliptical orbits move in a north-south direction.
Communications satellites act as relay stations in space. People use them to bounce messages from one part of the world to another. These messages can be telephone calls, TV pictures, or even Internet connections like the one you're using now. We can also use satellites as a relay system to send signals anywhere on Earth. Here you can see the overlapping footprints of four different satellites. If you wanted to send a signal from Africa to Southeast Asia, you could bounce, or relay, the signal using more than one satellite.
For a Glossery of Satellite terms go here.