Orbits 'R' Us!
When we talk about how Earth and the other planets travel around the Sun, we say they orbit the Sun. Likewise, the moon orbits Earth. Many artificial satellites also orbit Earth.
When it comes to satellites, space engineers have different types of orbits to choose from.
Satellites can orbit Earth's equator or go over Earth's North and South Poles . . . or anything in between. They orbit at a low altitude of just a few hundred miles above Earth's surface or thousands of miles out in space.
The choice of orbit all depends on the satellite's job.
Suspended animation
Image of Hurricane Fran made from GOES weather satellite data in September 1996.
The two GOES* weather satellites, for example, have the job of keeping an eye on the weather over North America. They need to "never take their eyes off" any developing situation, such as tropical storms brewing in the Atlantic Ocean, or storm fronts moving across the Pacific Ocean toward the west coast of the U.S. Therefore, they are "parked" in what is called a geostationary (gee-oh-STAY-shun-air-ee) orbit. They orbit exactly over Earth's equator and make one orbit per day. Thus, since Earth rotates once on its axis per day, the GOES satellite seems to hover over the same spot on Earth all the time.*GOES stands for Geostationary Operational Environmental Satellite.
In the cartoon at the right, we are looking down at the North Pole. Of course, this cartoon is not to scale! If Rusty were standing someplace on the equator and could see a geostationary satellite overhead (which would be pretty hard, since it would be 22,300 miles away!), the satellite would seem to be suspended above him all the time. Click on the cartoon to get an idea of what this would be like.
On the other hand, satellites whose job is to make maps or study all different parts of Earth's surface need an orbit that comes as close to passing over the North and South Poles as possible. This way, Earth turns under the satellite's orbit and Earth does most of the work of traveling! Also, the satellite should be close to Earth's surface (a few hundred miles up) to get a good view with its imaging and measuring instruments.
The lower the satellite's orbit, the less time it takes to make one trip around Earth, and the faster it must go. That's why a geostationary orbit must be so high. It has to go out far enough so that it can travel slowly enough to go around Earth only once per day.
In the cartoon to the left, the satellite passes nearly directly over the North and South Poles. In our animation, it goes around twice in one day. In reality, the satellite may orbit Earth once every hour-and-a-half or so, going around many times per day. An example of satellites in polar orbit are the three POES* satellites. Putting the images from the three satellites together, it takes only six hours to get pictures of just about every square inch of Earth. This information is used to help scientists understand weather, climate, oceans, volcanoes, and vegetation patterns around the world. In addition, the information helps in search and rescue and in spotting forest fires.
*POES stands for Polar-orbiting Operational Environmental Satellites.
Suppose two satellites are to be launched to the same altitude. However, one is to go into a polar orbit and one is to orbit the equator. Can you guess which satellite will take the most fuel to reach its orbit?
If you guessed the polar orbiting satellite, you are right.
At the equator, Earth itself is rotating from west to east at 1675 kilometers per hour (1041 miles per hour)! If the satellite is launched in the same direction as Earth is rotating, it gets quite a boost. If it is launched toward the north or south, it doesn't get to take advantage of this boost. Or, if the satellite is launched toward the east, it takes a lot of fuel in the spacecraft's thrusters to change the inclination, or tilt, of its orbit. A polar orbit has a high inclination.
Tornado in Cordell, Oklahoma, May 22, 1981. Credit: NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory (NSSL).
