Types of Orbits
What we have just described in the previous section is how a satellite orbits the Earth - it is just like the baseball that goes all the way around the Earth. Let's look at that in more detail.
Low Earth Orbits
Most satellites, the International Space Station, the Space Shuttle, and the Hubble Space Telescope are all in Low Earth Orbit (commonly called "LEO"). This orbit is almost identical to our previous baseball orbiting example, except that it is high enough to miss all the mountains and also high enough that atmospheric drag won't bring it right back home again.
1) From our previous discussion to this point, you should know that if an astronaut isn't moving fast enough, his orbit will bring him back to Earth's surface. What is the minimum speed he needs in order to stay at least as high as he is? At that speed, how long will it take the astronaut to go around the Earth once?
Advantages and Disadvantages of LEO
Low Earth Orbit is used for things that we want to visit often with the Space Shuttle, like the Hubble Space Telescope and the International Space Station. This is convenient for installing new instruments, fixing things that are broken, and inspecting damage. It is also about the only way we can have people go up, do experiments, and return in a relatively short time.There are two disadvantages to having things so close, however. The first is that there is still some atmospheric drag. Even though the amount of atmosphere is far too little to breath, there is enough to place a small amount of drag on the satellite or other object. As a result, over time these objects slow down and their orbits slowly decay. Simply put, the satellite or spacecraft slows down and this allows the influence of gravity to pull the object towards the Earth.
The second disadvantage has to do with how quickly a satellite in LEO goes around the Earth. As you can imagine, a satellite traveling 18,000 miles per hour or faster does not spend very long over any one part of the Earth at a given time. So what happens if we want a satellite to spend all of its time over just one part of the Earth? For instance, a weather satellite wouldn't be very effective for us in North America if it didn't have a long dwell time over us. (Dwell time = the time a satellite sits over one part of the globe.) Also, a communications satellite wouldn't work very well for us in North American if it spent most of its time over Africa or Asia.
There are two ways to accomplish this. One solution is to put a satellite in a highly elliptical orbit and the other is to place the satellite in a geosynchronous orbit.
Highly Elliptical Orbits
Remember Kepler's second law: an object in orbit about Earth moves much faster when it is close to Earth than when it is farther away. Perigee is the closest point and apogee is the farthest (for Earth - for the Sun we say aphelion and perihelion). If the orbit is very elliptical, the satellite will spend most of its time near apogee (the furthest point in its orbit) where it moves very slowly. Thus it can be above home base most of the time, taking a break once each orbit to speed around the other side.
With the highly elliptical orbit described above, the satellite has long dwell time over one area, but at certain times when the satellite is on the high speed portion of the orbit, there is no coverage over the desired area. To solve this problem we could have two satellites on similar orbits, but timed to be on opposite sides of the orbit at any given time. In this way, there will always be one satellite over the desired coverage area at all times.
If we want continuous coverage over the entire planet at all times, such as the Department of Defense's Global Positioning System (GPS), then we must have a constellation of satellites with orbits that are both different in location and time.
In this way, there is a satellite over every part of the Earth at any given time. In the case of the GPS system, there are three or more satellites covering any location on the planet.
Geosynchronous Orbits
Another solution to the dwell time problem is to have a satellite always sitting over the same location on the planet. The way we do this is to have the orbital period of the satellite exactly the same as the rotation period of the Earth, which is one day. This is called a geosynchronous orbit, or GEO for short.
In this case, the satellite can not be too close to the Earth because we already figured out that it would not be going fast enough to counteract the pull of gravity. If you recall, the space shuttle, in order to stay aloft, must circle the planet every 90 minutes.
We can use Kepler's third law to figure out how far out a satellite must be to spend all its time over one part of Earth. The answer is that a satellite has to be placed approximately 22,000 miles (36,000 km) away from the surface of the Earth in order to remain in a GEO orbit. That is a lot farther than a low Earth orbit, or a relatively close highly eccentric GPS-like orbit, so it costs more to get it there. However, then you only need one satellite to do the job and it is on the job 24 hours per day.
1) How fast is this satellite going relative to Earth?
By positioning a satellite so that it has infinite dwell time over one spot on the Earth, we can constantly monitor the weather in one location, provide reliable telecommunications service, and even beam television signals directly to your house. If you have satellite TV at home, notice that the small dish antenna outside is pointing at the same location in the sky at all times. There is a geosynchronous satellite sitting 22,000 miles away in that direction sending the signal to your house! The down side of a geosynchronous orbit is that it is more expensive to put something that high up and not possible to repair it from the shuttle. When a satellite is in LEO, the shuttle can repair it if needed, as we have done with the Hubble Space Telescope several times. So you only put something in GEO if you really need to have it in the same location in the sky at all times.
If it is hard to appreciate the value of these satellites, consider the following true story: One of the people responsible for this web site was on a small freighter (ship) on its way from Sweden to the US in November of 1959. The ship had just reached a particularly nasty part of the North Sea (not too far from the rocky coast of Scotland) when it was caught in a storm that had not been predicted or seen. For several days, the ship tossed back and forth on waves as big as it was - or bigger - tipping more than 30 degrees each way. (If you have seen "The Perfect Storm" then you have some idea of what that was like.) This ship made it to port some days later, but quite a few did not, in that storm. A couple of year later, the first weather satellites were launched, and no ship has been caught by a surprise storm since then!
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