by: Gary Davis
Any object that moves around the earth has an orbit. The orbit is defined by 3 factors. The first is the shape of the orbit, which can be circular or elliptical. The second is the altitude of the orbit. The altitude is constant for a circular orbit but changes constantly for an elliptical orbit. The third factor is the angle the orbit makes with the equator. An orbit that brings the satellite over the poles or close to it has a large angle. An orbit that makes the satellite stay close to the equator has a small angle.
Orbits depend on the mission the satellite was built for. The following orbits can be defined:
Low Earth Orbit
Satellites in low earth orbit (LEO) orbit the earth at altitudes of less than 2000 km (1242 miles). Satellites in LEO can get much clearer surveillance images and require much less power to transmit their data to the earth.
Medium Earth Orbit
At an altitude of around 10.000 km (6.000 miles) a satellite is in medium earth orbit (MEO). This altitude balances the advantages and disadvantages of LEO and GEO. MEO’s are used generally for navigational satellites and communications satellites
Geostationary Orbit
A satellite in geostationary orbit orbits the earth in exactly 1 day and is placed above the equator. The angle with the equator is 0 degrees. As a result the satellite seems to stand still as seen from the earth. These satellites are used for communications and Satellite TV.
Polar Orbit
An orbit that goes over both the North and the South Pole is called a Polar Orbit. The angle with the equator is 90 degrees. The advantage of these orbits is that they go over the poles. This may sound funny, but in reality most satellites never “see” the poles. Most polar orbits are in LEO, but any altitude can be used for a polar orbit.
Polar orbits are used a lot by navigation satellites which have to provide navigational information all over the world, including the poles.
Sun Synchronous Orbit
This is a very interesting type of orbit. Satellites in Sun Synchronous Orbit pass over the same locations on earth at the same time each day. Suppose a satellite in sun synchronous orbit passes over your house at 3 pm. The next time the satellite will pass over your house is 24 hours later at the same time the next day.
In order to do this the satellite has a very special orbit. Not only does it orbit the earth, but the plane of the orbit changes as well. It has to do this because each day the earth moves a bit through space around. After 3 months, the earth has moved 90 degrees of its orbit around the sun. If the satellites orbit wouldn’t have moved with it, it would show up 6 hours later than planned. (Actually the earth spins around its axis in approx. 23 hours, 56 minutes and 3 seconds and not in 24 hours. Because of the earth’s orbit around the sun, 1 day lasts 24 hours. A star day however is approx. 23 hours, 56 minutes and 3 seconds long.)
However, there is no need to actively change the plane of the orbit. The earth is not a perfect sphere but is a little bit wider around the equator. This is caused by the spinning of the earth. The gravitational difference this causes changes the orbit of a satellite. When the angle with the equator is chosen correctly (approx. 8 degrees of a polar orbit), an orbit is a sun synchronous orbit.
A very special type of sun synchronous orbit is called the dawn-to-dusk orbit. This orbit is above the earth where the sun comes up or goes down. A satellite in this orbit never enters the shadow of the earth but always receives sunlight.
Orbit Decay and Reentry
The earth’s atmosphere doesn’t stop at a certain altitude but really fades out into space. The higher you get the less thick it is until eventually there is no more atmosphere. Generally we say that the atmosphere is about 100 km (62 miles) thick, but in reality it extends much further out into space. Satellites experience friction from the atmosphere up to altitudes of 1000 km (620 miles).
Due to the friction, a satellite will loose speed and with that the altitude will decrease until eventually the satellite will fall out of orbit back to the earth. Depending on the altitude this happens sooner or later. A satellite at an altitude of 200 km (124 miles) will stay in orbit for just a couple of months. At 300 km (186 miles) a satellite can stay in orbit for a couple of years. Above 1000 km (620 miles) a satellite can stay in orbit for thousands of years.
Letting Satellites fall back to earth and burn up in the atmosphere is also a way of disposing of satellites.
By Gary Davis
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