There are approximately One thousand satellites orbiting earth at this time. Nature had gifted us with only one satellite - moon. We made a thousand artificial ones. You can sometimes spot an artificial satellite in the sky with naked eyes. You will see a distant spot moving speedily over the night sky. What distinguishes this spot from meteors is that it doesn't cease to exist suddenly, but will traverse the entire night sky and will finally vanish when it goes 'below' horizon.

The kind of satellite just described above is most likely to be a sun-synchronous satellite. It is called sun-synchronous because its orbit also revolves as the earth orbits the sun so that sun is always directly at the back of satellite. Or, better still, the argument of perigee of a sun- synchronous orbit rotates 0.98° ( = 360°/365 ) per day. You are not likely to be able to distinguish between a geostationary satellite and a star because both will seem to be absolutely still in the night sky.

Satellites can be classified into three major types based upon the distance from the earth's centre:

• Low Earth Orbit ( LEO )
• Medium Earth Orbit ( MEO )
• High Earth Orbit ( HEO )

A LEO satellite orbits the earth in orbits of radius about 200 to 1000 Km altitude (from the Earth's surface), MEO orbits have a radius of about 5000 - 20000 Km (Again from the Earth's surface), and a HEO has a radius of 36000 Km or more (again from the Earth's surface. Earth has a radius of about 6400 Km. You can add that to the distance and make the orbit radius from the centre) .

Now that the definitions are done, Let me note that MEO satellites are almost non-existent for the simple reason that they present no advantage whatsoever from any point of view. Most of the LEO satellites are used as spy satellites, meteorological satellites (satellites which collect data related to weather), remote sensing satellites, oceanographic satellites, cartographic satellites and recently, they are being used for communication also (Motorola's failed Iridium experiment).

HEO satellites are used for communications almost without exception ( Hubble Space Telescope being one though I'm not sure about that). Now the question that arises is that why LEO are used for such and such purpose and why not HEO ? Well, there are a horde of reasons.

Spy satellites (and most of the LEO satellites listed above) need to be close to earth to take clear pictures and sense data more accurately. The size of the most of the sensors used in remote sensing, oceanographic satellites will increase exponentially with distance if we want to maintain the same resolution.

Second thing is that the closer is a satellite to the earth, the greater its orbital velocity will be so that it will orbit the earth in lesser time. Which is beneficial because we can collect data for a larger area in lesser time. Also since the orbit of a sun synchronous satellite follows the sun (allow me to say that) a remote sensing satellite can cover whole of the earth in 20-30 days!

And the reason why most communication satellites are in a geostationary orbit (Which is a HEO) is that it allows us to maintain communications link with the satellite by simply pointing the antenna in one constant direction. This greatly simplifies our task of tracking the satellite. The other reason is that a satellite at a greater height can 'see' a larger area at any given time.