By the terms of the dictionary, a satellite is an object which revolves around another object. For example, the Moon is a satellite of the Earth, and the Earth is one of the Sun. The phenomenon interested mankind to investigate more on this idea and experiment on man-made satellite en-circling the Earth. It was thought that if such a satellite was put in the space, communication could be set up from one part of the world to the other. A communication satellite orbits around the earth and in effect is an artificial satellite, stationed in space for the purpose of telecommunication. These satellites use the geosynchronous orbits, Molniya orbits or low polar Earth orbits.

A geosynchronous orbit is essentially a geocentric orbit, which takes the same time as the Earth to complete one orbit. If this satellite could be seen from ground, it would seem that the satellite is stationary and not moving at all. Satellites in such orbits are useful for telecommunication applications. Satellites in Molniya orbit have a highly elliptical, set at an angle, taking 12 hours to make a complete the orbit. A satellite in a low polar Earth orbit passes right above both the poles, inclined at a certain angle relative to the equator.

How are the objects kept in orbit?

Since man have been able to lift his head towards the sky, he has looked up at the sky and wondered how that Sun is held up high above there, and why does not the Moon fall on us. It has only been about 300 years now, that we have developed the scientific reason as to the things we were wondering about in the past. It was in the late 17th century that Sir Isaac Newton put forth this fundamental law. The first law says that, every object of matter in this universe attracts another, with a force, which is proportional to the product of their masses and inversely proportional to the square of the distance between the two. Therefore, with the larger mass having greater power to attract, this attraction gets weaker as the distance between the two objects are increased.

Newton's law of gravity means that the Sun pulls the Earth and the other entire planet in this universe, and the Earth also pulling on the Sun. Since both are quite large masses, the force of attraction must be large. The question comes, if the Sun is pulling all other planets, why don't the planets fall on the Sun?

The explanation to this is, that the planets are moving sideways too at a great velocity. Therefore, taking the example of the Earth, by the time the Earth has fallen the 93,000,000 miles to the Sun, it has moved 93,000,000 miles sideways, far enough to miss the Sun. Likewise, by the time the Moon has fallen 240,000 miles to the Earth, it has moved 240,000 miles sideways, missing the Earth. This process is repeated continually and the Earth orbits the Sun and the Moon the earth, in a never-ending way. If any planet stops moving sideways and finds itself closer to the Sun, it will fall quickly without missing the Sun. This sideways movement, called the "angular velocity" prevents the Earth in falling into the Sun. The same is true for all planets en-circling the Sun. The Moon would fall on to Earth if it had not had that angular velocity.

If the gravity of the Earth were to be turned off, the Earth would leave its circular trajectory and travel straight with a high velocity, reaching 50 billion miles out from the Sun in a century's time. Hence, it is the gravitational forces of both the Sun and the Earth, that holds the Earth in a orbit around the Sun. The orbiting Earth may be compared to a piece of stone tied to a string, which is swung in a circle holding the string at one end. If you were to let go of the string, the stone, tied at the other end of the string, will fly off at a straight trajectory, just as Earth would do, if the gravity were to be turned off. This force is termed as the 'Centrifugal force.'

There can be a question about the time required to complete one orbit. Does that depend upon the distance at which the object is orbiting? After years of experiments, it was found out, that the greater the distance, the more time it takes to complete one orbit. This was deduced from the formula, that the time taken, is directly proportional to the distance of the object around which it is orbiting. Thus, the planet which is at a larger distance from the Sun, takes longer to complete one orbit. This time taken to complete one orbit is termed as 'orbital period.'

Artificial Satellites

Looking at the first law of Newton, it was understood that in principle it should be possible to put up a man-made satellite in the sky, which would orbit the Earth. As has been seen, this satellite would need a sideways velocity, like the earth, so that it does not fall back on Earth. If such a satellite is put up in the sky at a distance of 4,000 miles, it will have an orbital period of approximately 90 minutes. To miss falling on to the Earth, this satellite would have to have a sideways velocity of 17,000 miles per hour.

Let us consider firing of a cannon ball. If the firing is quite weak, the cannon ball describes a parabolic path and lands up few hundred yards away. If we bring a heavier cannon ball, and try and shoot it a bit further with greater force, the ball describes a parabolic path to the earth’s surface and lands up a few hundred miles away. This time the cannon ball goes over the Earth's curvature to describe its path. If now, the a super-heavy cannon ball is shot very forcefully to land a few thousand miles away, the ball would travel much further than what it would if the Earth was flat. Clearly it can be deduced that the Earth's curvature had some effect on the distance that this cannon ball traveled. Imagine that the velocity of this cannon ball is 5 miles per second, i.e., 17,000 miles per hour. As it falls to the Earth, it misses the Earth's surface and the Earth's gravitational force makes the ball continuously change its course in its the fall to the Earth. Thus this cannon ball starts orbiting the Earth. Till the concept of rocket came in, such velocity was unthinkable and putting up a satellite in the sky remained a dream. Eventually the technology evolved and the first artificial satellite 'Sputnik,' was launched by the Russians in 1957. This satellite was not much more than a basket ball and had a radio transmitter on board. It made 'Beep. Beep. Beep’ sounds and would appear and disappear, again to re-appear in 90 minutes time.

A few years before the Americans put John Glen into the orbit, on-board an artificial satellite, the Russians had already launched Yuri Gagarin, the first man in space. All these satellites were launched at the same altitude, give or take a few hundred miles, and both had the orbital time of 90 minutes. If these satellites were launched higher, for example at a height of 22,300 miles, the orbital time would have been orbiting the Earth every 24 hours. The skill required to launch the artificial satellites, so as to achieve a geosynchronous orbit, did not happen till 1963. This required the satellite to be launched at a much higher altitude.

When you, look back to the dawn of space travel, the landing on the Moon by Apollo in 1969, seemed to be a giant step forward in space travel. Since that time, weather forecasting has technologically improved with geostationary meteorological satellites, sending the pictures that we see on television everyday. The television broadcast has taken a leap with these satellites, where live telecasts are being beamed from one side of the world to the other. The satellite aided voice communication has had a remarkable effect in getting help in a remote area, where other communication is not available, delivering the highest speech quality with reliability.

With globalisation, factories and offices have come up in the remote part of the world. These locations are often in those parts where modern communication network does not exist. Despite this, business needs to go on and communication is vital. Satellite communication has virtually brought these places quite close, bringing the communication network virtually to any location around the world. This has been done without the need of an infrastructure of wireless network, fiber optics, or even copper cabling.