2000 years ago, the Greek astronomers thought that the earth was at the centre of the universe and the moon, the planets, the sun and stars were orbiting around it.
1. In the 6th century, it was Aryabhatta who stated that the rotation of the heavenly bodies as observed by us is due to the rotation of the earth around its axis. Aryabhatta discovered that the day and night are occurred due to the rotation of the earth. He also established that the lunar and solar eclipses were caused due to the shadows of the earth and the moon respectively.
2. In the 15th century, Nicholas Copernicus, the polish scientist showed that the sun was at the centre of the solar system and planets revolve around it. So the sun became the centre of the universe.
3. In the 16th century, Johannes Kepler discovered the laws of planetary orbits, but the sun still remained at the centre of the universe. It was only in the beginning of the 20th century, that a picture of our own galaxy became clear. The sun was found to be located in a corner of the galaxy.
There are many man-made satellites revolving around the earth. These are called artificial satellites. They are launched from the earth. They are much closer to the earth than the moon. The scientists have developed the technology for designing and fabricating artificial satellites in the last 50 years or so. They have also developed very powerful launch vehicles or rockets capable of carrying the satellites into the space and launch them.
At present only six countries in the world have the technology for developing artificial satellites and launching them into earth’s Orbit. India has built and launched several artificial satellites. Aryabhata was the first Indian experimental satellite launched in April 1975.
Some other Indian satellites are INSAT, IRS, Kalpana-I, EDUSAT, Chandrayan, Mangalyaan etc. Artificial satellites have many practical applications such as taking photographs of the earth, receiving and transmitting television, radio signals, research and defence. They can be used to forecast weather, can alert us to the arrival of monsoon, and can warn us of possible disasters like floods, cyclones, forest fires. They are also used for internet, telecommunication and remote sensing. Remote sensing (collecting information from a distance) is used to collect information about weather, agriculture, land and ocean features. Artificial satellites can be used for purely scientific work. For example they can carry instruments to make astronomical observations.
Hubble space telescope sends beautiful photographs of planets, stars and satellites. U.S.A. has launched a satellite recently to study x-ray radiations in space named Chandra. Artificial satellites can also be used to run advisory services for farmers and fishermen by providing information about land use, health of crops, density of fish in the seas, etc. Because of their ability to observe things from above the earth, artificial satellites are rightly called the ‘eyes in the sky’.
The speed of artificial satellites to be used for long distance communication is so adjusted that they compete one revolution around the earth in 24 hours. As a result, the satellite appears to be stationary with respect to the transmitting station on the ground. These satellites are called geostationary or geosynchronous satellites. A series of three such satellites placed at suitable points in space can cover the whole globe. Then, signals from any point can be transmitted to any other point on the earth. The minimum velocity required to launch an artificial satellite in an orbit around the earth is about 8 km/s. It is also necessary to lift the satellite to a height of about 200 km from the ground to give it a horizontal push. This is done to eliminate loss of energy due to friction suffered by the satellite, when it travelled through the earth’s atmosphere. Since artificial satellites need to be launched with very high velocities, only powerful launch vehicles i.e. specially designed rockets can do this job.
Rockets move forward by discharging gases at great speed through their exhausts. The essential properties of the fuel to be used, therefore, are:
(i) it must burn very rapidly but not explode, and
(ii) it must produce large volume of gas at high pressure and temperature. The fuel generally used is a mixture of liquid hydrogen (or kerosene sometimes) and liquid oxygen.
It is risky to store liquid fuel in the rocket before its actual launch. Solid fuels are better for storing because they do not pose any such danger. One common mixture which serves as solid fuel consists of powdered aluminium and ammonium per chlorate or ammonium nitrate.
Sometimes, satellites have to carry many instruments to carry out various tasks. The weight of instruments called payload makes the satellite very heavy. Such satellites have to be lifted with more than one rocket, firing one after another. Launch vehicles with more than one rocket are called multistage launch vehicles. Artificial satellites have generally two types of orbits. The geostationary satellites have orbits parallel to the equator. Such orbits are called equatorial orbits. Satellites used for weather forecast and remote sensing, have orbits passing over the poles of the earth. Such orbits are called polar orbits. These orbits are at a height of about a 1000 km from the earth’s surface.
The time period of a satellite in a polar orbit is usually less than two hours, so it makes several rounds in a day. Because of its nearness to the earth, a polar satellite can observe large areas of the earth around the point over which it passes. So, as the earth rotates under it, the satellite is able to scan the whole earth in a few days. By adjusting the height of the satellite, its time period is so arranged that the satellite arrives over a given point on the earth at the same time every time. This is very important from the point of view of remote sensing because the satellite is able to observe a given area repeatedly. Information about any changes that might take place in a given area can be found out by studying images sent by it on different days.
The useful life of a satellite depends crucially on the stability of its orbit. Therefore, it is constantly tracked. If any change in its orbit is noticed, it is immediately corrected by firing small rockets attached to the satellites. The orbit of an artificial satellite is generally elliptic, like that of a natural satellite. This is because the binding force in both the cases is gravitation. The picture and sound to be transmitted is first converted into electrical signals with a video camera. These electrical signals are then converted into a special type of waves and transmitted in air from a transmitting antenna so as to reach the artificial satellites.
The artificial satellites have special instruments installed in them, which receive the signals transmitted by the earth station. The signals so received are then amplified and retransmitted by the instruments fitted on the satellite. Since the satellite is located at great height, the signals transmitted by it can reach over a wide range of area on the earth. Antennas fixed in a number of stations on the earth including those of disc antennas receive the signals transmitted from the satellite and retransmit them. The TV sets in our homes receive the signals through the earth stations or disc antennas. Finally, the TV sets convert the signals in the form of image and sound.