ARTIFICIAL SATELLITES

An artificial satellite is a space-craft shot into space to make it orbit a heavenly body planet or moon etc. The first artificial satellite was launched from the earth by USSR on October 4, 1957 A.D. It was named Sputnik I. Since then hundreds of satellites have been sent into space.

Artificial satellites are of different types. Some of them tell us about weather while others point out the area with mineral deposits etc. Still others provide us with instant telephone calls or help us watch the events happening around the world (live) on our TV-sets. Not only this, satellites help astronomers with much needed space-information. They also help ships and aeroplanes to know about their exact positions.

Each satellite comes to occupy a special orbit around the earth. It is called a geo-stationary orbit. If a satellite is there about 3600 kilometers away from the earth, it orbits the earth in about 24 hours period for the earths own one rotation-round. So, it appears stationary in the sky. The electric power needed for the satellite to work comes from solar cells that convert sunlight into electricity. Rocket motors called thrusters keep the satellite in the correct position so that its dish-aerials point to the earth.

The earth's gravity makes a satellite stay in its orbit. Without it, it is sure to drift into space in a straight line.

While launching a satellite, its rockets must be controlled carefully to· give it the required speed to go up-about 11,100 kilometers an hour.

WHAT HOLDS THEM IN PLACE?

There is a force of mutual attraction acting between all material objects. This force is there in every object because of its mass-the amount of matter that it is made of. This mutual force is called gravitation.

Sir Isaac Newton discovered gravitation. The gravitational attraction exerted by an object on its surrounding objects is called its gravity. The gravity of an object directly depends on its mass. Clearly, if two unequal objects attract each other, the one with the larger mass will pull the other towards it. Moreover, the distance between the two has also much to do with this force of attraction. As an object moves farther from another object, the force of gravity acting on it goes on decreasing with the increase in distance.

All the heavenly bodies-stars, planets, asteroids and moons are mutually attracting each other by means of their respective forces of gravity. These forces depend on their masses and on how apart they are.

The moon is smaller than the earth. So, the larger gravity of the earth overcomes its lesser gravity and does not let it move farther away. As a result, it is forced to revolve round the earth at a certain distance from it. The path along which the moon moves is called its orbit.

It is this principle of mutual interaction of gravities that holds various heavenly bodies in their places. The sun's gravity is larger than that of any of the planets. So, each of them revolves round it.