In India, space exploration started in 1967 when India launched its first toy rocket RH-75. After it India launched its first artificial satellite namely Arya Bhatta on March 19, 1975. The first Indian who went into the space is Rakesh Sharma. Then, India launched its first experimental communication satellite namely APPLE (Ariane Passanger Pay Load Experiment) on June 19, 1981. Thereafter, India launched many satellites in series in the space like IRS-1A.IRS-1B, IRS-1C, INSAT-1, INSAT-2, INSAT-3 and Rohini satellites. India also developed and tested successfully Polar Satellite Launch Vehicle (PSLV) and Geosynchronous Satellite Launch Vehicle (GSLV).

In India all these activities for the development of space science are under the control of Indian Space Research Organization (ISRO). ISRO is an organization established by government of India to carry out research and developmental activities related with space science. The various tasks which have been assigned to ISRO are:

• To develop expertise to fabricate launch vehicles (rockets), rocket propellants and their control and guidance system.
• To prepare designs of artificial satellites and to fabricate artificial satellites.

To achieve these objects ISRO has developed Indian Space Research Programme. The main features of Indian Space Research Programme are:

1. Development of expertise in the field of planning, designing and fabrication of artificial satellites.
2. Development of launch vehicles.
3. Establishment of Earth Stations for launching, tracking, controlling and guiding satellites.
4. Development of facilities for use of space technology in the fields of communication, weather forecasting and spying.

Indian National SATellites (INSAT)

In India, the satellites which are launched by ISRO for the purpose of communication and weather forecasting are called INSAT. The main tasks of these satellites are:

1. Long distance telephone calls.
2. Radio and Television broadcasts.
3. Weather forecasting.

Test Your Understanding and Answer These Questions:

1. Name first satellite launched by India?
2. Name first Indian who went into space?
3. What is meaning of APPLE?
4. What is meaning of PSLV?
5. What is meaning of GSLV?
6. What is meaning of ISRO?
7. What is meaning of IRS?
8. What is meaning of INSAT?

Following are the important applications of artificial satellites.

1. Communication

The geostationary satellites are used for communication purpose like long distance telephone calls, telex, radio and T.V broadcasts etc. The satellites used for communication purpose are called communication satellites.

2. Spying

Artificial satellites are used by military for spying of enemy troops to locate their positions and movements.

3. Weather forecasting

Artificial satellites are used for weather forecasting. This is done by fitting special instruments and powerful cameras in the satellites which monitor various climatic factors such as air pressure, air temperature and humidity etc. The satellites used for weather forecasting are called weather satellites.

4. Collection of information about other planets and outer space

The artificial satellites are used for collecting information about other planets, outer space, stars, galaxies, asteroids etc.

5. Collection of information about natural resources of earth

Artificial satellites are also used for collecting information about natural resources of earth like oil wells, underground water, minerals, natural gas and coal deposits etc.

Test Your Understanding and Answer These Questions:

1. What are various applications of artificial satellites?

The technique which is used to get information about an object at a distance without being in physical contact with it is called remote sensing. The remote sensing is done through satellites called remote sensing satellites. The remote sensing satellites revolve around the earth in such a way that they pass over a particular area of the earth at the same local time. Such an orbit of a satellite is known as sun-synchronous orbit. When a remote sensing satellite passes over a particular area of the earth, it takes the photographs of that area which gives valuable information about that particular area of the earth. In India, the satellites launched for remote sensing are IRS-1A, IRS-1B and IRS-1C.

Applications of Remote Sensing Technology

The important applications of remote sensing technology are:

1. To know the area of earth covered by forests; to know the types of plants of a particular area;
2. To prepare the waste land maps;
3. Ground water surveys;
4. To know extent of pollution;
5. Drought assessment;
6. To detect deposits of fossil fuels and
7. For spying of enemy troops.

Test Your Understanding and Answer These Questions:

1. What is meaning of remote sensing technology?
2. What are applications of remote sensing technology?
3. What is meaning of sun synchronous satellites?
4. Give some examples of remote sensing satellites.

Satellite communication means sending, receiving and processing of information through satellites. For satellite communication, the information is sent to the satellites in the form of electromagnetic waves by transmitters located at the earth. The electromagnetic waves travel through the space with velocity of light (3 ×10⁸ m/s) and reach the satellite. The satellites receive these signals through special instruments called transponders. After receiving the signals, the satellite retransmits these signals in different directions either at same frequency or at different frequency. These signals are then received by the receivers located in various regions of the earth. Since the satellite communication takes place through space, so it is also called space communication.

Test Your Understanding and Answer These Questions:

1. What is the meaning of satellite communication?
2. What is space communication?
3. Give definition of transmitters, transponders and receivers.

The artificial satellites have generally three types of orbits:

• Polar orbits
• Equatorial orbits and
• Geosynchronous orbits

Polar Orbits

The orbits passing over the poles of the earth are called polar orbits. These orbits are present at the height of about 1000 km from the earth. The satellites which are used for remote sensing and weather forecasting have polar orbits. The time taken by the satellites in a polar orbit to complete one revolution around the earth is usually less than 2 hours. So, these satellites can revolve several times around the earth in a day.

Equatorial Orbits

The orbits parallel to the equator of earth are called equatorial orbits. Equatorial orbits are suitable for geostationary satellites.

Geostationary Satellites or Geosynchronous Satellites

If the position of a revolving satellite does not change with time with respect to earth, then such satellite is called geostationary satellites or geosynchronous satellites. And the orbit in which a geostationary satellite revolves around the earth is called geostationary orbit or geosynchronous orbit. In fact, geostationary satellites revolve around the earth with the same speed with which the earth revolves around its own axis, i.e. 24 hours. That is why these satellites appear to be stationary in the sky. These satellites are used for communication.

Test Your Understanding and Answer These Questions:

1. What is definition of orbit of a satellite?
2. What are different types of orbits of a satellite?
3. What is a polar orbit?
4. What is an equatorial orbit?
5. What are geosynchronous satellites?

Artificial satellites are placed in their orbits around the earth with the help of very powerful launch vehicles or rockets. For this purpose specially designed rockets are fabricated by scientists. In rockets special kinds of fuels called rocket propellants are used. These fuels burn very rapidly and produce large amount of gases. When rocket discharge out these gases with high speed, the rocket move upward. The fuels used in rockets may be both in solid form or liquid form. The most common example of liquid fuel used in rockets is mixture of liquid hydrogen and liquid oxygen, and that of solid fuel is mixture of powdered aluminium and ammonium nitrate.

Test Your Understanding and Answer These Questions:

1. What are Rockets?
2. What is meaning of rocket propellants?
3. Give some examples of rocket propellants.

A satellite is a small body which revolves around a planet. For example, moon is the satellite of the earth because it revolves around it. Satellites always revolve around the planets in closed elliptical paths. The closed elliptical path of a satellite around a planet is called its orbit.

Types of Satellites

Satellites may be natural or man-made. Natural satellites are those natural bodies which revolve around a planet. For example, moon is the natural satellite of earth. If however a man-made object is made to revolve around the earth, it is called artificial satellite. For example, Aryabhatta, Bhaskara, Rohini, Insat-1B, IRS-1A are artificial satellites because these satellites are placed in their orbits around the earth by human beings.

Principle of Satellite Launch

The principle involved in the launch of artificial satellites can be understand by assuming that you are standing on the top of a high tower and a stone is thrown by you parallel to earth with a certain speed. This stone will hit the earth at point A following a parabolic path. Now, throw the stone with greater velocity. The stone will now hit the earth at point B but after covering a longer distance. If you keep increasing the velocity of stone, it will hit the earth covering longer and longer distances and ultimately it will start revolving round the earth continuously without falling on the earth. The minimum velocity which is required to launch an artificial satellite in orbit around the earth is called orbital velocity It should be noted that the required minimum orbital velocity for an artificial satellite to place it in the orbit around the earth is about 8 km/s. The time taken by an artificial satellite to complete its one revolution around the earth is called time period.

Test Your Understanding and Answer These Questions:

1. What is the meaning of satellites?
2. What is definition of orbits?
3. What are types of satellites?
4. What is definition of orbital velocity?
5. What is definition of time period?
6. What are artificial satellites? Give examples.
7. What are artificial satellites? Give examples.

Various theories have been put forward by scientists to explain origin of universe. But here we will discuss only two theories related with origin of universe. These are:

• Big-bang theory and
• Steady state theory

Big-Bang Theory

In 1920, an American scientist Edwin Hubble proved that universe is dynamic in nature i.e. all the galaxies in the universe are rushing away from one another. From this, the scientists concluded that in the past all the galaxies were in the form of a dense ball of matter. About fifteen billions years ago, a very powerful explosion called big-bang must have taken place in this dense ball of matter which broke the ball into many fragments and scattered them in all the directions in the form of galaxies to form universe. This means that fifteen billions years ago there was no universe. It came into existence by a big-bang.

Steady State Theory

The steady state theory is contradictory to big bang theory. According to this theory, the universe is unchanging and it did not have a beginning and will not have an end.

Out of these two theories the Big-bang theory is preferred more by scientists as there are many drawbacks in steady state theory.

Test Your Understanding and Answer These Questions:

1. What is big bang theory?
2. What is steady state theory?

We have already studied in evolution of earth that the earth is made up of three layers. These are:

1. Crust
2. Mantle and
3. Core

Crust

Crust is the outermost solid and rocky layer of earth. The chief component of crust is silicon, which is present in the form of silicates. The thickness of crust varies from place to place. Its thickness is minimum (about 5 km) under the oceans and maximum (about 80 km) under the mountains.

Mantle

Mantle is the central layer of earth. It is mainly a solid layer having thickness of about 2900 km. the chief components of mantle are silicates or iron and magnesium. Inside the mantle, the temperature is in the range of 700 – 800⁰C. The pressure is also very high in mantle. Due to presence of high temperature and high pressure inside the mantle some rocks of mantle starts to flow. These flowing rocks are called magma.

Core

Core is the innermost layer of the earth. Core is composed mainly of iron and some nickel. The core of earth is divided into two parts; outer core and inner core. The inner core is made up of solid iron, whereas outer core is made up of liquid molten iron. The temperature at the centre of core is estimated to be in the range of 4000⁰– 5000⁰C.

Test Your Understanding and Answer These Questions:

1. What is the structure of earth?
2. Name three layers of earth.
3. What is the meaning of magma?
4. What is the temperature of core?
5. Name the outermost layer of earth?
6. Name the innermost layer of earth?

The earth was formed about 450 crore years ago from very vast clouds of gases and dust particles present in the form of disc around the sun. These particles of gases and dust collided and merged with each other to form small chunks of matter called planetesimals. These planetesimals then collided with each other and merged to form various planets including our earth. The gravitational force of earth was high, so it started attracting more planetesimals towards itself. When the planetesimals collided with earth, a large amount of heat gets produced. This resulted in rising the temperature of earth. Also decomposition of radioactive elements present in the earth produced enormous amount of heat. This enormous amount of heat caused the earth to melt after 80 crore years of its formation.

Layers of earth:

Subsequently, the earth started cooling and rearranged itself into three layers namely crust, mantle and core under the influence of gravity. During rearrangement, the heavy and medium weight materials (e.g. iron) sank down in the interior of earth and formed core and mantle respectively. While the lighter materials (e.g. silicon and aluminium) came up and formed crust of earth.

Test Your Understanding and Answer These Questions:

1. What are planetesimals?
2. How many years ago was earth formed?
3. Name different layers of earth?
4. What is structure of earth?
5. What are crust, mantle and core?