Satellites are the continuously orbiting objects around a bigger object, mostly a planet. Scientists from the early years have been trying to explore universe and understand the complex cosmological events. In their endeavor to explore the advanced communication techniques and predict the upcoming events by observing the universe from the space, scientists inspired by the natural satellites such as Moon, envisioned a revolutionary idea of launching artificial objects into the space known as satellites.

The idea for satellite was based on the Newton’s gravitational principles as it was observed that an object thrown at a very high speed (8 km/s) away from the earth surface starts rotating around the earth under the influence of the Earth’s gravitational pull. The satellite thus maintains its momentum tangentially but keeps falling on to the earth and therefore tracks a circular trajectory. But if the satellite is projected with an even higher velocity say above 11 km/s then it might escape away from the earth’s gravitational pull into the space. The atmosphere in the space is thin which allows the satellite to maintain its velocity and consequently, it does not fall on to the earth. The high velocity of 8 km/s was made achievable by the advanced rocket science and after a few unsuccessful attempts finally on 4^th of Oct 1957 Soviet Union launched its first artificial satellite named Sputnik-1. After the launch of 1^st satellite almost every year one country after the other launched their satellites into the earth’s orbits which totals around 10000, of those only 3000 are still functional and others have passed their term and are now reduced to solar debris.

The different technological needs gave way to the various techniques and design models of the satellites. Today we use satellites for personal, commercial, scientific research and many others ways to make our lives well-informed and comfortable. Satellites have transformed our lives, from communication to weather forecast and space research to navigation everything is controlled by satellites. The satellites are precisely placed in different orbital positions to perform the specified tasks perfectly. For example, a navigational satellite is geosynchronous in nature as for precise location tracking it needs to maintain a good line of sight to the earth surface. The earth satellite orbits are known as geocentric orbits and for the Sun and Mars, heliocentric and Areocentric respectively.  Broadly for earth, there are two types of orbits, polar orbits and geostationary orbits which are further classified into high, medium and low earth orbits. The geosynchronous orbits are the most common as it takes these satellites 24 hours to complete one revolution around the earth and the earth rotates in those 24 hours around its centre, hence, they remain in perfect sync with each other and therefore are widely used for broadcasting and communication. The low earth orbit is just a few hundred kilometres above the earth surface. The satellite is placed just outside the earth atmosphere to avoid air drag and maintain a good visibility to the earth surface at the same time. Hubble space telescope is one fine example of the low orbit satellites. Apart from that there are circular and elliptical orbits, the geosynchronous satellites are type of elliptical orbit satellites.

There are lot of satellites making companies such as Boeing and Lockheed to name a few.  The satellites are made of robust material to survive the tough atmospheric conditions as well as drastic pressure differences along its launch path.  The satellite has 4 primary parts.

Power System: This generates the required electricity for the maintenance and operation of the satellite.

Altitude control mechanism: A satellite has to monitor its altitude consistently and therefore a sound altitude sensing and controlling mechanism is employed.

Signal Antenna: For communication with other satellites, space stations and to the earth, it needs a high power receiver antenna to send and receive signals.

Payload: All the other essential stuff like food supplies, research equipments and rescue equipments come under the payload and need to be economically balanced as per the launching capacity of the rocket as per the payload.

And the launch facilities are located near equator to boost the velocity as at equator there is least gravitational pull and the satellite can be launched with less rocket fuel and with higher payloads.

The different types of satellites that have helped in enhancing our knowledge about the universe or in the form of communication, providing military support and Radio/Television entertainment are listed below, categorised according to their usage.

Astronomical Satellites: These are observational in nature and are used for sighting distant galaxies and understanding the solar events.

Communication satellites: From mobile signals to Radio wave signals, these satellites keep us connected and entertained. These are mostly geosynchronous satellites and placed in lower earth orbits.

Earth observation Satellites: These are mostly used for geographical exactness, environment monitoring and for area mapping.

Navigational Satellites: These are employed for precise GPS location monitoring and tracking. Therefore, it requires a clear line of sight to the earth surface.

Killer Satellites: These are for military usage and are used to destroy enemy spaceships and space attack machinery.

Weather Satellites: Their primary function is to monitor weather abnormalities ad forecast seasonal changes.

Space Stations: These are used for taking and placing humans in the satellite orbit to carry out the research and Observation work. One good example of this is the International space station.

Tether satellites: These are connected satellites and are coordinated in order to work together with the help of a thin cable connection.

Miniaturised Satellites: These are low mass and small size satellites. Also known as mini-satellites and these weigh around 500 – 1000kgs.

Recovery Satellites: These satellites facilitate recovery in biological forms as well as payloads on requirement basis.