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ARTIFICIAL SATELLITES
Definition:-
A satellite is basically an object which revolves around another object of a relatively larger size… they can be natural or artificial..
What we are going to discuss are Artificial Satellites..
NASA describes An artificial satellite as a manufactured object that continuously orbits Earth or some other body in space.
Most artificial satellites orbit Earth.
History of Artificial Satellites…
Artificial Satellites could be said as a useful by-product of the cold war. In 1955, the United States and the Soviet Union announced plans to launch artificial satellites…
On Oct. 4, 1957, the Soviet Union launched Sputnik 1, the first artificial satellite. It circled Earth once every 96 minutes and transmitted radio signals that could be received on Earth.
On Nov. 3, 1957, the Soviets launched a second satellite, Sputnik 2. It carried a dog named Laika, the first animal to soar in space.
The United States launched its first satellite, Explorer 1, on Jan. 31, 1958, and its second, Vanguard 1, on March 17, 1958.
The first living organism to travel in a spacecraft was a bitch named Laika…which went aboard Sputnik II..the second artificial satellite launched by Soviet Union
Sputnik II with Laika
Explorer I
The 1960s saw rapid increase in the number of spacecrafts being launched by US and Soviet Union which lead to ,what is known today , THE SPACE RACE…In August 1960, the United States launched the first communications satellite, Echo I. This satellite reflected radio signals back to Earth. In April 1960, the first weather satellite, Tiros I, sent pictures of clouds to Earth. The U.S. Navy developed the first navigation satellites. The Transit I B navigation satellite first orbited in April 1960. By 1965, more than 100 satellites were being placed in orbit each year.
Since then about 8000 artificial satellites have been launched till date…
The life and death of a satellite
STEP –I : Building a satellite
Every satellite carries special instruments that enable it to perform its mission. For example, a satellite that studies the universe has a telescope. A satellite that helps forecast the weather carries cameras to track the movement of clouds.
In addition to such mission-specific instruments, all satellites have basic subsystems, groups of devices that help the instruments work together and keep the satellite operating. For example, a power subsystem generates, stores, and distributes a satellite's electric power. This subsystem may include panels of solar cells that gather energy from the sun. Command and data handling subsystems consist of computers that gather and process data from the instruments and execute commands from Earth.
A satellite's instruments and subsystems are designed, built, and tested individually. Workers install them on the satellite one at a time until the satellite is complete. Then the satellite is tested under conditions like those that the satellite will encounter during launch and while in space. If the satellite passes all tests, it is ready to be launched
STEP-II : Launching the satellite
Space shuttles carry some satellites into space, but most satellites are launched by rockets that fall into the ocean after their fuel is spent. Many satellites require minor adjustments of their orbit before they begin to perform their function. Built-in rockets called thrusters make these adjustments. Once a satellite is placed into a stable orbit, it can remain there for a long time without further adjustment.
STEP III: Performing the mission
Most satellites operate are directed from a control centre on Earth. Computers and human operators at the control centre monitor the satellite's position, send instructions to its computers, and retrieve information that the satellite has gathered. The control centre communicates with the satellite by radio. Ground stations within the satellite's range send and receive the radio signals.
A satellite does not usually receive constant direction from its control centre. It is like an orbiting robot. It controls its solar panels to keep them pointed toward the sun and keeps its antennas ready to receive commands. Its instruments automatically collect information.
Satellites in a high altitude, geosynchronous orbit are always in contact with Earth. Ground stations can contact satellites in low orbits as often as 12 times a day. During each contact, the satellite transmits information and receives instructions. Each contact must be completed during the time the satellite passes overhead -- about 10 minutes.
If some part of a satellite breaks down, but the satellite remains capable of doing useful work, the satellite owner usually will continue to operate it. In some cases, ground controllers can repair or reprogram the satellite. In rare instances, space shuttle crews have retrieved and repaired satellites in space. If the satellite can no longer perform usefully and cannot be repaired or reprogrammed, operators from the control centre will send a signal to shut it off.
STEP IV: Falling from orbit
A satellite remains in orbit until its velocity decreases and gravitational force pulls it down into a relatively dense part of the atmosphere. A satellite slows down due to occasional impact with air molecules in the upper atmosphere and the gentle pressure of the sun's energy. When the gravitational force pulls the satellite down far enough into the atmosphere, the satellite rapidly compresses the air in front of it. This air becomes so hot that most or all of the satellite burns up.
Fig.: A satellite burning up while falling from the space
Types Of Artificial Satellites:-Artificial satellites are classified on the basis of their mission…
Scientific Research Satellites...
Scientific research satellites gather data for scientific analysis. These satellites are usually designed to perform one of three kinds of missions. (1) Some gather information about the composition and effects of the space near Earth. They may be placed in any of various orbits, depending on the type of measurements they are to make. (2) Other satellites record changes in Earth and its atmosphere. Many of them travel in sun-synchronous, polar orbits. (3) Still others observe planets, stars, and other distant objects. Most of these satellites operate in low altitude orbits. Scientific research satellites also orbit other planets, the moon, and the sun.
II. Weather Satellites…
Weather satellites help scientists study weather patterns and forecast the weather. Weather satellites observe the atmospheric conditions over large areas.
Some weather satellites travel in a sun-synchronous, polar orbit, from which they make close, detailed observations of weather over the entire Earth. Their instruments measure cloud cover, temperature, air pressure, precipitation, and the chemical composition of the atmosphere. Because these satellites always observe Earth at the same local time of day, scientists can easily compare weather data collected under constant sunlight conditions. The network of weather satellites in these orbits also function as a search and rescue system. They are equipped to detect distress signals from all commercial, and many private, planes and ships.
III. Communications Satellites..
Communications satellites serve as relay stations, receiving radio signals from one location and transmitting them to another. A communications satellite can relay several television programs or many thousands of telephone calls at once. Communications satellites are usually put in a high altitude, geosynchronous orbit over a ground station. A ground station has a large dish antenna for transmitting and receiving radio signals. Sometimes, a group of low orbit communications satellites arranged in a network, called a constellation, work together by relaying information to each other and to users on the ground. Countries and commercial organizations, such as television broadcasters and telephone companies, use these satellites continuously.
A communications satellite, such as the Tracking and Data Relay Satellite (TDRS) shown here, relays radio, television, and other signals between different points in space and on Earth.
IV. Navigation Satellite….
Navigation satellites enable operators of aircraft, ships, and land vehicles anywhere on Earth to determine their locations with great accuracy. Hikers and other people on foot can also use the satellites for this purpose. The satellites send out radio signals that are picked up by a computerized receiver carried on a vehicle or held in the hand.
Navigation satellites operate in networks, and signals from a network can reach receivers anywhere on Earth. The receiver calculates its distance from at least three satellites whose signals it has received. It uses this information to determine its location.
V. Military Satellites…
Military satellites include weather, communications, navigation, and Earth observing satellites used for military purposes. Some military satellites -- often called "spy satellites" -- can detect the launch of missiles, the course of ships at sea, and the movement of military equipment on the ground.
And some of them even carry weapons like missiles and have the capability to attack deep within enemy territory..
A MILSTAR satellite in orbit..
VI. Earth Observing Satellites…
Earth observing satellites are used to map and monitor our planet's resources and ever-changing chemical life cycles. They follow sun-synchronous, polar orbits. Under constant, consistent illumination from the sun, they take pictures in different colours of visible light and non-visible radiation. Computers on Earth combine and analyze the pictures. Scientists use Earth observing satellites to locate mineral deposits, to determine the location and size of freshwater supplies, to identify sources of pollution and study its effects, and to detect the spread of disease in crops and forests.
India In Space…
Aryabhata was India's first satellite, named after the great Indian astronomer of the same name. It was launched by the Soviet Union on 19 April 1975 from Kapustin Yar using a Cosmos-3M launch vehicle.
Aryabhata was built by the Indian Space Research Organization (ISRO) to conduct experiments related to astronomy. The 96.3 minute orbit had an apogee of 619 km and a perigee of 563 km, at an inclination of 50.7 degrees. Aryabhata was built to conduct experiments in X-ray astronomy, aeronomics, and solar physics.
The spacecraft was a 26-sided polygon 1.4 m in diameter. All faces (except the top and bottom) were covered with solar cells.
A power failure halted experiments after 4 days in orbit. All signals from the spacecraft were lost after 5 days of operation. The satellite re-entered the Earth's atmosphere on 11 February 1992.
Several other satellites have been launched since then namely the INSATs, IRS, the Chandrayana Mission etc..
Launch Vehicles built by the ISRO such as PSLV, GSLV have proved to be hugely successful in the commercial field.
A Few Satellites Worth Mentioning..
APOLLO 11
The  Apollo 11  mission was the first manned mission to land on the Moon. It was the fifth human Spaceflight of  Project Apollo and the third human voyage to the Moon or Moon orbit .
Launched on July 16, 1969, it carried Mission Commander Neil Alden Armstrong, Command Module Pilot Michael Collins, and Lunar Module Pilot Edwin Eugene 'Buzz' Aldrin, Jr. On July 20, Armstrong and Aldrin became the first humans to land on the Moon, while Collins orbited above.
Hubble Space Telescope
The Hubble Space Telescope (HST) is a space telescope that was carried into orbit by the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble.
Although not the first space telescope, the Hubble is one of the largest and most versatile, and is well-known as both a vital research tool and a public relations boon for astronomy. 
The HST is a collaboration between NASA and the European Space Agency
HST is the only space telescope that works in the visible and the ultraviolet range of light..
International Space Station
The International Space Station (ISS) is an internationally developed research facility currently being assembled in Low Earth Orbit. On-orbit construction of the station began in 1998 and is scheduled to be complete by 2011, with operations continuing until at least 2015. The ISS orbits at an altitude of approximately 350 kilometres (220 mi) above the surface of the Earth, travelling at an average speed of 27,724 kilometres (17,227 mi) per hour, completing 15.7 orbits per day. The station can be seen from the Earth with the naked eye, and, as of 2009, is the largest artificial satellite in Earth orbit, with a mass larger than that of any previous space station.
The ISS is a joint project among the space agencies of the United States (National Aeronautics and Space Administration—NASA), Russia (Russian Federal Space Agency—RKA), Japan (Japan Aerospace Exploration Agency—JAXA), Canada (Canadian Space Agency—CSA) and ten European nations (European Space Agency—ESA). The Brazilian Space Agency (AEB) participates through a separate contract with NASA. The Italian Space Agency (ASI) similarly has separate contracts for various activities not done within the framework of ESA's ISS projects (where Italy also fully participates). China has reportedly expressed interest in the project, especially if it would be able to work with the RKA, although as of 2009 it is not involved due to objections from the United States.
The ISS has been continuously staffed since the first resident crew, Expedition 1 , entered the station on 2 November 2000. This has provided an uninterrupted human presence in space for the last 8 years, 299 days.
Major Components of an Artificial Satellite and their Functions…
Flight computer and I/O Processor: Command and data ...This is the part of the satellite that controls all of the satellite's functions, like the satellite's brain. The flight computer directs the satellite's activity and the I/O processor sends control data to and from the flight computer.
Star Trackers and Reaction Wheels: Pointing control... Pointing control steers the satellite and consists of sensors to avoid collisions. The preciseness of the pointing control depends on the function of the satellite -- some satellites need to be in an exact altitude, while others can be but in a general area.
High-gain Antennas, Transmitter/Receiver, and Omni Antennas: Communications...The part of the satellite that communicates with Earth. Humans on the ground also use the communications system to convey instructions to the flight computer. In addition, pictures and other data collected by the satellite are sent back to Earth using the communications system.
Solar Arrays and Battery: Power supply..Solar arrays make electricity from solar power, batteries store the electricity, and distribution units send the power to where it is needed.
Digital Camera and Image Sensor: Mission payload... The payload includes everything the satellite may need for its mission, i.e. a communications satellite would have antenna reflectors to send telephone and TV signals in its payload.
Thermal Blanket: Thermal control... Thermal control protects the satellite's delicate electronics from the extreme temperature changes from sun to shade: 180 degrees above zero to 120 degrees below zero respectively.
Importance Of Artificial Satellites…
Artificial Satellites offer a wide range of possibilities for the future. With our changing weather and natural disasters like Hurricane Katrina and the Tsunami, and a bad hurricane season expected almost every year Artificial Satellites can help us to prepare for these hurricanes and other natural disasters far in advance to better protect our communities.
Weather satellites can predict such weather, and early notice can be what we need to save areas that will be targeted by these disasters.
In addition, solar power satellites would provide an inexhaustible energy source without our having to worry about using all of our natural resources, such as oil.
We can’t think about Communication without these satellites..
Researches in which the specimen needs to be isolated can be done aboard artificial satellites..as well as those pertaining to astronomy.
Remote Sensing is one of the areas where satellites have been extensively used.
Military Forces use satellites to keep an eye on movements of enemies.
In conclusion, Artificial Satellites can help to predict natural disasters to help us better protect targeted areas and to use a clean energy source.
The value of Artificial Satellites will go way up as they help us face the challenges that will arise in the future.