Satellites are the objects that have capable of exerting gravitational force. Artificial satellites are sent in the space for the study of many other planets including Earth. Some satellites can be used for communication signals, accurate surveys, weather report and other experiments. Natural satellites are the small bodies that revolve round the planet. The satellite of earth is Moon. Every planet except Mercury and Venus has their own satellite that revolves them. In July 2004, 101 satellites were discovered as the number of satellites present in our solar system. Earth has 1; Mars has 2, Jupiter 38, Saturn 30, Uranus 21, Neptune 8, and Pluto 1.The orbits of planets and satellites are different and sometimes they get very close to each other. It is easy to distinguish between the orbits of regular satellite and irregular satellites. Regular satellites have circular orbit and irregular satellites have elliptical or highly inclined orbits. It is said that the Earth’s Moon is a special case. The meteor of Mars sized collapsed with Earth that ejected the material that first formed a ring around the Earth and then coalesced to form the Moon. Pluto’s Charon is also said to have formed through similar collision.
Artificial satellites can be treated as the spacecraft that is set in the orbit about any planet. In most of the cases the spacecrafts are used to study about the Earth. These spacecrafts are used for observation, research or communication in space. The space shuttle travel in lower Earth orbits about 160 km above Earth surface and has orbital periods of 90 min. These satellites have lifetimes of days, weeks, months, or years, depending on their altitudes, their mass-to-drag ratios, and atmospheric drag variations caused by solar activity.
Earth is not a perfect sphere but its oval in shape. Its equatorial diameter is 42 km greater than its polar diameter. It is because of the rotation of the Earth. For low Earth orbits (LEO) satellites at altitudes below 6000 km causes the orbital plane to rotate eastward at exactly one revolution per year. The equatorial bulging is the result of permitting the plane of Sun synchronous orbits when viewed from Sun. If this Sun synchronous satellite crosses the equator in Brazil at 10:00 a.m. on January 1, it will also do so on June 30 or on any other day. The orbital plane remains fixed relative to Earth-Sun axis so the equatorial crossing time occurs at same time at any other longitude.
The rotation of Earth according to the fixed stars is 23 h 56 min 4 s. The satellite orbit having the same rotation period is called geosynchronous orbit. If the orbit of satellite is circular or equatorial then it is said to be geostationary as it remains fixed for the observer in the Earth. Its advantage is that during communication, once pointed geosynchronous orbit spacecraft does not requires re pointed. Further the GEO arc is used for weather report observation spacecraft. The three spacecrafts placed along the equator can scan continuously severe weather around the whole globe excluding the North and South poles where hurricanes and tornadoes are absent. There are many kinds of spacecrafts in the space. Some of them are-
· Communication satellite
· Navigation satellite
· Weather satellite
· Military satellite
· Scientific satellite
In 1962, the National Aeronautics and Space Administration (NASA) launched the first telephone and television satellite. Till today over 400 communication satellites has been launched. Navigational satellites are useful to find and locate the exact position by trapping the signal from the space. Any ship or plane lost can be found by such satellites. The weather satellite has special cameras that captures the position of clouds and warns us about the weather report. The military satellite is useful to locate the position of enemies and their hidden place. Whereas the scientific satellites captures the pictures of Earth and other planets for further study about the planets.
Every spacecrafts differs in shape and size. They are designed in such a way to fulfill the mission requirements whether it is communication satellite or weather or any other. The principal constraints are imposed by the dimensions and shape of the satellite payload provisions of the launch vehicle. There are some requirements of spacecrafts- one is the solar cell to charge its battery and continue its work at the time of solar eclipse too. The other is to have stabilization and control so that sensors and antennas can be directed towards required position as per the mission given.
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