When looked at the night sky many heavenly bodies like stars, moon, satellites, etc are observed in the sky. Satellites are small objects revolving or orbiting around a planet or on object larger than it. The most commonly observed and known satellite is the moon, the moon is the satellite of Earth, as the moon orbits around the earth in its fixed orbit.
Moon as the Satellite of Earth
Satellite
Satellites are objects that either occur naturally or man-made that used to go around a planet that is larger than it and rotates in its axis. There are various satellites including the moon present in the solar system. Therefore, on the basis of existence satellites are divided into two main categories:
Natural Satellites
Any celestial body present in space that revolves around a planet larger than it in a fixed orbit is called the natural satellites. The solar system consists of six planetary satellite system out of which 205 satellites are known to be natural satellites. These satellites are analogously larger and orbit in larger radii. Their surface are generally made up of various gases or Rocky Mountains.
e.g. The moon is the natural satellite of Earth similarly, Earth, Jupiter and Venus orbits around the Sun in their fixed orbit.
Artificial Satellites
Any object that people have made and launched into orbit using rockets is called the artificial satellite. Presently, there are over a thousand active satellites orbiting the Earth. The satellite’s size, altitude and design depend on the use of the satellite. These satellites are also fitted with delicate instruments and cameras, made to rotate around planets in pre-fixed orbits and are launched by rockets from the Earth.
e.g. Sputnik-I, launched on 4 October 1957, by the Soviet Union launched as the world’s first artificial satellite. After that USA send Explorer 1. Since then, almost 8,900 satellites from more than 50 countries have been launched. According to an estimate, some 5,000 satellites remain in the orbit. Out of these 5000, about 1,900 were operational, while the rest have exceeded the numbers of their useful lives and become space debris now. These satellites are useless and will revolve until their destruction.
However, Aryabhatta was India’s first satellite launched on 19 April 1975, after that many artificial satellites like INSAT, IRS, Edusat, GSat, Chandrayaan etc were launched in later days. Some other examples of artificial satellites on the basis of their uses are: GOES i.e. a weather satellite, ANIK i.e. a communication satellite, GPS i.e. a navigation satellite, TERRIERS i.e. a scientific satellite and MILSTAR i.e. a military satellite.
History of Artificial Satellites
The first-ever artificial satellite was successfully launched by the Soviet Union on October 4, 1957. This satellite was called Sputnik-I. Sputnik-I has weighed 183 pounds and about the size of a cricket ball. It took 98 minutes to orbit the earth. The launch of this satellite, Sputnik-I has been named as the start of the space age and the start of the US-USSR space race which spanned over the years of the 1960s. Another satellite launched on November 3rd in the same year when the Soviet’s launched Sputnik-II. Sputnik-II carried a much heavier payload along with a dog named Laika.
Immediately after Sputnik-I & II, the US Defense Department-funded another satellite project run by a man named Von Braun. Von Braun, his team and the US Army at Redstone worked on the Explorer Project. Explorer I was launched on January 31st. It consists of a small, scientifically important payload that is used to discover the belts of magnetic radiation around the earth. These were named after the principal investigator James Van Allen. This program continued to create a series of successful lightweight scientific spacecraft.
Types of Artificial Satellites
On the basis of the direction of the motion of the satellites and the distance from the Earth’s surface, the artificial satellites are broadly classified into two;
1. Geostationary Satellites: The artificial satellites that rotates in the same direction as the earth in their pre-fixed orbits which is around 35, 800 km away from the Earth’s surface are called the geostationary or geosynchronous satellites.
2. Polar Orbiting Satellites: The artificial satellites that orbit around the north-south orbit passing over the north and the South Pole and is approximately 500-800 km away from the Earth’s surface are called the Polar satellites.
Geostationary Satellites
Geostationary Satellites
- These satellites are high orbit satellites that orbit around the earth in approximately 36000 km away orbits.
- The geostationary satellites are called so because geostationary satellite revolves in an equatorial linear orbit due to which they orbit around the Earth at the same rate as the Earth rotates so that the satellites are over the same spot on Earth all the time and so appeared stationary in reference to the Earth.
- Also, the revolutionary time period of geostationary satellites is around 24 hours which is equal to the rotational period of planet Earth.
- These satellites are generally used as communication and weather satellites.
- Some examples of such satellites are INSAT of India, Geostationary Operational Environmental Satellite (GEOS) of USA, Himawari of Japan, Meteosat of Europe etc.
Polar Satellites
Polar Satellites
- These satellites are low orbit satellites that orbit around the earth in approximately 500-800 km away orbits.
- A polar-orbiting satellite passes near to or directly over the pole on each orbit. With the earth turning underneath it this means that the polar-orbiting satellite sees a different bit of the earth each time.
- Also, the revolutionary time period of geostationary satellites is around 24 hours which is equal to the rotational period of planet Earth.
- Such satellites are used to study the universe, help forecast the weather, transfer telephone calls over the oceans, assist in the navigation of ships and aircraft, monitor crops and other resources, and support military activities.
Applications of Artificial Satellites
Some important uses of artificial satellites are:
- Telecommunication: The satellites are used for telecommunication purposes through telephone, television, mobile phones, internet services, etc. where signals are received from different locations across the globe and transmit to other parts of the world.
- Monitoring: This is used to secure important information about geological and meteorological areas even agriculture monitoring i.e. crop production, disease and failure in particular areas.
- It helps scientists to stay updated about droughts and food production and to estimate the loss from these calamities.
- Artificial satellites help in discovering underground water reserves and contribute to water management.
- It also helps in identifying the exact location of an airplane, a ship, a person, and an object.
Sample Problems
Problem 1: Differentiate between natural and artificial satellites.
Solution:
The differences between natural and artificial satellites are,
Natural Satellites
Artificial Satellites
1. These satellites are naturally occurring objects. They are man-made. 2. They can not be controlled by humans and so have no use. They can be controlled by humans and so have wide applications. 3. The natural satellites can not communicate on earth or with other planets. The artificial planet can communicate with instruments on earth. 4. The natural satellite is made up of natural materials like gases, rock, minerals, water, dust etc. The artificial satellite is made up of metal and other electronics materials. 5. The natural satellites like Planets are opaque bodies with no light of their own. They also receive heat and light from sun like moon. The artificial satellites are objects humans propel from earth in order to orbit around the earth, the electrical power required by satellite is provided by panels of solar cells and small nuclear reactors.
Problem 2: How both geostationary and polar-orbiting satellites are different from each other.
Solution:
The difference between the geostationary and polar satellites are as follows:
Geostationary Satellites
Polar-orbiting Satellites
1. Geo-stationary satellites revolve in equatorial orbits. Polar satellites revolve in polar orbits of the earth. 2. These satellites complete one circle of the earth in 24 hours. These satellites take less time to complete one circle of earth. 3. These satellites are placed at a height of 36000 km from the Earth surface. These satellites are placed at a height of 500-800 km from Earth surface. 4. The main use of this satellites is in telecommunication and weather forecasting. This satellite is used for remote sensing, weather, science and environment related studies.
Problem 3: List five indian artificial satellites.
Solution:
India’s first satellite was Aryabhatta. It was built by ISRO and launched by the Soviet Union on 19 April 1975. It was named after the mathematician Aryabhata. In 1980, Rohini was the first ever satellite to be placed in orbit by an Indian-made launch vehicle, SLV-3. Some other Indian satellites are INSAT, IRS, Kalpana-1, EDUSAT, etc.
Problem 4: What is satellite communication?
Solution:
The technique of transferring data or information from one place to another through a communication satellite is called the satellite communication. The communication satellite is an artificial satellite that is responsible for transmission of signals.
Problem 5: Name some more types of satellites on the basis of their applications.
Solution:
On the basis of the applications of satellites in different fields, the satellites are categorized as:
- Navigation satellites,
- Communication satellites,
- Weather satellites,
- Military satellites,
- Earth Observation satellites,
- Astronomical satellites,
- International Space Station,
- Remote Sensing satellites,
- Navigation satellites,
- Global Positioning satellites etc.