Satellites

A satellite is anything that orbits a bigger object. For example, Earth orbits the sun, making it a satellite. Similarly, the moon orbits Earth, so it is also a satellite. Both Earth and the moon are natural satellites.

However, when most people mention "satellite," they are usually referring to man-made satellites. These are devices created by humans and sent into space to circle around Earth or other celestial bodies.

Thousands of these man-made satellites exist. Some capture images of our planet, others take pictures of different planets, the sun, and various space objects. These images help scientists understand more about Earth, our solar system, and the universe. Additionally, some satellites are used to transmit TV broadcasts and telephone calls globally.

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Artificial Satellites

A man-made object revolving around a planet is called an artificial satellite.

The world's first artificial satellite, the Sputnik 1, was launched by the Soviet Union in 1957. Since then, thousands of satellites have been launched into orbit around the Earth; also some satellites, notably space stations, have been launched in parts and assembled in orbit. 

An Artifical Satellite : Sources NASA

Artificial satellites are used for various purposes. Depending on their use, the satellite may be further classified as:

Communication Satellite

A communication satellite is a geostationary satellite which revolves around the earth at a height of about 36,000 km with a period of revolution exactly equal to 24 hours so that its position with respect to earth remains stationary.

Remote sensing Satellite

These collect information about objects without actual physical contact with that object. These are placed in sun-synchronous orbits around the earth.

In a sun-synchronous orbit, the satellite passes over a particular area of the earth at approximately the same local time so that whenever the satellite passes over this particular region of earth, the position of sun with respect to that region remains approximately the same.

In order to understand the applications of these satellites, it is important to understand the characteristics of their orbits.

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Role of Artificial Satellites

1. Satellite pictures help us in forecasting weather.
2. Communications satellites in space allow us to watch events around the world “live” on our television sets.
3. These satellites also allow us to make instant phone calls overseas.
4. Satellite pictures also point out areas such as mineral deposits.
5. Some satellites gather information about outer space.
6. Certain satellites monitor the ultraviolet radiations coming from space.
7. Satellites are also used to gather military information / intelligence.
8. Some of the satellites sent up by India are:

Orbit of a Satellite

A satellite always moves in a fixed plane. This is called the orbital plane, and in the case of a satellite orbiting the Earth this plane always passes through the centre of the Earth.

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The following variables characterize the flight of a satellite, and they are known as its orbital elements:

• Apogee: The point at which satellite in an Earth-centered orbit is farthest to the Earth’s surface is known the apogee.
• Perigee: The point where a satellite in an Earth-centered orbit is closest to the Earth’s surface is known as the perigee.
• Orbital inclination: The angle between the plane of the orbit of a satellite and the plane of the equator is called inclination of the orbit.

Apogee, Perigee and Inclination are the three parameters which define or characterize an orbit

Types of Orbits

If the orbital inclination is zero degrees, the satellite’s orbit is following the Earth’s equator.

A larger orbital inclination means an orbit nearer the Earth’s poles. So an orbit with an inclination of 90o will be a “polar orbit” passing over the Earth’s North and South Poles.

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Geostationary Orbit

A satellite that appears to remain in the same position above the Earth is called a “geostationary satellite.”

The orbit is circular and its inclination is 0o, which means that it is above the Earth’s equator. The altitude is approximately 36,000 km, and the satellite travels at 3 km/s.

The satellite’s orbital period is about the same as the earth’s rotation period, roughly 24 hours. Many weather observation satellites and broadcast satellites use this kind of orbit.

Applications of geostationary orbit:

1. Global or long distance communications such as TV transmission, radio signal beaming, long distance telephone calls, telex, fax, etc.
2. Weather monitoring, and forecasting.

Polar Orbit

An orbit passing through the sky near the North and South Poles is called a polar orbit.

An orbit with an inclination of 90o, or close to it, is called a “polar orbit.” Because the earth is rotating as the satellite follows a polar orbit, the satellite can survey the whole of the earth’s surface, including the poles, in a few days.

Many observation satellites that need to cover the entire earth are in polar or near-polar orbits.

Sun-synchronous Orbit

The satellite’s orbital plane and the sun’s direction (orientation) are always the same. In a sun-synchronous orbit, the direction of rotation of the orbital plane and its period (the rotation angle per day) are the same as the earth’s orbital period (the earth’s rotation angle per day). The whole orbital plane of a satellite going around the earth completes one revolution in one year.The sun-synchronous orbit is a polar orbit, but with an inclination larger than 90^o

If the inclination were 90^o, then the satellite’s orbital plane would not complete a full rotation. This orbital inclination varies with the satellite’s altitude. For example, in the case of a circular orbit with an altitude of 800 km, an orbital inclination of 98.4o gives a Sun-synchronous orbit.

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Applications of satellites in Sun-synchronous orbit

It enables the remote sensing cameras of each satellite to take photographs of a particular region of the earth with nearly the same illumination every time it passes over that region. In this way, repeated survey of vast, even inaccessible areas of the earth can be carried out in a very short-time.