# What is Artificial Satellite?

In simple words, if an object in space revolves around a star we call it a planet and if it revolves around a planet then we call it a satellite. When it comes to the classification of satellites, it is divided into two main categories, one is natural satellites and the other is artificial satellites

We all know that the satellites that are found in nature are called natural satellites. For example, the moon is the only natural satellite of the planet earth, similarly, Jupiter has 67 such natural satellites, and so on.

But all the satellites that are created by humans are called artificial satellites. In this tutorial, we will uncover the details of artificial satellites. So let's start...

As we discussed earlier the motion of various planets or satellites can be determined by Newton's law of motion and Newton's law of gravitation. From this scientists believe that, if any object launched from the earth can be given the proper speed at the proper distance from the earth, then the object will orbit the earth like the moon.

Putting this idea into practice made it possible to launch the first artificial satellite Sputnik-I on 4th October 1957. Today technology has advanced so much that it is possible to place artificial satellites around not only the earth but also other planets.

A satellite is launched vertically or eastwards from the earth's surface by means of a rocket. With the help of rockets arranged behind the satellite, the direction of its motion is changed in such a way that the satellite can attain a certain level of horizontal velocity after reaching a certain height. Then it is placed in a certain orbit and starts to circle the earth.

## Applications of Artificial Satellites

Basically, artificial satellites are used for - 1. Communication 2. Weather forecasting 3. Military use for spying on enemy troops, and 4. Collection of information about the natural resources of the earth, other planets, and outer space.

## Types of Artificial Satellites?

Based on the satellite's direction of motion and distance from the earth's surface, artificial satellites are classified into two categories - Geostationary satellites and Polar satellites.

Geostationary Satellite:

Artificial satellites that revolve in the same direction as earth in their pre-determined orbits about 35,800 km above the earth's surface are called geostationary or geosynchronous satellites

Some examples of such satellites are INSAT (Indian National Satellite System), GEOS (Geostationary Operational Environmental Satellite) of USA, Meteosat of Europe, etc.

These satellites are called high-orbit satellites. Basically, they are used for communication, and weather forecasting like measuring cloud top temperatures, water vapor,  measuring land temperature, and facilitating cyclone path prediction, etc.

Polar Satellite:

Artificial satellites that orbit the earth in a north-south orbit crossing the north and south poles and are about 500-800 km above the earth's surface are called polar satellites.

For example, Landsat, NOAA, SPOT, and ERS are a few examples of polar satellites.

These satellites are called low-orbit satellites. Basically, they 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, support military activities, etc.

Actually, any artificial satellite obeys Kepler's law. Hence, the orbit of an artificial satellite may also be elliptical or circular like the orbit of a planet around the Sun or the orbit of a satellite around a planet.

However, since the eccentricity of the orbits of artificial satellites is quite low in most cases, assuming the orbits are circular does not lead to significant errors in the calculations.

## Orbital Velocity of Artificial Satellites

The velocity at which the satellite orbits the earth is called the orbital velocity of the satellite. Now we will determine the formula for the orbital velocity of an artificial satellite orbiting the earth.

So let us consider the mass of the earth as capital M, the radius of the earth capital R, the mass of the artificial satellite as small m, the orbital velocity of the satellite as small v, and the height of the orbit from the surface of the earth is small h

Hence the distance of the satellite from the center of the earth i.e. the radius of the orbit r = R + h

Let's assume the orbit to be circular then the centripetal force

${\color{Red} \frac{mv^{2}}{r}=\frac{mv^{2}}{R+h}}$

In fact, the gravitational force between the earth and the satellite provides this centripetal force for orbiting the earth. From Newton's law of gravitation, we know the gravitational force

${\color{Red} F=\frac{GMm}{r^{2}}}$

${\color{Red} \therefore \;\frac{mv^{2}}{r}=\frac{GMm}{r^{2}}}$

${\color{Red} Or\;v^{2}=\frac{GM}{r}=\frac{GM}{R+h}}$

${\color{Red} Or\; v=\sqrt{\frac{GM}{R+h}}\to (1)}$

This is the formula for the orbital velocity of an artificial satellite orbiting the earth.

We also know the acceleration due to gravity at the surface of the earth

${\color{Red} g=\frac{GM}{r^{2}}}$

${\color{Red} Or\;GM=gR^{2}}$

Now by substituting the value of GM in equation number (1) we get

${\color{Red} v=\sqrt{\frac{gR^{2}}{R+h}}}$

${\color{Red} Or\;v=R\sqrt{\frac{g}{R+h}}}$

This is also the formula for the orbital velocity of an artificial satellite orbiting the earth.

## Period of Revolution of Artificial Satellites

The time it takes to travel one complete orbit around an object is called the period of revolution of artificial satellites.

If the period of revolution of the satellite = T

Distance traveled by the satellite = circumference of the orbit = 2𝜋r = 2𝜋(R+h).

${\color{Red} \therefore \;T=\frac{2\pi (R+h)}{v}\to (2)}$

Now substituting the value of v in equation no (2) we get

${\color{Red} T=\frac{2\pi (R+h)}{R\sqrt{\frac{g}{(R+h)}}}}$

${\color{Red} Or\;T=\frac{2\pi }{R}\sqrt{\frac{(R+h)^{3}}{g}}}$

This is the formula for the period of revolution of artificial satellites.

## Weightlessness in Artificial Satellites

Astronauts always feel weightless in an artificial satellite of the earth. But on a natural satellite like the Moon, astronauts do not experience weightlessness, because the Moon has more mass and exerts a gravitational force on them.

In artificial satellites, the mass of the satellite is less, so it exerts a less gravitational force on the object due to which the astronaut feels weightless.

But it must be remembered that the attractive force of the earth acting on the artificial satellite or the astronaut inside it can never be zero.

If this force were zero there would be no question of the satellite orbiting the earth, because then it would not be possible to provide the centripetal force necessary for orbiting.

So an object being weightless means that the reaction force acting on it is zero, but the gravitational force is not zero.