Earth's Satellites

What is a Satellite?

  • An object orbiting around the sun, earth or any other colossal body is known as a satellite. There are two major types of categorization when it comes down to satellites, one is natural and the other is man-made.
  • Some of the examples of natural satellites are planets, moons, and comets. Jupiter has 67 natural satellites. The earth has one permanent natural satellite, the moon we know, which causes the tides in the sea. Sometimes other objects (like asteroids) can enter into temporary orbits of the earth and become a natural satellite for a span.
  • Apart from these, the earth has many man-made satellites which are placed in the orbit and are used for different applications in communications and information gathering. As the term itself states, an artificial satellite is one that is put in our space by human efforts and follows the orbit of natural satellites.
  • Since they have a very large view field, they can collect data a lot faster than instruments which can be used at ground level. Apart from this, their view into space beyond earth is not blocked by clouds, dust and other obscurities, due to which a satellite can view space a lot more efficiently than telescopes on earth.

Satellites

  • Currently, there are more than 2,500 man-made satellites orbiting the earth. Most of these are of Russian origin. You may wonder why none of these satellites collide with each other, considering the volume. Actually, it is quite possible for this to occur. Although care is taken to launch a satellite in specific orbits such that collisions never occur, these orbits can vary in nature. There are many international organizations in place to prevent such occurrences. However, in 2009, a couple of Russian and American satellites did collide for the first time!
  • The satellites are launched with a specific objective in mind pertaining to several uses such as communications, research in scientific areas, forecasting the weather and intelligence. Once out in the space, all the different types of satellites follow similar physics principles and are governed by the same math equations.
Based on their purpose, there are two kinds of artificial satellites. They are geostationary satellites and polar satellites.

Types of Satellites:

Geostationary Satellite:

These satellites are placed into orbit at a distance of around 35,800 km from the earth’s surface. They rotate in the same direction as the earth and one revolution of such satellites is the same as one day on earth (roughly 24 hours). This means that, as seen from earth, these satellites will appear to be at the same spot throughout. Hence, the name “geostationary” satellites.

Most common uses for geostationary satellites are communications and weather based applications.

Polar Satellite:

Polar satellites revolve around the earth in a north-south direction around the earth as opposed to east-west like the geostationary satellites. They are very useful in applications where field vision of the entire earth is required in a single day. Since the entire earth moves below them, this can be done easily. They are used in weather applications where predicting weather and climate based disasters can be done in short time. They are also used as relay stations.

Polar Satellite

The International Space Station (ISS) which was launched into orbit in 1998. It is a habitable artificial satellite and sometimes can be seen on nights with a clear sky. It functions as a lab, observatory, and a landing base for possible expeditions.

Projectile Nature of a Satellite:

  • The main thing one can understand about a satellite is that at the end of the day, they are projectiles. Any object, that only has the force of gravity acting upon it, is known as a satellite. The gravity’s force is the only thing that affects a satellite once it is launched into the orbit.

Motion of Satellites

  • To understand this concept clearly, we will use the example of launching a satellite from the top of Newton’s Mountain which is a hypothetical location well above the influence of the drag force of air. Newton was the first scientist to give the concept that if an object is launched with the adequate speed it will start orbiting the earth. This object would experience a gravitational pull that would try to pull it downwards when it travels in a horizontal direction tangentially to the earth.
  • If the launch speed is slower than the escape velocity it will fall back to the earth. The lines A and B of the diagram represent those types of projectiles.
  • If a projectile is shot off at an escape velocity with the perfect speed it will fall into an orbit outside the earth and will start revolving around the earth; the dotted line C represents such an object. If launched at a higher speed, the object will still revolve around the earth but will now have an elliptical orbit; the dotted line D represents such an object.
  • It can also be possible that the object is shot at such a speed that it escapes the gravitational pull of the earth and become a free body; the solid line E represents such an object. The objects C and D never fall back to the earth even though being pulled towards it continuously, as our earth is a circular body.

Velocity Needed for an Object to Orbit the Earth in a Circular Pattern:

This entire observation raises a very basic question, that how much velocity is necessary for shooting a body out of the earth’s lower atmosphere and establishing it into the outer one still in the range of the gravitational force. We get the answer by observing the most basic aspect of the earth, measuring its curvature. It has been measured that for every 8000 meters that one goes along the horizon of the earth, the surface dips down by about 5 meters. Thus, applying basic mathematics we get the assumption that if a projectile wants to orbit around the sun, it will have to be at such a speed that it travels 8000m for every 5m of downward fall. It was observed that if an object is launched horizontally it will fall by around 5 meters in the first second. Thus, we get to the conclusion that an object that is launched with a velocity of around 8000m/s will orbit the earth in a circular pattern. This is only applicable when the object experiences the insignificant amount of atmospheric drag. The launched object will travel at a speed of around 8000 meters in a second and will drop around 5 meters but as the earth is spherical and has a curvature that drops 5 meters every 8000 meters, the object will never touch the ground.

Stay tuned with Byju’s to learn more about satellite, satellite communication and much more.


Practise This Question

Two identical satellites are at 7R and R away from earth surface, the wrong statement is (R = Radius of earth)