Earth satellites: Types and Uses of Satellite, Geostationary & Polar Satellite, Orbital velocity- Gravitation

Any object revolving around the sun or earth is known as a satellite. Similarly, any object revolving around the earth is know as earth’s satellite. Basically, satellite has been divided into two categories: Natural Satellite and Artificial Satellite. One of the best example of natural satellite is the Moon. Let’s study about the category of satellite:

Natural Satellite

A natural satellite is a satellite that exists naturally in the sky and circles around a celestial body. It is an object that orbits around a larger body with no human involvement in its production or deployment. The moons of a planet are often regarded as a natural satellite.

Due to its rotation motion around the earth, the moon is often known as Earth’s natural satellite. Our solar system consists of 240 known moons with 163 moons orbiting around the planet, four orbiting dwarf planets, and dozens more orbiting various other solar bodies.

Artificial Satellite

Artificial refers to something that is not natural, an artificial satellite is a man-made satellite. Around 1,100 man-made satellites are orbiting the planet. Hence, a man-made spaceship that orbits the planet is known as an artificial satellite. It's made up of a computer and two solar panels that get their energy from the sun. They have cameras and other scientific gear to aid in the collection of data by scientists. For example, GOES is an artificial satellite which tells us about the weather conditions. Another example is ANIK. This artificial satellite helps to manage the communications.

History of Satellites

The first satellite, Sputnik 1, was launched by Russia in 1957. A total of 8900 satellites from 40 countries were launched following this. Around 5000 of them are still in orbit, 1500 of them are operational, while the rest have reached the end of their useful lives and are drifting across space as mere garbage. Till date India has launched approximately 100 satellites.

Depending on the purposes, these satellites are put in space at various altitudes facing various planets. 

• 2612 of currently functioning satellites are in a low orbit level (2000 km)
• 139 in a medium orbit level (20000 km)
• 562 in a geostationary orbit level (36000 km)
• The remaining 59 are in an elliptic orbit level (> 36000 km).

Uses of Satellites

The following are some of the applications of satellites:

• Forecasting the weather

Satellite image-feeds of the Earth helps to monitor climate conditions on the planet, predict extreme weather situations such as hurricanes, cyclones, storms, and successfully manage disasters.

• Military Satellites

Military satellites are used to spy on, survey, and track the enemy's space objects. They scan the space for the enemy's item and transmit photographs and other data to the host country.

• Navigation

This facility allows us to connect automobiles, employees, and everything with their owners by tracking and tracing the whereabouts of every object in the world via satellites.

• Direct-to-Home(DTH) and Radio Broadcasting:

Live TV shows can be accessed from Satellite without the usage of any cable. Transponders receive a fixed-frequency signal from the broadcaster and send it to each of our homes on a distinct frequency. Radio follows the same principle so that the broadcasts can be received from any part of the globe.

• Telephone:

Wireless telephone contact with anyone in any remote part of the globe is possible due to the presence of satellites and it works in all weather.

Types of Satellites based on their Functions

Geostationary Satellite: This satellite is an earth-orbitting satellite which revolves in the same direction as that of the earth and takes roughly 24 hours. Geostationary satellite is placed at an altitude of around 35,800 km. This satellite appears at the same spot throughout the day as it revolves with the earth and thus it got the term Geostationary. 

Polar Satellite: Polar satellites are mainly used to forecast weather and climatic conditions. This satellite revolves round the earth in a north south direction. Polar satellites are also useful in applications where the earth’s filed version is required. 

Projectile Nature of Satellites

• The most important thing to remember about satellites is that they are projectiles in their core. The term "satellite" refers to any object that is only affected by gravity. Once a satellite is launched into orbit, it is only affected by the force of gravity.

• To better comprehend this notion, consider launching a satellite from the summit of Newton's Mountain, which is a hypothetical position well above the impact of the air drag force. Newton was the first scientist to propose that if an item is launched quickly enough, it will begin to orbit the planet. When this item travels horizontally tangentially to the earth, it will experience a gravitational pull that will try to draw it downwards.
• It will fall back to Earth if the launch speed is slower than the escape velocity. Those projectiles are represented by lines A and B in the illustration.
• A projectile launched at the perfect speed will fall into an orbit outside the planet and begin circling it; the dotted line C depicts such an item. The object will still spin around the planet if launched at a faster speed, but it will now have an elliptical orbit; the dotted line D symbolizes such an object.
• The object could alternatively be launched at such a high speed that it escapes the earth's gravitational influence and becomes a free body; the solid line E symbolizes such an object. Because our earth is a spherical body, the items C and D never fall back to the earth despite being constantly attracted towards it.

How to Determine Time Period of Earth Satellite?

When a satellite moves around the earth, a centripetal force act on it. Let’s derive the expression for total time taken by a satellite for one rotation:

Here, the distance above the earth’s surface has been denoted by ‘h’

The centripetal force will act towards the center and now another gravitational force is there between earth and satellite, where

m=mass of the satellite

Me=mass of the sun

Since, satellite covers a distance of 2π ( R_e+ h ) in one revolution,

The above stated result is the total time taken by a satellite to revolve around the earth.

The Velocity Required for an Object to Circularly Orbit the Earth

This observation presents a fundamental question: How much velocity is required to propel a body out of the earth's lower atmosphere and into the outer one while remaining within the gravitational force's range? 

We find this answer by measuring the curvature of the earth, which is the most basic characteristic of the planet. The surface of the earth dips down by around 5 meters for every 8000 meters traveled along the horizon. Applying fundamental mathematics, we may deduce that if a projectile wants to orbit the sun, it must travel at a speed of 8000 meters for every 5 meters of downward fall. If an object is hurled horizontally, it will fall approximately 5 meters in the first second.

As a result, we may deduce that an object launched at a velocity of roughly 8000 meters per second will orbit the Earth in a circular pattern. This only applies when the item is subjected to a negligible degree of air drag. The launched object will fly at a speed of roughly 8000 meters per second and drop around 5 meters, but the object will never contact the ground since the earth is spherical and has a curvature that lowers 5 meters every 8000 meters.