The moon and all the artificial satellites are constantly falling towards Earth just like a ball falls towards ground when thrown up. This gives you an idea of how amazing and yet astonishing the satellite motion is. Satellites are based on simple classical physics with some aspects from the Relativity theory of Albert Einstein.
The first question that needs to be answered precisely is “What is a satellite?”. So, in physics, any object that is orbiting Earth, sun, or any other cosmological planet. Satellites can be natural, such as moons. Many artificial (man-made) satellites have also been developed like the GPS satellites or the International Space Station (ISS). They are used for the purpose of communication, research, spying, weather forecasting etc. Here we are to explore the science behind its motion .
THE START OF A SATELLITE MOTION
In order to understand satellite motion, consider the motion of projectile. Just as projectile, satellite is given initial force and the only force controlling its motion will be gravity.
Imagine going to a very tall mountain and throwing the projectile horizontally from the top. The projectile will travel horizontally, but due to the gravity it will be pulled down and will ultimately follow a curved path. Now consider throwing the satellite at greater speed. It will obviously travel farther. At this point remember that Earth is roughly a sphere and has a curvature of its own. Now increase the speed of projectile, it will go farther and farther away. Ultimately a point will come when curvature of projectile will be equal to curvature of Earth. The projectile will be so fast, it will travel so forward that the point it drops, the Earth have curved away.
This is the start of satellite motion, a projectile moving so fast that it is constantly falling towards the Earth but when it drops the Earth would have curved away. This projectile will be moving will a constant horizontal velocity and an acceleration of “g” towards Earth in a curved path. And bang!! This is your satellite.
THE LAUNCH SPEED
One may ask a question that how fast do I have to throw an object so that it stats to orbit the planet? The answer to this question emerges from the curvature of the Earth. For every 8000 meters measured along the horizon of the earth, the earth’s surface curves downward by approximately 5 meters.
Thus, for a satellite to orbit, it is projectile motion should be such that, it should travel 8000 meters horizontally for every 5 meters of downwards vertical fall. And the projectile falls 5 meter in the first second of the launch, so it must travel 8000 meters horizontally in that first second. Consequently, the speed of satellite should be 8000m/s, to allow it to orbit the Earth near the surface. If the speed is greater than this, the satellite will take an elliptical orbit.
ALTITUDE AND ORBITAL VELOCITY
The orbital velocity depends upon the altitude (height above Earth’s surface). The nearer the satellite is to the surface, the stronger the gravitational pull and thus consequently more speed is required to maintain the orbital motion. At an altitude of 124 miles (200 kilometres), the required orbital velocity is a little more than 17,000 mph (about 27,400 kph). To maintain an orbit that is 22,223 miles (35,786 kilometres) above Earth, the satellite must orbit at a speed of about 7,000 mph (11,300 kph). The satellite moving with 7000 mph take 24 hours to complete a revolution, same as the time for Earth to complete one rotation. Such satellites stay at fixed position relative to Earth’s surface and are called Geo-Stationary satellites.
Generally, higher the satellite, slower its speed and thus longer it takes to complete an orbit. The life of a satellite depends on number of factors, one of them is drag. At lower altitudes, a satellite runs into traces of Earth’s atmosphere, which creates drag. The drag causes the orbit to decay until the satellite falls back into the atmosphere and burns up. At higher altitudes, where the vacuum of space is nearly complete, there is almost no drag and a satellite like the moon can stay in orbit for centuries.
Satellite is one of the most brilliant invention in the human history. It enabled us to seek thing further than the visible sky. The invention changed our perspective of ourselves, our planet, and the entire universe. The satellites, the rockets and all the space shuttles are pollens of human race spreading throughout the universe. The simple science is going to change the entire universe, the entire existence.