Satellite concepts

Hello friends. I’m back with another string of thought. This time its related to astronomy. Lets start with ‘Circular motion’. That’s a punch-word to get you started. Circular motion is strangely very common in our universe. Stars revolve around the center of their galaxies; planets around stars; moons around planets; and even electrons around atoms. Plus they’re all rotating about their axes! Is the universe also rotating? Maybe, but its difficult to say because we’re within the non-inertial frame of the universe and any material force accelerating us is balanced out by a pseudo-force in the opposite direction. But let’s leave that aside for now. Let’s consider satellites of earth. They’re all revolving circularly around the earth. A popular way of classifying satellites is from the height ‘h’ of their orbit above earth’s surface. Since gravitational pull by earth (which depends on h) provides the centripetal force (which depends on velocity v of the satellite) required for the satellite’s circular acceleration round the earth, a satellite at a specific ‘h’ has a unique ‘v’ . A little calculation can show you that the time period T of a satellite is only a function of ‘h’. A special type of satellite is geosynchronous satellite whose revolution period equals earth’s rotation period ie. 24 hrs. Since T= f(h), so 24 hr = f(36000 km). This is more than five times earth’s radius into the sky. We also have LEO satellites which hover within 1000 km from earth’s surface. The ISS is also situated in an LEO orbit. The ISS is a great system to understand a few concepts. First lets ask how does the ISS get there? Just like any other satellite is launched. How are satellites launched? Using SLVs commonly called ‘rockets’. So a rocket carries the satellite as a payload. It has either solid or liquid propellant (fuel) alongwith liquid oxygen. They’re mixed together and the combustion results in an upthrust as per Newton’s 3rd law. Note that air-breathing engines have been developed in some jet planes (such as ramjet & its improved form scramjet) because they fly in the troposphere. Since rockets go well inside the upper atmosphere and even further where air is rare, air-breathing engine is difficult. So a rocket is launched. Midway a part of it containing an exhausted fuel tank is jettisoned (which is called the first stage). The second stage carries & parks the payload into orbit, and then falls down and burns out in the atmosphere.

Lets consider ISS. It consists of several modules, each of which can be considered a satellite. Each was launched by some rocket. Now to bring food & supplies to ISS, the Soyuz spacecraft is used which consists of a rocket and its payload ‘capsule’. The rocket withers off as discussed. The capsule reaches the orbit and stays attached to the ISS. Even astronauts can use that capsule to re-enter earth safely. For that no fuel is required, gravity does the job. Special heat-shields are installed. And when it approaches earth’s surface, parachutes open and the astronauts descend. None of the capsule’s parts remain usable though. Before Soyuz spacecraft, NASA’s Space Shuttles were used for the purpose where its capsule landed back on earth just like an aircraft lands in an airport ! But space shuttles were disbanded by NASA in 2010. Thus it can be seen that rockets as a whole do not re-enter and land on earth in any re-usable way. However Space-X’s Falcon 9 became the first rocket to land back completely in a re-usable state in 2017.