Section 4.1 - Introduction to Orbital Mechanics

What's an Orbit?

Ancient astronomers created detail catalogs of the objects they saw in the night sky, but for centuries they struggled to predict their motions. Orbital mechanics (aka astrodynamics) is the application of physics and ballistics to model the movement of celestial bodies and spacecraft in flight

TRUE or FALSE: Objects orbit the Earth because they aren't feeling the pull of gravity, hence the term "zero-G"

FALSE - an object stays in orbit because the inertia of its forward velocity is perfectly matched by the downward pull of gravity.

Think of it this way - if you throw a ball, it moves forward but eventually hits the ground due to Earth's gravity. But if you throw the ball faster and faster, it would travel farther and farther before it hit the ground. If you threw the ball fast enough, the ball would continuously fall, but its massive forward motion would cause it to continuously miss the Earth's surface, resulting in a perpetual circular motion. And if you increased the speed faster still, gradually the orbit becomes more and more elliptical until it reaches escape velocity and leaves Earth completely!

 Orbital trajectories of 21 spacecraft. Image credit: Universe Today

This unit will discuss the different parameters and types of orbits commonly used for crucial civilian, military, and scientific applications and will include some calculations of our own. Like the previous unit, true understanding requires advanced math far beyond my capabilities, but what's shown here can be understood by a lay person, given enough effort!