What is an Orbit? – Lesson 1

An orbit is a closed “path” around which a planet or satellite travels.  In the simplest sense, orbits are a type of "racetrack" in space that a satellite "drives" around.

How fast would a car have to “drive” to stay in orbit?

Remember from your physics class, the centripetal acceleration of a car around a circular track is:

a = v2/r

And the force of gravity is:

1)  F = GMm/r2 = ma

2)  F/m=a = GM/r2

3)  GM/r2 = v2/r

4)  v = √(GM/r)

So for a “car” at a 9,000 km altitude (as shown):

G = 6.674 x 10-11 m3 kg-1 s-2

Mearth = 5.972 x 1024  kg

rcar=rearth+raltitude= 6,378.14+9,000 km=15,378.14 km

v = √(GM/r) = 5.09 km/s !!!!

Lower altitude orbits travel even faster!

There are many different types of orbits which are described by various orbital elements.  Before jumping into specific orbits, we will cover Kepler’s three laws of orbital motion.