Canadian Space Agency
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Table of Contents
Orbital Mechanics
The Physics of Orbits
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1. Forces
Two objects in space are mutually attracted by gravitational forces created in space-time by their masses. The magnitude of the force was first formulated by Isaac Newton, and the equation
Fg = GMm/r2
has become known as Newton's Law of Universal Gravitation.
Newton's law holds equally for Earth orbiting satellites, the Moon, planets orbiting the Sun, stars orbiting the centre of the Galaxy and even galaxies orbiting a common centre of mass in clusters of galaxies.
It is through the analysis of orbital dynamics that we know much about the nature of the Universe. Orbital studies have revealed such things as the masses of the planets, and some of the properties of black holes; and have raised the mysterious "missing mass" problem in the Universe.
2. Energy
To move an object away from a gravitationally attracting mass requires energy.
The process generally consumes the kinetic energy of an object launched outwards from the gravitationally attracting body.
Since the gravitational force decreases as 1/r2 it takes less and less energy to move outwards as the distance from the attractive mass increases. This effect can be seen in the graph to the left.
The total amount of energy required to move an object totally beyond the gravitational effect of an attracting mass is called the gravitational binding energy.
A payload launched with less kinetic energy than the gravitational binding energy will either fall back to the Earth or enter an elliptical orbit around the Earth (depending upon the direction of the velocity vector).
