# Astrodynamics 1

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Astrodynamics combines Newtonian physics of celestial mechanics, orbital mechanics, and ballistics into a science which can be applied to satellites, rockets, and spacecraft.

# Orbital Elements

## Mean Motion

Mean Motion (revs/day) required for a body to complete one orbit.

In it's simplest form mean motion can be expressed as "mean_motion" = n = "rev"/ P,where P is the period and rev is a representation of the time to complete one orbit.

The Orbit time can be expressed in units of "revolutions"/"time", "radians"/"time", "degrees"/"time"

In the two-line element sets used by the both the government sector and commercial industry to specify a satellite's orbit, mean motion is captured in units of "revolutions"/"day"

can be expressed as a function of orbital period as:

n = "1 rev" / P = "360 deg" / P = (2pi " radians")/ P

## Orbital Period

P = 1/n, where n is the mean motion.

## Kepler's Laws

### Kepler's 3rd Law

Kepler's 3rd law of planetary motion states, the square of the periodic time is proportional to the cube of the mean distance, or

mu = a^3/P^2

where a is the semi-major axis or mean distance, P is the orbital period as above, and μ is a constant for any particular gravitational system.

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