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.