Background:
Orbits

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Planets, comets, asteroids, and anything else moving through the solar system are held on their orbits by the Sun's gravity. Studying the motions of the planets, Johannes Kepler discovered that the motions of objects orbiting the sun could be described by three laws which we now call "Kepler's Laws":

  1. Objects move around the sun on elliptical orbits, with the Sun at one focus of the ellipse.


    Eccentricity describes the flattening of an elliptical orbit; a circular orbit has an eccentricity of zero, while a highly-elongated orbit has an eccentricity approaching one.


  2. An imaginary line connecting the object to the Sun sweeps out equal areas in equal times. This essentially means that objects move very quickly when they are near the Sun, and more slowly when they are further away.
  3. There is a relationship between the orbital period of an object (P) and the length of the semi-major axis of the orbit (a). For objects orbiting the sun, if P is measured in years and a in astronomical units, P2/a3 = 1.

Note that these laws can be applied (with minor modifications) to planets around other stars, moons around planets, and spacecraft moving through the solar system. In fact, knowing these laws, we can calculate the proper orbit and "launch window" to send a space probe to other planets. First, though, we need to learn about a special type of orbit called a "Hohmann transfer orbit."