A lthough most known planets have many moons or satellites, the Earth's own Moon has been studied in the most detail with his close proximity. The Moon is one-fourth the size of Earth and so the pull of gravity on the lunar surface is approximately one-sixth of that on Earth.
Recent expeditions, mainly by the satellite Clementine have led to the discovery of a frozen ice formations at the poles of the Moon; however, the Moon does not have free water nor an atmosphere, which causes a static state of weather.
Revolving about the Earth approximately every 27 days, the Moon is 384,403 km (238,857 mi).
While the origin of our companion Moon remains in debate, the most probably theory of formation is the planetesimal impact. Stated within this theory is that the Moon originally grew from the dust particles of an original nebulae that formed the Earth and the rest of the Solar System.
As the Earth had begun its first rotations, the Moon spun off and because of its smaller mass, it was grabbed by gravitation into eternal revolution about the Earth. This theory has been repeatedly been proven by the similarity between the densities of the Earth's upper crust rocks and Moon rocks.
Still, with NASA budget cuts, missions to our closest neighbor have become rare, leaving many Moon colonization plans to be canceled.