The origin of the Moon has been a subject of research for many years and theories about its creation abound. Hypotheses vary from non-scientific proposals, such as that the Moon is a spaceship, to the currently favored idea that it was formed more than 4.5 billion years ago from a dense cloud of debris created when another planet struck the Earth in its early stages of development. Scientists even have a name for this hypothesized astronomical body, which they believe contained about 10 percent of the mass of the Earth—they call it Theia, after the Greek goddess mother of the Moon goddess Selene.
The initial evidence in favor of this theory was obtained during the Apollo Moon missions, nearly five decades ago. Astronauts brought back about half a tonne of Moon rocks from these missions, and analysis has revealed a striking similarity between the chemical compounds found on the Moon and those found on Earth. Other discoveries also support this theory, including evidence that shows a match between water samples collected from crystalline structures in lunar rocks and water collected on Earth.
The Earth-Theia collision hypothesis, also known as the Giant Impact Theory, has been around for awhile. It reflects the current scientific consensus on this question. But it has not gone unchallenged, by either planetary scientists or by the latest available data.