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What Are Four Ways That Scientists Believe the Moon Was Formed?

On July 20, 1969, astronaut Neil Armstrong took "...one small step for man, one giant leap for mankind," when he became the first man in history to walk on the moon, more than 238,000 miles from the the Earth. The moon, which has no atmosphere, orbits Earth at a speed of 2,288 miles per hour, and can complete one orbit in an average of 27 days. Though unproved, there are several theories about the moon's formation.

Capture Theory
- The capture theory postulates that the moon was formed somewhere in space, then moved through space until it came into contact with the Earth's gravitational force and began its orbit around the Earth. Though this theory could account for why the moon's composition is so radically different from the Earth's, some questions remain.
  
  Some scientists who doubt this theory have wondered why the moon was not drawn by the sun, which has a stronger gravitational pull than Earth. In addition, some scientists say that if this theory were true, the moon would have crashed into the Earth or drifted far out of Earth's orbit.
Giant Impactor Theory
- The giant impactor theory, formulated in the 1970s, proposes that soon after the solar system was formed, the Earth was struck by a meteor the size of a small planet. The resulting expulsion of debris created the moon, with most of the matter coming from the mantle of the Earth. This theory helps to explain why the moon's density is much less than the Earth's, since it is composed in part by debris from the Earth.
Fission Theory
- The fission theory suggests that the moon was originally part of the Earth during the birth of the solar system, but then detached from the Earth and became a separate formation. Supporters of this theory cite the similarity between the moon's composition and the mantle of the Earth, and theorize that a rotating Earth spun off matter that resulted in the moon's creation.
  
  Doubters of this theory point out the fact that there is no fossil evidence of the Earth's ability to spin fast enough to shed layers.
Co-Formation or Condensation Theory
- The co-formation or condensation theory posits that the moon and the Earth were condensed from a solar nebula at the same moment in the same place, and that the moon began to orbit around the Earth.
  
  This theory answers the question of why the moon is in its fixed location near the Earth, but falls short when it comes to explaining the reason why the moon and Earth are not composed of the same material.

Astronomy