Scientists generally agree that the moon was created when an object crashed into the earth, sending fragments into orbit. Despite this consensus, much disagreement remains over the question of what the colliding object looked like and exactly how the collision formed the moon. Scientists first proposed “giant-impact” theories in the 1970s, believing a massive planet had collided with the earth, but this idea was soon discredited.
  Other scientists believed the object was small, perhaps only one-tenth the mass of the earth about the size of Mars. A decade later, the theory of Mars-sized invader planet took the place of the giant-impact theories, and scientists began to consider medium-sized planets.
  Two scientists in the Untied States have now proposed a new model that disproves the Mars-sized hypothesis. Robin Canup and Erik Asphaug found that an object with more than one-tenth the earth’s mass would have crushed our less-than-invincible planet, while anything smaller, perhaps, an asteroid, would have been too small to produce the necessary debris.
　They reached this conclusion by adjusting variables such as the size of the incoming planet, the angle of impact, and the mass of the earth, until they determined which combination resulted in the moon as we know it today. They surmise that the impact nearly destroyed the earth, which tenuously held together in an oblong shape until gravitation pulled it back into a sphere hours or days later. Some of the material from the impact was flung into space, eventually clumping together to form our lunar satellite.
  Not all scientists believe Canup and Asphaug have had the final word. Canup and Asphaug's model is based on an equation developed in 1962 that fails to adequately distinguish between the ejection behaviors of solids, gases, and liquids in the hours after an impact. A more sophisticated equation is now being developed, and simulations utilizing the new equation will be run in the future.