In astronomy, impact generally refers to the collision of two solid bodies, possibly both of significant size, or possibly one much smaller than the other. Craters are evidence of impacts. Impacts are assumed in theories of planet formation, both to explain how planets grew, assuming one of the bodies gained the material from the other, and how bodies came to have their constituents, assuming impacts removed the outer surface removed by one or more impacts (impact erosion), which can leave the body denser if the densest material had already fallen toward the center of the body. It is assumed both scenarios occur, depending upon the velocity of the impact(s).
The favored theory of the Moon's formation is that it results from a giant impact, the impact of a Mars-sized planet with Earth (known as the giant-impact hypothesis).