Figure 9.10The number of planetary impacts has varied over the history of the solar system. After the planets formed, they continued to collide with the residual matter that still cluttered the solar system. These collisions tapered off over the first 500 million years of planetary development. However, there was another period of frequent impacts, known as the Late Heavy Bombardment, which peaked about 3.9 billion years ago. (Ga, billion years ago.)