Ever since Galileo Galilei observed that the lunar surface is rough, continuous efforts have been devoted to understanding the reason for this roughness. Today, we have access to the detailed structure of the surface terrain of the moon, as shown in Fig. 1.1. A lot of circular structures called craters appear almost all over the surface: these pits are evidence of ancient impacts. Numerous astronomical objects have impacted the moon and have left craters. Astronomical impacts have been one of the most important and ubiquitous processes since the formation of the solar system. Thus, an in-depth understanding of impact cratering is a necessary key to shedding light on the history of the solar system. However, a fundamental understanding of this concept remains in development mainly because the physical basis for impact phenomena remains in its infancy. Moreover, the actual planetary-related phenomena are very complex processes. Therefore, various approaches in addition to the physics of impact would also be necessary to fully explain the entire phenomenology of planetary impacts. The physics of impact is only a part of these various approaches. Nevertheless, many unsolved problems remain even in the very fundamental physics of impact.