I submit that impact of solid bodies is the most fundamental of all interstellar processes that have taken place on the terrestrial planets: without impact, Earth, Mars, Venus, and Mercury would not exist.

Simply put, the collision of smaller objects is the process by which the terrestrial planets were born. On the surface, that the geological record of the earliest history of impacts on the terrestrial planets has been lost, is troubling. As the process is self-erasing, to a certain extent, the earliest record would have been lost even if processes of melting and internal evolution of the planets had not occurred. But much of the record of the last stages of accretion of the planets is preserved, especially on the moon, Mercury, and Mars. In fact, the last stage of accretion is still going on, albeit at a very slow rate.

This is fortunate, because we can study many aspects of the processes of planetary birth by investigation of the nature of small bodies that still exist, the dynamics of their orbital evolution, and the effects that they produce when they ultimately collide with a planet. If impact and accretion were not still occurring, it would be hard to come to grips with a number of difficult problems of planetary origin and early evolution.