Believe it or not, impact cratering is the most common process affecting the bodies in the solar system -- none of the planets, planetary satellites, asteroids, comets, meteoroids, or smaller objects are immune to its effects. Even the Earth has been pounded by large asteroids and comets; evidence of almost 200 such collisions has been discovered and studied by geologists and other scientists, with new craters being added to the list almost every year. Locations of known craters on the Earth; inset is Meteor Crater in Arizona, which is about 1.2 km across -- a small one. The possibility that mass extinctions were caused by asteroid or comet impacts is just one of the potential consequences of such events. Other effects are less obvious to nonspecialists, but are no less important in the evolution of the planets and their satellites, or even the solar system. Gaining acceptance by the scientific community, for instance, is the suggestion that the Moon was formed from the debris ejected during a collision between the Earth and an object roughly the size of Mars. Of course, this can't be proven (yet!), but it can handle a variety of geochemical observations of the Earth and Moon that are difficult to explain with any other single theory of the Moon's origin. Aristarchus is the largest fresh crater in this picture, just above the center of the frame. It's about 40 km in diameter, and because it's so fresh, it's is one of the most studied craters on the Moon. It has nicely terraced walls and a small but noticeable central peak, both of which indicate that the present appearance of Aristarchus is due to modification of the original, bowl-shaped cavity that existed during the formation of the crater. These modification processes are very complex, and they occur because a simple, bowl-shaped form is unstable under the influence of gravity and the spring-like rebound of the target. This picture is here, though, to show how the ejecta from a typical lunar crater changes with distance from the crater. The ejecta formation immediately around the crater take the form of a hummocky, wreath-like deposit. This unit is composed of ejecta and local material that mixed with the ejecta when it reimpacted the Moon. Inset - At much greater distances from Aristarchus (as far as the red box, for instance), chunks of ejecta have spread out and have enough energy to make craters of their own when they reimpact the lunar surface. These are called "secondary craters" and appear here as the complex clusters just to the left of the 6.7-km diameter Aristarchus C (the large crater to the right). The question is, how fast were they really traveling when they hit? As we learn more about the consequences of impact in the Great Scheme of Things, we appreciate how varied those effects can be on other facets of planetary science. One such aspect of impact cratering is the ejection of material from the growing crater. Check out the Apollo 15 photographs below of of Aristarchus Crater and its surroundings on the Moon. The differences in the appearance of the deposits of ejecta -- the material thrown out of the crater as it grew -- are striking as you look progressively farther from the crater's rim. In places far enough from the crater, you'll notice that clusters of much smaller, irregularly shaped craters begin to appear, and a little farther out they're everywhere. These are called "secondary craters," and they're caused by pieces of ejecta that were traveling fast enough to make their own craters when they landed back on the Moon. In fact, there's pretty good evidence that, in some cases, secondary craters can be found on the other side of the Moon from their parent crater!