Impact Crater (definitions)

The floor of the crat er can be bowl shaped or with a central uplift, while around it the walls form a raised rim. The crater is usually surrounded by an ejecta blanket, while its floor is covered with breccia, a course-grained rock, composed of broken rock fragments. During the impact both the target rock and the impactor are melted in what is called impact melt which is then dispersed in various impact deposits and ejecta, including some small particles called tektiles.


The word asteroid means "star-like", even if these minor bodies of the Solar System don't emit light on their own, but are visible only because they reflect sunlight. The size of asteroids range from dust particles to significant bodies hundreds of miles in diameter (Ceres, the largest observed is 913 km of diameter). Globally, the total mass of all the asteroids is less than that of the Moon.

Asteroids are found in different places in the solar system: most of them orbit around the Sun, grouped in the main belt, while others are farther objects, with highly unpredictable orbits, such as the Trojans, which lie on the orbit of Jupiter, or such as the Centaurs, in the very outer solar system. Asteroids that, for some dynamical mechanism, closely approach the Earth are named Near-Earth Asteroids (NEAs) and form a class of particular interest.