Asteroids and Comets
An asteroid is any of the thousands of small irregularly shaped bodies of stone, metal, and ice that revolve about the sun. Most asteroids orbit the Sun in the Asteroid Belt located between Mars and Jupiter. However many large objects have been observed passing through Earth's orbital path. Asteroid collisions with Earth were frequent in Earth's early history, but are now extremely rare events. The extinction of the dinosaurs and many other species is mostly blamed on the environmental catastrophe created by an asteroid impact about 65 million years ago, defining the end of the Cretaceous Period (and Mesozoic Era).
A comet is a celestial body thought to consist chiefly of if ices of ammonia, methane, carbon dioxide, and water, and dust (Figure 1.75). Comets are observed only in that part of its orbit that is relatively close to the sun, having a head consisting of a solid nucleus surrounded by a nebulous cloud of gas and debris (a coma) up to 2.4 million kilometers (1.5 million miles) in diameter. The coma turns into an elongated curved vapor tail arising from the coma when sufficiently close to the sun. There may be more than 100 million comets in the outer Solar System.
A meteor is a bright trail or streak that appears in the sky when a meteoroid is heated to incandescence by friction with the earth's atmosphere.
A meteorite is a stony or metallic mass of matter that has fallen to the Earth's surface from outer space (Figure 1.76).
Asteroids and Comets
A bolide is a large meteor (or asteroid or comet) that explodes in the atmosphere (Figures 1-77 and 1-78). About a dozen significant (recorded) bolide events happen each year. A recent bolide explosion involved the Chelyabinsk meteor that blew up over Russia on February 15, 2013. The explosion occurred high in the atmosphere, but the atmospheric shock wave blew out windows, doors, and injured over a thousand people on the ground (see YouTube video).
An atrobleme is an eroded remnant of a large crater made by the impact of a comet or asteroid (large meteorite). Because of weathering and erosion processes, impact craters are relatively short lived on the Earth's surface (with exceptions for large impacts or in arid regions). Currently there are almost 200 known craters distributed on all continents. Others have been discovered by oil drilling through sedimentary cover.
Can you explain why are there so many craters on the surface of the Moon but not on surface of the Earth?