If an incoming meteoroid survives its passage through the atmosphere as a meteor and actually impacts the Earth, it is known as a ‘meteorite’. Of the several thousand tonnes of interplanetary debris to enter the Earth’s atmosphere every day, only about 1 tonne makes it to the Earth’s surface, and the majority of this is in the form of microscopic dust particles. In particular, objects smaller than 10-6 m are slowed by collisions with molecules in the upper atmosphere to a degree where ablation does not occur during their fall to Earth. These land as micrometeorites.
Meteorites (which can measure metres across), are mostly fragments of asteroids and can be traced back to the different classes of asteroids that exist in the main asteroid belt. Their survival through the atmosphere depends on their initial mass, composition and speed of entry, and their superheated arrival means that they are typically smooth and encased in a black ‘fusion crust’.
Meteorites are classified according to their composition and whether their parent body was differentiated or not. The three primary classes are:
||The largest meteorite ever discovered weighs 60 tonnes and was found in Namibia. However, meteorites much larger than this have impacted the Earth and vapourised as they formed meteor craters. The most famous is the Barringer Crater located near Winslow, Arizona. It is thought that this 1,200 m diameter, 200 m deep crater was formed about 50,000 years ago by a meteorite measuring about 50 m across and weighing of order 300,000 tonnes.|