On stony meteorites, the heat-affected zone is at most a few mm deep in iron meteorites, which are more thermally conductive, the structure of the metal may be affected by heat up to 1 centimetre (0.39 in) below the surface. As it decelerates, eventually the molten surface layer solidifies into a thin fusion crust, which on most meteorites is black (on some achondrites, the fusion crust may be very light-colored). If the meteoroid maintains a fixed orientation for some time, without tumbling, it may develop a conical "nose cone" or "heat shield" shape. They can be sculpted into various shapes during this process, sometimes resulting in shallow thumbprint-like indentations on their surfaces called regmaglypts. Following the passage of the fireball, it is not unusual for a dust trail to linger in the atmosphere for several minutes.Īs meteoroids are heated during atmospheric entry, their surfaces melt and experience ablation. Whistling and hissing sounds are also sometimes heard but are poorly understood. These sounds can be heard over wide areas, with a radius of a hundred or more kilometers. Explosions, detonations, and rumblings are often heard during meteorite falls, which can be caused by sonic booms as well as shock waves resulting from major fragmentation events. Flashes and bursts of light can occur as the object breaks up. Various colors have been reported, including yellow, green, and red. The fireball that occurs as the meteoroid passes through the atmosphere can appear to be very bright, rivaling the sun in intensity, although most are far dimmer and may not even be noticed during the daytime. Several phenomena are well documented during witnessed meteorite falls too small to produce hypervelocity craters. (The very first example of a stony meteorite found in association with a large impact crater, the Morokweng crater in South Africa, was reported in May 2006.) Such events are generally so energetic that the impactor is completely destroyed, leaving no meteorites. Very large stony objects, hundreds of meters in diameter or more, weighing tens of millions of tons or more, can reach the surface and cause large craters but are very rare. Although such disruption events are uncommon, they can cause a considerable concussion to occur the famed Tunguska event probably resulted from such an incident. In contrast, even relatively large stony or icy bodies such as small comets or asteroids, up to millions of tons, are disrupted in the atmosphere, and do not make impact craters. Examples of craters caused by iron meteoroids include Barringer Meteor Crater, Odessa Meteor Crater, Wabar craters, and Wolfe Creek crater iron meteorites are found in association with all of these craters. The most frequent hypervelocity cratering events on the Earth are caused by iron meteoroids, which are most easily able to transit the atmosphere intact. The force of such collisions has the potential to cause widespread destruction. The kind of crater will depend on the size, composition, degree of fragmentation, and incoming angle of the impactor. Large meteoroids may strike the earth with a significant fraction of their escape velocity (second cosmic velocity), leaving behind a hypervelocity impact crater.
The impact pit made by a 61.9-gram Novato meteorite when it hit the roof of a house on 17 October 2012. Extraterrestrial meteorites have been found on the Moon and on Mars. Meteorites smaller than 2 mm are classified as micrometeorites.
Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy. Meteorites have traditionally been divided into three broad categories: stony meteorites that are rocks, mainly composed of silicate minerals iron meteorites that are largely composed of ferronickel and stony-iron meteorites that contain large amounts of both metallic and rocky material. Meteorites that are recovered after being observed as they transit the atmosphere and impact the Earth are called meteorite falls. For geologists, a bolide is a meteorite large enough to create an impact crater. Once it settles on the larger body's surface, the meteor becomes a meteorite. It then becomes a meteor and forms a fireball, also known as a shooting star astronomers call the brightest examples " bolides". When the original object enters the atmosphere, various factors such as friction, pressure, and chemical interactions with the atmospheric gases cause it to heat up and radiate energy. Ī meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. The 60- tonne, 2.7 m-long (8.9 ft) long Hoba meteorite in Namibia is the largest known intact meteorite.
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