The question is very simple. There are many similar questions, such as why eggs always fall in the chicken coop? Although netizens asked this question to cause trouble, this does not prevent me from using this to popularize science.
First of all, we need to explain that the small celestial fragments falling on the ground are considered meteorite . Many meteorites actually do not fall in the crater, but around the crater. The craters are the impact point. Under the huge impact force, except for the small fragments scattered around, almost all the rest have turned into powder.
The two concepts of meteorite and crater are strongly correlated, because the meteorite (more strictly speaking, small celestial bodies) hit the ground first, and craters will be generated on the ground, that is, there is a causal relationship between these two concepts.
meteorite is irregular in shape. It is an alien object that crashes into planets, dwarf planets and satellite and reachs its surface. These alien objects mainly include asteroids, comet and meteoroids.
Some small extraterrestrial bodies have loose structures and are relatively small in size. Before they have fallen to the ground, they have been ablated or disintegrated and exploded into many debris due to violent friction with the air in the sky. Among them, fragments with too small mass have weak energy when hitting the ground, and they cannot produce meteorite craters with considerable diameters.
Human understanding of craters began in the early century, and Daniel Balingjie was the first person to confirm certain geological forms on the earth as craters (impact craters). At present, some craters on the earth have a diameter of hundreds of kilometers. In some places with abundant precipitation, the craters left by small celestial bodies sometimes form lakes, and some craters with larger diameters are also called craters .
Some people say that the moon blocks many small celestial impacts for the earth, which is far-fetched. Geological activities on the earth's surface are relatively active. Coupled with vegetation, weathering and other effects, the number of discernible craters existing on the earth is naturally far less than that on the moon.
meteorites are small celestial fragments from outer space. According to their composition, they can be divided into three categories: stone meteorites (average density of 3 to 3.5 grams per cubic centimeter ), iron meteorites (average density of 7.5 to 8.0 grams per cubic centimeter) and stone-iron mixed meteorites.
The meteorites on Earth mainly come from asteroid belt between the orbit of Mars and Jupiter orbit. Although the number of small celestial bodies in this asteroid belt is as high as hundreds of thousands, their total mass is very small, far less than that of the earth.
At the beginning of the birth of the solar system, there were many small celestial bodies wandering in space, and the probability of the earth being hit by large-mass small celestial bodies was much higher than it is now.
65 million years ago, the extinction of dinosaurs was likely caused by the impact of the asteroid on Earth. The energy released by that impact was amazing. At the beginning of the last century, the Tungus incident that occurred in the Siberia region was most likely caused by the impact of small celestial bodies.
These impacts are amazingly destructive! Therefore, it is necessary to track and monitor large-mass small celestial bodies that may collide with the earth. Scientists are doing this now and plan to use technical means to change the orbits of small celestial bodies that may collide with the earth in advance.
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