The Moon is full of craters everywhere. Same is the case with Earth, but the craters caused by asteroid impacts have been worn away by the wind, rain and other natural phenomena. However, there is a big difference between the Moon and the Earth. Space bodies with no atmosphere allow small asteroids or comets to hit the surface without any hindrance. On the other hand, Earth’s thick atmosphere protects us from the masses of small asteroids, or meteors which pelt our planet every day. Those shooting stars in our night sky are basically the burning debris of these meteors. Even 33-feet sized meteoroids are destroyed in the Earth’s atmosphere on entering. However, bigger asteroids manage to reach the Earth and some have caused widespread carnage, with one causing the extinction of dinosaurs. NASA mentions that there are more than 100 ring-like structures on Earth, which are recognized as definite impact craters.
NASA Ames Research Center estimates that there are around 2,100 asteroids larger than 1 kilometres and maybe 320,000 larger than 100 meters. This is the asteroid size that caused the devastating Tunguska event.
Now, the question is, what will happen if any such asteroid crashes to Earth? NASA says that the crash of any of these large asteroids in the wrong place would be a catastrophe!
However, it would not threaten civilization.
Can asteroids or meteoroids pose a risk to humans on Earth?
NASA predicted that an impact by an asteroid larger than 1-2 kilometers is capable of degrading the global climate, which could lead to widespread crop failure and loss of life. This can bring a global environmental catastrophe as the entire population of the Earth will be at risk.
However, an object larger than about 5 kilometers will have the potential to cause mass extinction.
This all may sound scary, but NASA assured that, “As noted earlier, no human in the past 1000 years is known to have been killed by a meteorite or by the effects of one impacting. An individual’s chance of being killed by a meteorite is small.”