The presumptive scenario at the end of the Cretaceous around 65 million years ago is well known. A celestial body hit the earth off the coast of Mexico and led to a catastrophic species extinction. And it can not be ruled out that the earth will be hit again by such catastrophes. Scientists of the Lincoln Near-Earth Asteroid Research Program (LINEAR) suspect that the alien threat is greater than previously thought. With their telescopes in the American New Mexico, they counted more than 1, 100 asteroids, which have a diameter of more than a kilometer. This is twice as much as previous estimates. All are in orbits that could once come dangerously close to Earth.

Even chunks with a diameter of about one kilometer can have devastating consequences. Billions of people could become victims of flood disasters and global climate change. From a purely statistical point of view, we have to expect such an impact every 500, 000 to ten million years. Alone 400 pieces of it were able to identify the astronomers so far. Events to the extent that they once sealed the fate of the dinosaurs may be expected once in 100 million years. If an asteroid were larger than 10 kilometers, any life would be destroyed.

The probability of a fatal hit is still low, which of course does not mean that such catastrophes are ruled out. After all, in 1996, an approximately 500-meter-long asteroid approached the Earth up to 450, 000 kilometers near. From there it was just 70, 000 kilometers to the moon. For warnings, let alone defensive measures, it would have been too late in the event of a case. The astronomers discovered him only four days before his flyby.

Every year, about 20, 000 meteorites fall on the earth's surface. Most are hardly larger than pebbles, a variety of geologically preserved craters, such as the Nördlinger Ries, but show that even with great events must be expected. On June 30, 1908, witnesses in the sky over the Siberian Tunguska region observed a pale-blue fireball. Shortly thereafter, a blast wave leveled some 2, 000 square kilometers of forest. The barometric pressure fluctuations were still measurable in England. Today it is believed that this was an asteroid about 50 meters in diameter, which exploded at 6, 000 meters altitude.

Joachim Schüring and Lincoln Near-Earth Asteroid Research Program (LINEAR). Image: NASA display

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