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Collision course TEXT
Florian Aigner
Celestial disasters: distances in space are enormous, and the universe is largely a void. Nevertheless, collisions occur – again and again. A collision is inevitable: the Andromeda Galaxy is racing towards us at a speed of about 110 kilometers per second. While it is still about two and a half million light years away, in four or five billion years it will reach our Milky Way and merge into a new, larger galaxy. That’s less dramatic than it might sound. The stars that occupy the centers of their galaxies today will then be forced into completely new orbits around the center of the giant new galaxy. Some will probably even be jettisoned into the vastness of intergalactic space. And nobody knows what will happen to our sun in the process. But the chances of two stars or planets crashing into each other somewhere during this massive meeting of galaxies are extremely slim. Galaxies consist mainly of empty space. The distances between the stars are so unfathomable that galaxies can merge without any of their celestial bodies ever coming into contact. On a smaller scale, however, collisions occur constantly. That can be seen, for example, on the surface of the moon, which is dotted with impact craters of various sizes. The Earth is exposed to exactly the same bombardment as the moon – only you don’t see the damage to its surface. We have an atmosphere that incinerates smaller objects as they enter. We have weather, wind and vegetation that disguise impact craters. And we have plate tectonics that melt the Earth’s substructure over hundreds of millions of years and create new plates. Brighter than the sun On February 15, 2013, people near the Russian city of Chelyabinsk learned how frightening it can be when a celestial object literally comes down to Earth. A meteor with a presumed diameter of some 20 meters passed through the atmosphere there at a speed of approx. 70,000 kilometers per hour. Because it entered at a fairly shallow angle, it exploded in the air before it could hit the ground. The flash this generated
was brighter than the sun – clearly visible from 100 kilometers away. A huge shockwave damaged thousands of buildings, with some 1,500 residents being injured – many of them by shattered window glass. So that’s a good lesson to learn: if you are observing an explosion in the sky, keep away from the windows! Just as thunder often follows a flash of lightning, the shockwave from an explosion often reaches us after the radiance has waned. The Chelyabinsk meteor was, however, harmless compared to the mysterious phenomenon in Tunguska, central Siberia, in 1908. What happened there is still not entirely clear because the region along the Tunguska River is remote and sparsely populated. There is no doubt that there was a huge explosion. Its flare was visible a full 500 kilometers away. The shockwave was registered and measured around the world, and trees were uprooted within a radius of 25 kilometers. An actual impact crater was never found, but it is believed that an asteroid measuring between 30 and 80 meters across exploded a few kilometers above the Earth’s surface. It is difficult to imagine the carnage that would have been unleashed had this occurred over a densely populated city. A catastrophe of completely different proportions was sparked by the asteroid that wiped out the planet’s dinosaurs 66 million years ago. Measuring approximately 10 to 15 kilometers across, it created an impact crater with a diameter of 180 kilometers. The consequences were devastating: rock was propelled into the higher layers of the atmosphere, with much of it falling back to Earth as a hail of rubble. Huge wildfires broke out. Compared to the earthquakes that this asteroid likely triggered, even the worst seismic events in human history would have seemed trivial. Huge tsunamis devastated the coastal areas, dust darkened the sky and caused a global cold snap. The planet’s ecosystems veered out of kilter, resulting in a mass extinction of its species. A sea of stars and a bowl of cherries Meteoroid hits on Earth are therefore nothing unusual. But why is it, then, that stars so rarely
