On June 30, 1908, seismic stations all across Europe registered an enormously powerful shock wave, which originated from a location near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia. The so-called Tunguska event ever since has challenged the fantasy of scientists, who related it to the impact of a meteor or comet fragment, or even have developed theories that speak of black holes, anti matter or less exotic geothermical hypotheses.
In the morning of June 30, 1908, inhabitants of the sparsely populated area north west of the famous Lake Baikal noticed a blue light, as bright as the sun making its way quickly across the sky. Just a few minuted after, a crash was heard moving from east to north along with a gigantic shock wave. It knocked down entire woods and crashed windows hundreds of kilometers away, even though an explosion was not seen. The shockwave itself is approximately comparable with an earthquake of 5.0 on the Richter scale and as a consequence of the light passing through high-altitude ice particles, the night sky above Asia and Europe was incredibly illuminated.
Surprisingly, the scientific interest on the event was hardly even noticed and the first serious expedition to search for an explanation was sent 17 years later. Leonid Kulik, a Russian mineralogist eventually convinced the government to sponsor his trip. His hypothesis was, that a meteorite impact caused the damage but he was just not able to find a crater and prove his theory.
Today’s leading theory considering the cause of the explosion is an air burst of an object passing Earth about 10 kilometers above its surface. Eugene Shoemaker calculated that events like these occur about every 300 years. It was highly discussed, if the event was caused by an asteroid or comet. The comet hypothesis is supported by the glowing of the sky above Europe and Asia and gained lots of acceptance in the 1960’s. in 1983, scientist Zden?k Sekanina published his idea, an asteroid could have been the object they were looking for, which was later supported by further astronomers who noticed that the object presumably came from the asteroid belt. For years, supporters of the asteroid and comet theory battled each other with new findings. Vladimir Alexeev led an expedition on 2010. His team found a crater about which he was able to prove that a gigantic piece of ice caused most damage, which highly supports the comet hypothesis.
At yovisto academic video search, you may enjoy a short talk by Phil Plait about how to avoid asteroids and comets on Earth in the future.
References and Further Reading:
-  Quasi Three-Dimensional Modeling of Tunguska Comet Impact (1908)
-  Tunguska at Wikidata
-  Tunguska Website
-  The Tunguska Event 100 Years later at NASA
-  Caroline Herschel – Comet Sweeper , SciHi Blog
-  Christiaan Huygens and the Discovery of Saturn Moon Titan , SciHi Blog
-  Edmund Halley and his famous Comet , SciHi Blog
-  The Arecibo Radio Telescope – Looking for Extraterrestrial Signals, SciHi Blog