On the morning of June 30, 1908, the ground trembled in Central Siberia, and a series of flying fireballs, causing a “frightful sound” of explosions, were observed in the sky above the Stony Tunguska River. Strange glowing clouds, colorful sunsets, and a weak luminescence in the night were reported as far as Europe.
Likely many thousand people in a radius of 1.500 kilometers (or 900 miles) observed the Tunguska Event. However, due to the remoteness of the affected area, eyewitness testimonies were collected only more than half of a century after the event, and most were second-hand oral accounts. In 2008, unpublished material collected by Russian ethnographer Sev’yan Vainshtein resurfaced, including some first-hand accounts of the event.
Despite its notoriety in pop-culture, hard scientific data covering the Tunguska Event is sparse. Since 1928 more than forty expeditions have explored the site, taking samples from the soil, rocks, and even trees flattened by the explosion, with ambiguous results. Some seismic and air-pressure wave registrations, recorded immediately after the blast, survive to this day and surveys of the devastated forest mapped some thirty years later. The explosion was powerful enough to flatten more than 80 million trees.
Based on the lack of hard data, like a crater or a meteorite, and conflicting accounts, many theories of widely varying plausibility were proposed over time.
At the time of the event, international newspapers speculated about a volcanic eruption. Russian scientists, like Dr. Arkady Voznesensky, Director of the Magnetographic and Meteorological Observatory at Irkutsk, speculated about a cosmic impact. In 1927, Russian mineralogist Leonid Alexejewitsch Kulik of the Russian Meteorological Institute, explained the event as the mid-air explosion of a meteorite, based on the lack of an impact crater on the ground and the discovery of interplanetary dust in peat layers at the site. In 1934, Soviet astronomers, based on Kulik’s work, proposed that a comet exploded in Tunguska. Such an icy body, entering the atmosphere, would rapidly heat up and evaporate explosively in mid-air without ever hitting the ground. The resulting explosions could have been powerful enough to flatten trees without leaving a crater or much other evidence.
A new paper tries to answer the question about the nature of the impactor by simulating the effects of a meteorite – made of rock and metal-alloys – and a comet – made mostly of ice and traces of interplanetary dust – entering Earth’s atmosphere at a speed of 12 miles per second (20km/sec). Friction with the atmosphere heats these objects, but while rock and iron vaporize at around 3.000°C, water evaporates at only 100°C. Based on calculations of the authors, an icy body large enough to cause such a large explosion would have traveled at least 186 miles (300 kilometers) through Earth’s atmosphere before vaporizing completely. Some reports of the Tunguska Event describe a series of explosions in the sky, suggesting that the impactor traveled some 435 miles (700km) through the atmosphere.
According to the authors of the simulations, this observation excludes the possibility that the Tunguska Event was caused by an icy body. Only a 656 foot-wide (200 meter-wide) stone-iron asteroid could have traveled so far trough Earth’s atmosphere. To explain the lack of any crater on the ground, the researchers suggest that the asteroid didn’t actually hit Earth, but traveled through Earth’s atmosphere, causing a series of air-bursts in midair – explaining the series of explosions and the flattened trees – and leaving behind a plume of dust in the upper atmosphere – explaining reports of glowing clouds over Europe after the event.