What really happened in Tunguska?

Astronomers roll up explosion and possible originators

The cause of the Tunguska event in 1908 is still controversial - but the most likely is the explosion of a cosmic bolide in the Earth's atmosphere. © Marharyta Marko / iStock
Read out

Enigmatic to this day: Astronomers have gained new insights into what caused the enigmatic explosion in the Siberian Tunguska on June 30, 1908. Thus, the most likely originator is not a comet, but a 50 to 80 meter rock asteroid that exploded five to fifteen kilometers in height. Simulations prove that even a massive chunk would be almost completely torn in such an explosion - which explains the missing crater.

The Tunguska event is still puzzling because its cause is still unclear. It seems clear that on June 30, 1908, a massive explosion over the Siberian taiga occurred, resulting in the destruction and destruction of trees covering more than 2, 000 square kilometers. Only in the center of the area remained tree trunks, deprived of their branches, like telegraph poles standing vertically. In places, the trunks showed traces of extreme radiant heat, in addition, tiny glazed rock debris and altered isotope values ​​were found in the vicinity of the explosion.

Tunguska Falls overturned trees, taken in 1929. © Leonid Kulik / Historic

Cause still debatable today

But what was the cause of the explosion? Initial hypotheses were based on a volcanic eruption or the impact of a meteorite. In both cases, however, the event would have left a crater - and one such was never found. There is also no debris from a meteorite that may have burst in the air. "These observations led to the hypothesis that the author was a comet because such an ice-rich object is more likely to burst in the atmosphere than a rock asteroid, " explain Darrel Robertson and Donovan Mathias of the NASA Ames Research Center.

According to popular theory, the Tunguska event was triggered by a comet that exploded above ground in a few kilometers. But whether it really was a comet, how big the object was and what kind of energy the explosion released is still a matter of controversy. A new opportunity to find out more, however, opened to researchers in February 2013 when an asteroid exploded over the Russian city of Chelyabinsk.

New model simulates cosmic hits

Based on the Chelyabinsk explosion, Robertson and Mathias have now developed a geophysical model that can more accurately simulate the effects of an atmospheric explosion of various celestial bodies than ever before. They have also reconstructed the Tunguska explosion and its possible originators. They specifically searched for objects and events that could reproduce the observed pattern of fall of the trees and the absence of a crater and other classical impact marks. display

The result: The object that exploded over Tunguska must have been around 50 to 80 meters tall. The explosion occurred at five to 15 kilometers altitude and released about ten to 30 megatons of energy, as the researchers report. The result was a pressure wave that swept through the Siberian taiga at 40 to 50 meters per second, as the researchers report.

Rather a rock than a comet

But what was that for an object? Contrary to popular belief, the cosmic bolide must not have been an ice-rich comet even a rock meteorite comes as a trigger of the Tunguska explosion in question, as the model calculations showed. "Our simulation shows that many meteors may have generated the subversion pattern of the trees - including both typical rock asteroids and typical short-period comets, " the researchers report. "In many of these atmospheric explosions, no fragments reached the ground that fits in with the absence of an obvious crater in Tunguska."

According to the researchers, a comet is even less likely than a rock asteroid: "At a shallow entry angle, the comet would have to be unrealistically stable, but a steep entry angle would not match the comet-typical angles of ten to 50 degrees, "explain Robertson and Mathias. They almost completely exclude a long-period comet, because its higher velocity had already torn it to a much greater height.

Thus, the new results support an earlier study, according to which even stony asteroids can burst into the atmosphere almost without any suppres- sion. Porous chunks explode, so to speak, from within and are torn into tiny pieces. The same could have happened with Tunguska, the researchers suspect.

Only every few thousand years

Reassuring, however: In another study, a research team has now re-examined the probability of a Tunguska event and gives the all-clear. For while previous estimates of such an event ran out every few hundred years, the scientists now estimate the intervals based on the new size estimates on one to two and a half millennia. The likelihood of experiencing such an explosion during our lifetime is not zero, but rather low.

Nevertheless, the scientists urge vigilance and emphasize the importance of asteroid surveillance programs and early warning systems. "Tunguska is the greatest cosmic event modern humanity has experienced, " says David Morrison of the Ames Research Center. "And it is characteristic of the kind of impact against which we are most likely to protect ourselves in the future." (Icarus, 2019; doi: 10.1016 / j.icarus.2018.10.017; doi: 10.1016 / j.icarus.2019.04 .006)

Source: NASA

- Nadja Podbregar