Tunguska explosion in 1908 caused by asteroid grazing Earth

A massive explosion flattened extensive forests in a remote part of Eastern Siberia along the Tunguska River early on June 30, 1908. Curiously, the explosion left no crater, creating a mystery that has puzzled scientists ever since — what could have caused such a huge blast without leaving any remnants of itself?

Daniil Khrennikov of Russia’s Siberian Federal University and colleagues have now developed a new model of the incident, which may finally solve the mystery. According to Khrennikov and colleagues, the explosion was generated by an asteroid that grazed the Earth, entered the atmosphere at a shallow angle, and then passed it back out into space.

“We argue that the Tunguska event was caused by an iron asteroid body, which passed through the Earth’s atmosphere and continued to a near-solar orbit,” they say. If they are correct, the theory suggests Earth escaped an even larger disaster by a hair’s breadth.

First, some context. Scientists have long speculated about the Tunguska impact. The most popular theory is that the explosion was caused by an ice body, such as a comet, entering the atmosphere. The ice then rapidly heated up and evaporated explosively in mid-air but without ever hitting the ground.

Little evidence

Such an explosion could have been powerful enough to flatten trees without leaving a crater. And it would have left only vapor in the atmosphere as evidence.

However, some of the other data supports this theory. There were only a few eyewitness reports of the event. These describe how “the sky split in two,” a huge explosion and widespread fire. However, they show that the impactor traveled 435 miles (700 km) through the atmosphere before exploding that morning.

Fallen trees at Tunguska in 1927. Wikimedia Commons

So Khrennikov and colleagues modelled the effect of meteorites made of rock, metal, or ice traveling at a speed of 12 miles per second through the atmosphere (20 kilometers per second). (Meteorites enter the atmosphere at a speed of at least 11 kilometers per second.)

Friction with the atmosphere immediately heats these objects. However, although iron vaporizes at around 5,432 degrees Fahrenheit (3000 degrees Centigrade), water vaporizes at only 212 degrees Fahrenheit (100 degrees C). So icy meteorites do not last long.

Indeed, Khrennikov and colleagues calculate that an icy body large enough to generate such a large explosion would have only traveled 186 miles (300 kilometers) into the atmosphere before fully vaporizing. This shows that the Tunguska meteorite was not made of ice.

Instead, Khrennikov and colleagues say a different scenario fits the facts. They believe the explosion was caused by an iron meteorite the size of a football stadium. This must have gone through the upper atmosphere, heated up fast, and then returned to the Solar System. The shock wave from this trajectory was what flattened trees.

The shock wave would have caused an explosion of about the right magnitude, and any vaporized iron would have condensed into dust that would be indistinguishable on the ground. Importantly, no visible asteroid remnants would have emerged from this scenario.

It could also explain reports of dust in the upper atmosphere over Europe after the impact.

Lucky miss

If Khrennikov and his colleagues are correct, Earth had a lucky near miss that morning. A direct impact with a 656 foot-wide (200 meter-wide) asteroid would have devastated Siberia, leaving a crater 2 miles (3 kilometers) wide. It would also have had catastrophic effects on the biosphere, perhaps ending modern civilization.

In the event, the Tunguska impact is thought to have killed perhaps three people because the region is so remote. It could clearly have been much worse.

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