New Research Suggests Tunguska-Like Events Happen Less Often Than We Feared

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The forest of Tunguska, Siberia, picture taken by Russian explorer Evgeny Krinov twenty years after an explosion flattened estimated 80 million trees.

Evgeny Krinov

In September 2019, the asteroid 2006 QV89 will approach Earth’s orbit. Astronomers estimated on preliminary data that the asteroid had a 1-in-7,000 chance of hitting Earth, but more recent observations confirmed that it will fortunately miss. Measuring 40 meters in diameter, an impact would release the energy of 100 Hiroshima bombs, almost like the explosion over Tunguska on June 30, 1908. The Tunguska Event flattened more than 80 million trees across 2,000 square kilometers of Siberian forest. Scientists still are not sure what happened. No impact crater or fragments of a meteorite were ever found. Scorched trees with all their branches blown off over an eight-kilometer radius suggest some massive object exploding in mid-air.

This scenario is supported by contemporary reports describing a fireball descending on the forest of Tunguska, the presence of impact-related minerals like nanodiamonds, metallic- and silicate spherules in sediments, and the mapped distribution of the flattened trees. Based on the energy of the explosion (estimated in 10–30 megatons, equivalent to the blast energy of the 1980 Mount St. Helens eruption), old models propose either an 800 meters in diameter comet or a 30 to 90 meters in diameter large meteorite exploding.

Tunguska represents a category of medium-sized impacts for which we have almost no record. It is still small if compared with the impact that created Meteor Crater some 50,000 years, yet more massive than any recent meteorites, like the Chelyabinsk meteor. On February 15, 2013, an asteroid with just 20 meters in diameter exploded 24 kilometers above the Russian city of Chelyabinsk.

Recovered fragment of the Chelyabinsk meteor.

D,Bressan

The resulting airburst caused widespread havoc and injured 1500 people. If the Tunguska object reached Earth just four hours later, instead of hitting a swamp it would have destroyed the city St. Petersburg, at the time populated by 150,000 people.

Using the age of fossil craters of large impacts and historical accounts of small meteors, scientists extrapolated the missing data for medium-sized impacts. Based on such estimates, a Tunguska-like event happens every 100 to 1,000 years. Using computer models to simulate the energy released by airburst of objects exploding mid-air, from stony asteroids of different size to an ice comet, new NASA research was able to characterize the properties of the Tunguska object in more detail. It was probably a stony (rather than icy) asteroid, measuring between 50 to 80 meters in diameter. It whipped through the atmosphere at 54,700km/h and exploded mid-air at an elevation of 5 to 10 kilometers above ground. In our solar system more than 40,000 asteroids with a diameter larger than 100 meters can be found. Based on current data on the asteroid population and the new size calculated for the Tunguska object, the research also reassesses the probability of such impacts on Earth. Impacts of mid-size rocky objects like Tunguska occur less frequently than previously thought—on the order of millennia, rather than centuries.

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