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Home / Science / Recent research on Tunguska has revealed that such events occur less frequently than we thought

Recent research on Tunguska has revealed that such events occur less frequently than we thought



  The famous Tunguska event of 1908 scorched an eight-kilometer row of trees, dropping many in a particular radial pattern from the center of the blast.
Enlarge / The famous Tunguska event of 1908 scorched an eight-kilometer row of trees, dropping many in a particular radial pattern from the center of the blast.

Photo12 / Universal Images Group via Getty Image

Last month, the 111th anniversary of the explosion Tunguska event, an explosion that flattened on June 20, 1908 trees on an area of ​​half a million acres of Siberian forest made. Since then, scientists are puzzling over details. We now have new evidence of what was happening at the time, in the form of new data obtained from a well-documented rare meteorite eruption near Chelyabinsk, Russia, in February 2013. These data support the hypothesis that the Tunguska event was most likely due to an asteroid impact. The results are described in a series of scholarly articles commemorating the event, published in a special edition of the journal Icarus of 15 July 1945/96. Seismometers around the world have registered the Tunguska impact, the Richter scale reaching 5.0 points in some places. But there were not many human eyewitnesses, as it was a remote place – first-hand observations came mainly from a few Russian settlers and Evenki natives. They described a gleam of light across the sky, followed by another flash of light and a loud noise accompanied by a shockwave. "Suddenly the sky seemed to be divided into two parts, high above the forest, the entire northern sky seemed to be completely covered in blazing fire," recalled a farmer named Sergei Semenov. He had had breakfast only 64 km before the impact. "At that moment I felt a big heatwave, as if my shirt had caught fire." The shock wave was strong enough to knock him off the chair.

Celestial Fall

Nevertheless, the impact was so remote that no one investigated more than a decade. Only in 1927, the Russian mineralogist Leonid Kulik led a scientific expedition to the region. His Evenki leaders believed the blast had been a punishment for their thunder god Agda. Kulik, on the other hand, believed it was a meteor and was surprised to find no impact crater. But the trees had been scorched within eight kilometers, and all the branches had been blown away. Kulik made three more expeditions, where he discovered small swamps that resembled potholes. He thought these might be impact craters, but found an old stump at the bottom of a crater as he dewatered it.

In the decades that followed, researchers continued to discuss and pinpoint the most likely source of the Tunguska event. There are two options. The British astronomer F.J.W. Whipple suspected that it was a comet exploding in the atmosphere over Siberia. He cited the glowing night sky over the region for several days after impact as evidence of dust and particles in the tail of a comet. Critics countered that the comet was on such a low trajectory that it would have dissolved before it reached the Earth's lower atmosphere.

  The Chelyabinsk meteorite event in February 2013 had a trail of contrails, which was recorded in this picture about a minute later. The house-sized asteroid entered the Earth's atmosphere. "src =" https://cdn.arstechnica.net/wp-content/uploads/2019/07/chelyapic-640x427.jpg "width =" 640 "height =" 427 "srcset =" https: //cdn.arstechnica .net / wp-content / uploads / 2019/07 / chelyapic.jpg 2x
Enlarge / The meteorite event in Chelyabinsk in February 2013 had a contrail, as shown in this image about a minute after entry of the house-sized asteroid into the earth's atmosphere.

There is a general consensus that it was most likely asteroid-like object. The orbital modeling of its atmospheric trajectories showed a 83% probability that the Tunguska object follows an asteroid-like trajectory, possibly from the asteroid belt. Scientists have analyzed fragments of the terrain and resin of nearby trees and found large amounts of materials commonly found in rocky asteroids. There was no crater because the object crumbled before it fell to the ground.

Reopening of the books

And then in 2016 came the Chelyabinsk meteorite event. This time, there were advanced tools monitoring the event, and many eyewitnesses. The shock wave of the explosion destroyed windows and damaged local buildings. About 1,600 inhabitants were injured. NASA's planetary defense officer Lindley Johnson called it "a cosmic wake-up call," highlighting the need for better systems for detecting large asteroids from collision with Earth. NASA sponsored a workshop at the Ames Research Center in Silicon Valley, where it "re-examined the astronomical cold of the Tunguska impact event of 1908". The articles published in Icarus are the result of workshop discussions.

"Tunguska is the greatest cosmic influence that modern man experiences," said Ames planetary scientist David Morrison. "It is also characteristic of the type of impact we are likely to face in the future."

From the combination of computer models with fireball video and maps of the area, scientists concluded that the Chelyabinsk object is most likely a stone asteroid the size of a five-story building that broke 24 km above the ground. The resulting shockwave was as strong as a 550-kilo-tonne atomic blast – and the Tunguska object was probably much larger. Based on the Chelyabinsk models – supplemented by surveying reports from the Tunguska region shortly after the event – the scientists came to the conclusion that the Tunguska object is likely to be rocky (rather than icy) with a diameter of between 50 and 80 meters. , It whipped through the atmosphere at 34,000 miles per hour (about 54,700 km / h), producing an amount of energy equivalent to the eruption of Mount St. Helens in 1980.

These models, as well as recent data on the asteroid population, also enabled researchers to calculate how likely such events might occur. The good news is that this research suggests that mid-sized rocky bodies, such as those likely to cause damage in Tunguska, are less common than previously thought – on the order of millennia rather than centuries.

NASA continues to strive to improve its systems to detect possible asteroid impacts. "With so few observed cases, there is still a great deal of uncertainty about how large asteroids can break up in the atmosphere and how much damage they can do to the ground," said Lorien Wheeler, an Ames researcher at the NASA project evaluating the Threat of asteroids. "Recent advances in computational modeling and analysis of events in Chelyabinsk and other meteors, however, help us better understand these factors so that we can better assess potential asteroid threats in the future."


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