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Meteor that blasted millions of trees in Siberia only 'grazed' Earth, new research says

A mysterious blast in 1908, thought to have been caused by a meteor, flattened a Siberian taiga forest. This photo was taken in 1938, during an expedition by Russian mineralogist Leonid Kulik, investigating the event.
A mysterious blast in 1908, thought to have been caused by a meteor, flattened a Siberian taiga forest. This photo was taken in 1938, during an expedition by Russian mineralogist Leonid Kulik, investigating the event.
(Image: © Sovfoto/Universal Images Group via Getty Images)

A new explanation for a massive blast over a remote Siberian forest in 1908 is even stranger than the mysterious incident itself.

Known as the Tunguska event, the blast flattened more than 80 million trees in seconds, over an area spanning nearly 800 square miles (2,000 square kilometers) — but left no crater. A meteor that exploded before hitting the ground was thought by many to be the culprit. However, a comet or asteroid would likely have left behind rocky fragments after blowing up, and no "smoking gun" remnants of a cosmic visitor have ever been found. 

Now, a team of researchers has proposed a solution to this long-standing puzzle: A large iron meteor hurtled toward Earth and came just close enough to generate a tremendous shock wave. But the meteor then curved away from our planet without breaking up, its mass and momentum carrying it onward in its journey through space.

Related: Crash! 10 biggest impact craters on Earth

On the morning of June 30, 1908, the sky above Siberia flared so bright and hot that a witness standing dozens of kilometers from the site thought that his shirt had caught fire, said Vladimir Pariev, co-author of the new Tunguska study and a researcher with the P. N. Lebedev Physical Institute of the Russian Academy of Sciences in Moscow. 

Following the bright light, which lasted for about 1 minute, was an explosion that smashed windows and knocked people off their feet in a town more than 35 miles (60 km) away, the BBC reported. "The sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire," another witness said in a testimonial. Energy released by the blast was later estimated by scientists to be 185 times greater than that of the atomic bomb dropped on Hiroshima in 1945, according to NASA.

Initial explanations for the blast included volcanic eruptions and mining accidents, according to NASA, but those claims were not supported by physical evidence. Other later suggestions were more far-fetched, such as a crashed UFO or a black hole collision with Earth — a study describing the black hole hypothesis was published in the journal Nature in 1973 (and was soundly debunked in another Nature study published just a few months later).

The most widely accepted scientific explanation is that a rocky asteroid or comet entered Earth's atmosphere and then disintegrated with a bang about 3 to 6 miles (5 to 10 km) above the ground, Pariev told Live Science in an email. But such an explosion should have strewn the ground with rocky debris, which no one has ever found. By comparison, a meteor that exploded over Chelyabinsk, Russia, in February 2013 broke into fragments that were discovered within a week, Pariev said.

What if, the researchers questioned, the Tunguska meteor were made of iron rather than rock? Could a massive iron meteor "graze" Earth's atmosphere, approaching close enough to generate a powerful shock wave, then yank free of the planet's gravitational pull and escape without fragmenting?

Related: Top 10 ways to destroy Earth

To test that hypothesis, the scientists calculated meteor paths using computer models. They looked at objects that were as small as 164 feet (50 meters) across and as large as 656 feet (200 m) in diameter. Objects were made of rock, ice or iron, and approached in a trajectory that brought them within 6 to 10 miles (10 to 15 km) of Earth's surface. 

The scientists' calculations showed that space bodies made of rock and ice would completely disintegrate under the enormous pressures generated by their passage through the tropospheric altitudes. "Only asteroids made of iron larger than 100 m [328 feet] in diameter can survive and not get cracked and fragmented into many separate pieces," they said. 

The researchers estimated that the Tunguska meteor likely measured between 328 and 656 feet (100 and 200 m) in diameter, and hurtled through Earth's atmosphere at roughly 45,000 mph (72,000 km/h). During its fiery passage, the meteor would lose some of its mass. But iron shed by a meteor traveling at such speeds would have escaped as gas and plasma, oxidized in the atmosphere and then dispersed on the ground, becoming nearly indistinguishable from terrestrial iron oxides, according to the study.

Prior studies have calculated the power of shock waves produced by meteors based on the object entering Earth's atmosphere at a very steep angle "and either hitting the ground or exploding in midair," Pariev said. 

In the case of the Tunguska meteor, the iron-rich space object could have entered Earth's atmosphere at a very shallow angle — about 9 to 12 degrees tangential to the surface. It then would have grazed through the atmosphere, creating a shock wave at an altitude of around 6 to 10 miles (10 to 15 km) above the ground, capable of flattening trees for hundreds of kilometers and scorching the surface. But because of the meteor's mass and momentum, it didn't break up; it then exited the atmosphere and returned to space, the researchers reported.

Related: Space-y Tales: The 5 strangest meteorites

However, some lingering questions about this scenario remain, said Mark Boslough, a research professor at the University of New Mexico and physicist with Los Alamos National Laboratory. 

Boslough, who was not involved in the study, told Live Science in an email that if an object "skimmed through the atmosphere" and didn't blow up, the resulting shock wave would be significantly weaker than an explosion's blast wave.

"An object that survived such a transit through the atmosphere could not have descended close enough to the surface for a sonic boom to do the kind of damage that was observed at Tunguska," Boslough said. 

What's more, the pattern of felled trees at the site is radial — emanating from a single point of tremendous energy release, he said. That's something you'd expect to see after an explosion rather than a sonic boom, "even if it had been strong enough to blow trees over." Boslough added that eyewitness accounts at the time of the incident "are consistent with an object that was descending toward the surface before it exploded." 

While the study authors didn't numerically calculate the impact of a shock wave that a "grazing" iron meteor of this size could produce, their estimates still suggest that such a wave would be powerful enough to flatten trees and damage the ground as the Tunguska event did, Pariev said in the email.

"Detailed calculations of the shock waves from a grazing asteroid is the subject of our ongoing research," he added.

The findings were published online in the March issue of the journal Monthly Notices of the Royal Astronomical Society.

Originally published on Live Science.

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  • Think twice
    It's interesting this hypothesis. It could explain what I saw once when I was young , with a friend. We both saw and could not explain what looked essentially like a rock shaped object maybe slightly smaller than the space the full moon takes in the night sky... brightly coloured and lit , irregular shaped and tumbling . It traversed across sky not slow but not nearly as fast as a meteor and left no trail and no bang or flash. More sedate than anything else. Just after dark, around sept, and in mid sixties. I can't remember exact year. South Ontario near Ottawa. If I remember correctly it was going roughly east west. I've tried recently to look up any unusual events at that time and place but nothing specifically like that . Although there was the flaring bolide of sept 17 1966 which matches time and place. Reading this article ,..maybe it was an asteroid that grazed earth, flouresced
    and fragmented slightly to produce the observed seperate event of the bolide?
    Reply
  • G Ollie
    Not a large weather balloon, perhaps, falling and tumbling (being irregular shaped) and being lit up much brighter higher up by the sun's rays (as it would be for us on the earth's surface at any dusk on dawn)? Was just thinking about a possible explanation
    Reply
  • mvk
    In lieu of these findings, I'd say it was a small iron meteor with a perpendicular vector towards earth at twice the speed. Going 40km/s, it would have disintegrated into plasma and created one hell of a blast 🤷‍♂️
    Reply
  • dohnjoe
    mvk said:
    In lieu of these findings, I'd say it was a small iron meteor with a perpendicular vector towards earth at twice the speed. Going 40km/s, it would have disintegrated into plasma and created one hell of a blast 🤷‍♂️

    Well, I'm glad we got this sorted.
    Reply
  • dfjchem721
    Have to agree with Mark Boslough, that such a grazing event would not produce a radial dispersion of trees, etc.

    From the article:

    Boslough:

    "What's more, the pattern of felled trees at the site is radial — emanating from a single point of tremendous energy release, he said. That's something you'd expect to see after an explosion rather than a sonic boom, "even if it had been strong enough to blow trees over." Boslough added that eyewitness accounts at the time of the incident "are consistent with an object that was descending toward the surface before it exploded."

    end quote

    The only logical conclusion is the original, well-conceived postulate that a loose, rocky asteroid detonated due to heating and a vast over-pressure caused by its high speed entry into the atmosphere, and the inability of the asteroid to maintain its structural integrity from the over-pressure. That no sizable debris was found is likely an indication of the force of the detonation, which likely vaporized the asteroid. Fine debris isolated from the site would appear to finish this story (see below).

    A quote from wiki*

    "Expeditions sent to the area in the 1950s and 1960s found microscopic silicate and magnetite spheres in siftings of the soil. Similar spheres were predicted to exist in the felled trees, although they could not be detected by contemporary means. Later expeditions did identify such spheres in the resin of the trees. Chemical analysis showed that the spheres contained high proportions of nickel relative to iron, which is also found in meteorites, leading to the conclusion they were of extraterrestrial origin. The concentration of the spheres in different regions of the soil was also found to be consistent with the expected distribution of debris from a meteoroid air burst. Later studies of the spheres found unusual ratios of numerous other metals relative to the surrounding environment, which was taken as further evidence of their extraterrestrial origin.
    Chemical analysis of peat bogs from the area also revealed numerous anomalies considered consistent with an impact event. The isotopic signatures of carbon, hydrogen, and nitrogen at the layer of the bogs corresponding to 1908 were found to be inconsistent with the isotopic ratios measured in the adjacent layers, and this abnormality was not found in bogs located outside the area. The region of the bogs showing these anomalous signatures also contains an unusually high proportion of iridium, similar to the iridium layer found in the Cretaceous–Paleogene boundary. These unusual proportions are believed to result from debris from the falling body that deposited in the bogs. The nitrogen is believed to have been deposited as acid rain, a suspected fallout from the explosion."

    end quote

    Another quote from wiki*:

    "Since the second half of the 20th century, close monitoring of Earth's atmosphere through infrasound and satellite observation has shown that asteroid air bursts with energies comparable to those of nuclear weapons routinely occur, although Tunguska-sized events, on the order of 5–15 megatons, are much rarer. Eugene Shoemaker estimated that 20-kiloton events occur annually and that Tunguska-sized events occur about once every 300 years. More recent estimates place Tunguska-sized events at about once every thousand years, with 5-kiloton air bursts averaging about once per year. Most of these air bursts are thought to be caused by asteroid impactors, as opposed to mechanically weaker cometary materials, based on their typical penetration depths into the Earth's atmosphere. The largest asteroid air burst to be observed with modern instrumentation was the 500-kiloton Chelyabinsk meteor in 2013, 0which shattered windows and produced meteorites."

    end quote

    The Tunguska event almost certainly resulted from the terminal penetration of a large rocky asteroid that exploded due to the heat and vast over-pressure on the body as it raced into the atmosphere. Cannot see how a high altitude shock wave could produce a radial pattern of destruction. Only an air-burst overhead would cause such a pattern. And there is no doubt that isotope ratios (common to asteroids) found in the area of destruction are not going to be deposited by a simple high altitude shock wave.

    For an excellent overview on this bolide, and other theories, see:

    * https://en.wikipedia.org/wiki/Tunguska_event
    Reply
  • Think twice
    G Ollie said:
    Not a large weather balloon, perhaps, falling and tumbling (being irregular shaped) and being lit up much brighter higher up by the sun's rays (as it would be for us on the earth's surface at any dusk on dawn)? Was just thinking about a possible explanation
    Yes possibly one could contest it as a weather balloon but I would be sceptical of this as my memory and my understanding of weather balloons and how they could break up and explode in fireballs are not consistent.I think about this sighting of mine many years ago almost daily it was so strange. It definitely looked structurally sound and solid. Just like an irregular shaped coloured rock tumbling through the sky. The only reason I mentioned it here was the premise in the article. It implies that if an asteroid can graze the earth atmosphere and break up, then one that grazes it at a higher altitude...won't break up, be visible still, be heated so as to Glow and have small disintegrations occur. And my memory places it almost exactly at the same time in the so called "Ottawa Bolide" well documented . My memory has it just past dusk early September mid sixties. Going roughly E-W ( Ive checked google streets for the original location I observed) ...a look at Google maps shows it was consistent with the Ottawa bolide which although stated as northerly, if one looks at google maps, it's trajectory was S. Ontario to lake Michigan which actually is e-w. What's more important for me is that if we have many documented fireballs of asteroids that break up in the narrow window of a few tens of miles above our surface, there must be many many more asteroids that randomly skim our earth's surface between tens and hundreds of miles altitude...And go unnoticed.
    Reply
  • dfjchem721
    Think twice said:
    It's interesting this hypothesis. It could explain what I saw once when I was young , with a friend. We both saw and could not explain what looked essentially like a rock shaped object maybe slightly smaller than the space the full moon takes in the night sky... brightly coloured and lit , irregular shaped and tumbling . It traversed across sky not slow but not nearly as fast as a meteor and left no trail and no bang or flash. More sedate than anything else. Just after dark, around sept, and in mid sixties. I can't remember exact year. South Ontario near Ottawa. If I remember correctly it was going roughly east west. I've tried recently to look up any unusual events at that time and place but nothing specifically like that . Although there was the flaring bolide of sept 17 1966 which matches time and place. Reading this article ,..maybe it was an asteroid that grazed earth, flouresced and fragmented slightly to produce the observed seperate event of the bolide?


    Reviewing possible meteors/bolides observed in Ontario with a mid-60s time-frame leaves only the one you suggested for 17 September, 1966, which was moving from the south-east to the north-west, starting around 8:48 PM EDT, as you noted ("just after dark").

    It, or a fragment, is believed to have splashed into Lake Huron about 10 mile off Kincardine. While I cannot find anything about a definite explosion, it is often called the "1966 Southern Ontario Bolide" (1). Since there were reports of loud detonations (2), it seems likely it exploded, and the sounds were both from sonic booms and breakup of the meteor. I can understand why you remember it almost every day. It must have been quite spectacular!

    (1) https://pineriverobservatory.wordpress.com/2013/02/16/canadas-own-big-meteor-of-1966/
    (2) http://adsabs.harvard.edu/full/1966JRASC..60..257H
    Reply