Solving the Tunguska mystery

A meteor that exploded over Russia earlier this year could finally help to explain what flattened millions of trees in 1908

It’s just after 7 on a quiet Tuesday morning in June 1908 when a dazzling fireball streaks across the Siberian sky. Minutes later an immense blast topples 80 million trees and knocks people off their feet 60 kilometres away. It’s the violent end of an alien dogfight, with one spaceship destroyed in mid-air and the other turning and vanishing into space.

His voice trembles as 79-year-old retired Russian physicist Viktor Zhuravlyov tells me this rather unorthodox theory of what happened that day at Tunguska. The enigma has fascinated scientists for more than a century. Something exploded over the Siberian taiga – but what?

Back in 1959, when he had just graduated from Tomsk State University, Zhuravlyov joined one of the earliest expeditions to the region, marching for three days from the nearest town of Vanavara to get there. But neither he nor the hundreds of other researchers who have visited the area ever found a crater or any debris.

And so they keep guessing. Was it an icy comet? An asteroid? The Earth spewing out a huge cloud of methane gas? Antimatter? A black hole colliding with our planet? Aliens? All of these, and more, have been considered. But they all have a little something that just doesn’t add up, keeping Tunguska shrouded in mystery.

Another fire in the sky may finally lift the veil. Earlier this year, thousands of people in the Chelyabinsk region of Russia watched as a large fireball shot across the morning sky before exploding. The video footage and measurements from government sensors have given us an unprecedented insight into what happens when a large meteor has Earth in its sights. Events at Chelyabinsk look set to help us solve the mystery of Tunguska once and for all.

So what do we really know about Tunguska? Scientists have information on the damage the blast caused and the strength of the seismic waves generated. But the rest is based on estimates with various degrees of uncertainty. It is assumed that the event was an explosion between 5 and 10 kilometres above ground, releasing at least 1000 times more energy than the atomic bomb that destroyed Hiroshima and creating an enormous shock wave that knocked down trees and triggered fires.

If the initial newspaper reports could be relied on, there wouldn’t be a mystery. On 10 July 1908, The South Telegraph in Rostov-on-Don declared with confidence that a meteorite had crashed to Earth. It even carried eyewitness accounts: “The meteorite fell with an extremely loud noise and deafening impact… A train in the area stopped and let the passengers out, the crowd went to see the faraway traveller. But they couldn’t get a close look at the meteorite because it was scorching hot… Later, when it cooled down, other people examined it, and according to them, the meteorite is almost entirely buried underground, only its top is visible – a rocky whitish mass.”

It took years, though, before the first scientific expedition was sent to the site. At the time, Russia had other problems – the revolution of 1905, the first world war and the October Revolution of 1917. A fallen meteorite was not a priority. Finally, in 1921, Russian mineralogist Leonid Kulik arrived in central Siberia and tracked down eyewitnesses. Their accounts convinced him that the Tunguska fireball was indeed a meteorite. But when he returned in 1927 and finally reached the site it was supposed to have crashed into, he was in for a surprise. The whitish mass mentioned in the eyewitness accounts was nowhere to be found. Nor were any other fragments of space rock, for that matter. Kulik couldn’t find a crater, either.

What he did see were damaged trees – millions upon millions of them. Some were still standing, branchless like telegraph poles, but most had fallen in a strange, butterfly-like pattern, with their roots pointing towards the epicentre.

His reports sparked curiosity around the globe. After the second world war, in which Kulik fought and died, Moscow sent another expedition to the site. The trip of 1958 was led by Russian geologist Kirill Florensky from the Vernadsky Institute of Geochemistry and Analytical Chemistry in Moscow. “At the end of the 1950s, there was an idea that it was either an icy comet or an asteroid that got completely fragmented, and that we should be looking for some material from it,” says Mikhail Nazarov, the current head of the institute’s Laboratory of Meteoritics. “Scary thing is, had the body entered the atmosphere just 5 hours later, it would have hit St Petersburg and destroyed it.” Having searched the woods and swamps in vain for anything even remotely resembling meteorite fragments, Florensky’s team concluded that there had been no impact; the Tunguska blast was most probably an airburst – an explosion in the atmosphere.

But of what? Remember, this was the era of the first reported UFO sightings. Many people, including scientists, were easily excited by apparent anomalies and came up with adventurous explanations. The suggestion of a fight between alien spaceships didn’t seem that far-fetched to some.

As recently as this March, mathematician Vladimir Rubtsov from Kharkiv, Ukraine, posted a paper on the arxiv server hypothesising that the event was caused by neither a comet nor an asteroid, but some new unknown space body – maybe even an alien spaceship. He died of cancer, aged 64, shortly after completing his book The Tunguska Mystery, which details the many different explanations for the event. “My husband was passionate about Tunguska till the day he died. Solving the mystery was extremely important to him,” says his widow Natalia Kamentseva.

Most researchers have tried to find a more conventional explanation. In 2008, for example, Natasha Artemieva at the Planetary Science Institute in Tucson, Arizona, used computer modelling to see whether an airburst could produce the damage seen at Tunguska. Her calculations suggest that the airburst of a 40-metre-wide object would result in shock waves that cause a distinctive butterfly pattern of felled and damaged trees.

Not everyone agrees with the airburst theory. Giuseppe Longo and his team at the University of Bologna and the Institute for Marine Geology, both in Italy, are convinced that there is a crater and claim to have found it. They went to Siberia in 1999 and headed straight to Lake Cheko, which Longo believes was not on any map of the area prior to 1908. Unfortunately for Longo, it seems that locals knew of the lake long before the blast.

Nazarov has also been to the area. He went there in the 1980s to analyse peat bogs, which contain layers of dead plant material that can be easily dated. “In the layer of the bogs corresponding to 1908, we found much more iridium,” says Nazarov. “It’s an element that is typical for space bodies but is usually found in very small concentrations on Earth.” This lends weight to the idea that an object from space rapidly decelerated in the lower atmosphere and then exploded, spraying the Earth with elements from out of this world. Other scientists have confirmed the presence of iridium at the Tunguska site. But was it a comet or an asteroid that did the damage? Nazarov’s money is on a comet because of the lack of fragments, but others, including Longo, favour an asteroid.

After going around in circles for over a century, researchers now have an opportunity to solve the enigma. Fast forward to 15 February 2013 and the Chelyabinsk region near the Ural mountains. Once again a bright fireball lights up the morning sky. Seconds later it explodes. The shock wave from the blast blows out windows, injuring more than 1200 people.

This time, though, there are rock fragments, which are picked up and analysed. They show that the Chelyabinsk blast was caused by a stony asteroid that exploded at an altitude of about 20 to 25 kilometres. The meteor came in at a shallow angle, probably “right from the asteroid belt”, says David Kring, principal investigator for NASA’s Center for Lunar Science and Exploration in Houston, Texas.

The event was exceptionally well recorded, thanks to the dashboard cameras used by many drivers in Russia to counter police corruption and settle traffic accident disputes. The explosion also produced infrasound waves, inaudible to human ears but loud and clear to the worldwide network of sensors designed to detect nuclear explosions. These measurements show that the energy of the Chelyabinsk airburst was equivalent to about 400 kilotonnes of TNT, some 30 times more than the atomic bomb dropped on Hiroshima. Based on this, researchers have calculated that the meteor weighed about 10,000 tonnes as it entered the atmosphere and measured about 18 metres in diameter. Most importantly, the meteor’s fireball and blast showed a striking similarity to what happened at Tunguska.

Armed with all this data, scientists hope to time travel – in a way. They want to use Chelyabinsk “to better constrain what happened in Siberia in 1908”, says astronomer Bill Cooke, head of NASA’s Meteoroid Environment Office at the Marshall Flight Space Center in Huntsville, Alabama.

Both a comet and an asteroid could have produced an airburst large enough to trigger a shock wave able to uproot pine and cedar trees. However, a comet would produce “more blast damage on the surface than an asteroid exploding at the same altitude”, says Kring.

Researchers plan to use the precise measurements of Chelyabinsk’s energy, the altitude of its explosion and the damage inflicted, integrating these readings with an analysis of the asteroid’s fragments to create a computer model. Once that’s done, they will run computer simulations of the event that can then be applied to Tunguska.

The only exact data about Tunguska is the seismic record and the extent of the damage, although there is also a good guesstimate of its altitude based on measurements of the shock wave and data from nuclear tests. For the rest, there is uncertainty. “Was it 2 megatonnes of energy or 20 megatonnes of energy? We don’t know,” says Kring. “But now we will – by applying the Chelyabinsk model and calibrating the aerial damage with energy as a function of altitude.”

And once scientists know the energy, it will be straightforward to determine the mass of the body, too. “I think that for a lot of people, the observations of Chelyabinsk will lend support to the notion that Tunguska was actually produced by a weak stony asteroid as opposed to a comet,” he says. However, Gareth Collins at Imperial College London argues that it is too early to discard the comet theory. After all, comets are known to carry a lot of energy, with one telling example being Shoemaker-Levy 9, which slammed into Jupiter in 1994 and left the planet with visible scars.

Marc Boslough of the Sandia National Laboratories in Albuquerque, New Mexico, is already deep into crunching the numbers and may have a definitive answer soon. Even before the Chelyabinsk meteorite hit, he was running computer simulations of what happened at Tunguska, using estimated data.

Now he is confident he can improve his models. “I think that Chelyabinsk will increase our confidence in our previous models of Tunguska,” he says, adding that he has already plugged the data from Chelyabinsk into one of his previous Tunguska simulations. The next step will be applying the new model to study Tunguska in much more depth.

But if Tunguska was indeed an asteroid, how is it possible that a huge space rock exploded without leaving any traces at all? Small meteoroids explode and fragment daily, but until this year, some had been reluctant to accept the idea that large space rocks could do the same.

Heavy metal

One possible explanation lies with iron. Meteors containing a large amount of iron tend to survive their journey through Earth’s atmosphere. Take the Sikhote-Alin meteorite which fell in the mountains of Russia’s far east in 1947. It is over 90 per cent iron, and when it hit the atmosphere it fragmented into just a few pieces, the largest weighing 1750 kilograms.

Contrast that with the Chelyabinsk meteorite, which is classed as an “LL chondrite”, meaning it has a low metal content, and is low in iron in particular. This weakens the structure of such stony meteors significantly, so they can break into myriad fragments. “Compare a piece of rock with a piece of iron: if you hit each with a hammer, the rock shatters, the iron may get a dent,” says Detlef Koschny, head of the Near Earth Objects team at the European Space Agency, based in Noordwijk, the Netherlands.

As a stony meteor perforates the atmosphere faster than the speed of sound, the heat generated ablates material away. The rock then enters into hypersonic shock, leading to an explosion before it reaches the ground. “We saw this with Chelyabinsk and this is what apparently happened with Tunguska, based on eyewitness records,” Koschny says.

The lesson from Chelyabinsk and other fireball events is that you had better get to the site fast if you want to find something. “With Tunguska, the first expedition happened almost 20 years after the blast,” says Koschny. “But meteorites change with time. They get weathered, all sorts of stuff starts growing on them, they’re being modified when they just sit on Earth. So unless you collect a lot of stuff and analyse everything in the lab, you’ll never find anything.”

At the very least, Chelyabinsk could help eliminate all the other theories. “Of course it’d be much more fun if it were a UFO or a secret American weapon, but I’m afraid it probably wasn’t,” says Koschny.

(Article written by Katia Moskvitch and first appeared in New Scientist Magazine).

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