Early one ordinary morning, around 7am local time, one of the most extraordinary events in recent times took place in Siberia. On the 30th June, 1908, near the Podkamennaya Tunguska River, something – very large and moving very fast – detonated in the air about 3 miles up. It’s difficult to describe the scale of the explosion.
We can offer figures such as “equivalent to 15 – 20 megatons of TNT – 1,000 times as powerful as the Hiroshima bomb – but how many people can relate to that? The blast is estimated to have been as powerful as a magnitude 5.0 earthquake – which some people can certainly understand. It knocked people to their feet 40 miles away, and broke windows for hundreds of miles. On the recently-completed Siberian Express Railway, 400 miles from the epicentre, a driver was forced to bring his train to a halt as the ground shook, and the tracks ahead of him rippled widly.
As ever, perhaps, human testimony brings it closest to home:
“We had a hut by the river with my brother Chekaren. We were sleeping. Suddenly we both woke up at the same time. Somebody shoved us. We heard whistling and felt strong wind. Chekaren said, ‘Can you hear all those birds flying overhead?’ We were both in the hut, couldn’t see what was going on outside. Suddenly, I got shoved again, this time so hard I fell into the fire. I got scared. Chekaren got scared too. We started crying out for father, mother, brother, but no one answered. There was noise beyond the hut, we could hear trees falling down.
Chekaren and I got out of our sleeping bags and wanted to run out, but then the thunder struck. This was the first thunder. The Earth began to move and rock, wind hit our hut and knocked it over. My body was pushed down by sticks, but my head was in the clear. Then I saw a wonder: trees were falling, the branches were on fire, it became mighty bright, how can I say this, as if there was a second sun, my eyes were hurting, I even closed them. It was like what the Russians call lightning. And immediately there was a loud thunderclap. This was the second thunder. The morning was sunny, there were no clouds, our Sun was shining brightly as usual, and suddenly there came a second one!
Chekaren and I had some difficulty getting out from under the remains of our hut. Then we saw that above, but in a different place, there was another flash, and loud thunder came. This was the third thunder strike. Wind came again, knocked us off our feet, struck against the fallen trees. We looked at the fallen trees, watched the tree tops get snapped off, watched the fires.
Suddenly Chekaren yelled ‘Look up’ and pointed with his hand. I looked there and saw another flash, and it made another thunder. But the noise was less than before. This was the fourth strike, like normal thunder. Now I remember well there was also one more thunder strike, but it was small, and somewhere far away, where the Sun goes to sleep.”
- Chuchan of the Shanyagir tribe
The devastation was utter. 80 million trees in the heavily forested area were felled, set to fire, or annihilated. More than 800 square miles were destroyed.
And yet, we were enormously fortunate for two reasons.
First of all, the Tunguska event was actually fairly small. Estimates of the size of the impacting object range from perhaps 50 – 100 metres across. The Space Weather site lists so-called near-earth objects – NEOs – which are tracked because they pass close to the Earth. A quick look through the table of upcoming ‘near misses’ shows many objects at or around this size, and a few much, much larger.
Secondly, it struck in the middle of nowhere – in one of the most sparsely populated places on the planet. If you were to plan to have something like this happen, the wilds of Siberia are about the safest and most remote place on Earth. Imagine drawing a 15-mile wide circle centred around Big Ben, The Eiffel Tower, or Time Square. Now imagine every building, every shop, church, museum, every man, woman and child within that circle, instantly destroyed.
Events such as Tunguska helped inspire the ongoing search for NEOs. With sufficient warning, we have options and opportunities to take action against threats. If we don’t know they’re there, because we’re not actively searching, then we would prove to have evolved no further than the dinosaurs. (The image below is of the minor planet Tunguska – no connection to the impact event, but just one of the hundreds of thousands of asteroids which are being tracked by the Catalina Sky Survey. Tunguska does not present a risk of collision with the earth).
The one significant puzzle still outstanding is: what exactly caused the explosion? Puzzlingly, for such a large event, no meteoritic material – no material of any kind – has been retrieved, despite a number of expeditions to the area. This is unusual – a large event like this would be expected to generate a good deal of meteorites for analysis.
This has caused some to speculate that, rather than an asteroidal object, the Tunguska impactor was in fact a comet – far less dense, and having so little structure that the material was completely annihilated in the explosion. However, speculation has been rife, and proposed theories range from the ridiculous to the sublime. A short list of some of them is given below (based on Surendra Verma’s book “The Tunguska Fireball“):
You can take your pick, but as astronomer Dr Vitalii Bronshten said: “We astronomers know two types of solar system bodies – comets and asteroids. We do not know of anything else.”