Tunguska: Half Solved

COLD OPEN0:00 to 0:35

In June 1908, a man named Semenov was sitting on his porch at a Siberian trading post when the sky to the north caught fire. The heat almost burned the shirt off him. The blast threw him off the porch, from about 65 kilometers away.

Out there, millions of trees went down. And there was no crater.

More than a century later, NASA says direct evidence of the object that caused it is still elusive and still debated. So what's settled, what isn't, and who died?

WHAT'S SETTLED0:35 to 2:30

Whatever came in that morning did its damage from the air. NASA puts the explosion somewhere between 6 and 10 kilometers up. That's why there's no crater under it. The blast came down from above.

You can read it in the trees. Across more than 2,000 square kilometers, the trunks lie in a radial pattern, all pointing away from one spot.

But right under the explosion, the trees stayed standing. NASA's Don Yeomans described them as upright, with their limbs and bark stripped away. Directly underneath, the blast came straight down, so nothing pushed the trunks sideways. And the shock wave moved fast enough to snap the branches off before the force could pass into the trunk. Thirty-seven years later, the same kind of branchless trees turned up at Hiroshima.

The heat was enough to set the forest alight. Britannica's explanation is that the blast wave that followed quickly put those fires out, so the forest was badly charred, but the fire never took hold.

And the reach of it. Instruments in the region recorded ground motion equal to a magnitude 5 earthquake. Barometers in England picked up the air wave as it went past.

For a while afterward, clouds formed so high that they caught sunlight from beyond the horizon, and people in parts of Europe and Asia could read a newspaper outdoors at midnight.

So the event itself is well documented. The arguing started with what the first scientist to reach the site didn't find.

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NINETEEN YEARS2:30 to 3:35

The first scientist to reach the site was a mineralogist named Leonid Kulik, and he didn't get there until 1927. Physicist Giuseppe Longo of the University of Bologna gives the reasons. The site was extremely remote, and then came World War One and the Russian Revolution.

The people who lived there, Evenki reindeer herders, weren't keen to talk about it either. Many believed the blast was a visitation by the god Ogdy, who had cursed the place.

Kulik went in expecting a meteorite crater. He found flattened forest and swamp. No crater, and no meteorite to bring home.

And when there's no object, there's room for everyone's theory. Over the years people blamed a tiny black hole, a chunk of antimatter, and, in a 1946 science fiction story, a Martian spaceship whose nuclear engine blew up. That last one, to be fair, started out as fiction.

Hold on to that missing object, because every argument in the rest of this story comes back to it. The first one is about size.

HOW BIG3:35 to 4:50

The usual estimate was 10 to 20 megatons of TNT. That's around a thousand times the bomb that destroyed Hiroshima.

Then, around the hundredth anniversary, physicist Mark Boslough at Sandia National Laboratories ran new supercomputer simulations. They included something older models had thrown away, the downward push of the air squeezed under the incoming object. His answer was 3 to 5 megatons. Reports of his work put the object anywhere from about 20 to 50 meters across.

NASA's current estimate for the object is bigger, 50 to 100 meters.

So why does that argument matter? Because if a smaller rock can flatten a forest this size, rocks that size come along more often. In Boslough's own words, collisions like this are "not as improbable as we had believed."

How often, then? Planetary scientist Philip Bland put Tunguska-sized airbursts at about once every 500 years.

And size is at least something they can model. What the object was made of is harder.

WHAT IT WAS, AND WHERE IT WENT4:50 to 6:55

The two candidates are a stony asteroid or a piece of a comet. In 2008, Boslough said the field had pretty much accepted a carbonaceous asteroid, but he wasn't sure it was "a slam dunk."

Then there's the question of where the rest of it went.

In 2007, an Italian team that included Longo pointed at Lake Cheko, about 8 kilometers northwest of the epicenter. It's about 50 meters deep, it has a cone-shaped bottom unlike the other lakes nearby, and it sits right along the estimated flight path. Their sonar found either a buried object or a dense layer of sediment about 10 meters under the middle of the lake bed. Their idea was that a chunk survived the blast and punched the lake into the ground.

The next year, Gareth Collins of Imperial College London and his colleagues pushed back. A hole like that would normally come with others, from the rest of the breaking object, and Cheko has none. There's no layer of thrown-out material around it. And aerial photos from 1938 show trees more than 30 years old on its shore, which points to a lake older than the blast.

That fight never really ended. The Italian team was still answering its critics in 2023.

In 2020, a team led by Daniil Khrennikov at Siberian Federal University went further. Their model says the object was iron, came in at a shallow angle, skimmed through the atmosphere, and kept going, back out into orbit around the sun.

Which would be one answer to where the rock went. It left.

Even a write-up of their paper on the site Astrobites notes that it goes against most of the research on Tunguska. So that one's open too.

Every one of these fights is about the object. The question that should've been the simplest is about people.

WHO DIED6:55 to 8:25

For a long time, the standard line was that no human deaths were confirmed. As recently as 2008, a centenary article featuring NASA's Don Yeomans said hundreds of reindeer died, but there was no direct evidence any person had.

But the Evenki had already given names.

In 1926, a Soviet official named Innokenty Suslov, who was organizing a large Evenki assembly, asked people in the area what had happened. A woman named Akulina said she was asleep in her tent with her husband Ivan and an old man when something shoved the tent, then shoved it again and knocked them all to the ground. The wind tore the cover away.

When she found Ivan, he'd been thrown about 40 meters, by Suslov's estimate. His arm had broken over a log, and the bone was showing through his shirt.

At the assembly itself, another old man said that when he told his father, Lurbuman, what he'd seen, his father got a fright and died. They buried him that same day.

Then the assembly named the dead. Lurbuman. Akulina's husband, Ivan. And a shaman, Uyban, who lost consciousness and died.

In 2019, Peter Jenniskens of the SETI Institute and his colleagues went back through more than 700 eyewitness accounts. They found at least 30 people inside or near the zone where trees fell. Many lost consciousness. At least three died. One of them was Ivan, the man with the broken arm.

And "at least" is the honest word. Suslov's interviews came 18 years after the fact, and the 2019 researchers say not everything relevant survived in the written record.

WHERE IT STANDS8:25 to 9:20

So here's where it stands. Something from space came in over Siberia, released its energy in the air, and flattened a forest. That much is settled. What it was, how big it was, where the rest of it went, and exactly how many people it killed are still open. Each of those gaps comes back to the same two problems. Nobody with instruments got there for 19 years, and nobody has a piece of the object that everyone accepts.

So if you've ever wondered whether the death toll is three people, or three people we know about, it's the second one.