What Caused Siberia’s Mysterious Tunguska Event?

On the morning of June 30, 1908, a massive explosion over a remote part of Siberia sent shock waves through the ground, flattened trees for hundreds of square miles around and triggered wildfires. The few local reindeer herders who witnessed the event described a fireball trailing smoke across the sky, followed by a flash brighter than the sun and a thunderous boom.

Explanations of the Tunguska event have abounded since it happened—including the idea that it was an exploding alien spacecraft. Scientists now generally believe a giant asteroid was responsible: The space rock, estimated to have been up to 300 feet across, exploded about 3 to 6 miles above Earth’s surface.

Still, many questions remain.

“After more than a century of investigations, the ongoing debates and the enduring scientific interest in the Tunguska event demonstrate that we still do not fully understand what happened,” says geophysicist Luca Gasperini of Italy’s National Research Council, who has led two expeditions to the area. Gasperini stresses that understanding what happened in 1908 is critical in case similar phenomena happen again. A Tunguska-like event in a more populated area could, he argues, “produce devastating and possibly global consequences.”

The event is named after Siberia’s Tunguska River that's in a region so remote that almost no one lived there in 1908 and few people live there today. Gasperini traveled to the area in 1999 and was struck by its isolation. “The overwhelming sensation was that I had reached the end of the world,” he says.

In fact, the first reports that something big had happened in June 1908 didn’t appear in Russian newspapers until a few weeks later. Bright lights had been seen in the sky, windows in remote towns had been shattered and loud rumblings like thunder had been heard throughout the region. It would be nearly 20 years before the first scientific investigators arrived in 1927. Even then, however, what researchers found was astonishing.

Photographs show the blast had leveled more than 800 square miles of forests, toppling more than 80 million trees. Soviet mineralogist Leonid Kulik, who led the expedition, thought the blast had been caused by a massive meteorite hitting the ground. But he never found a crater, meteorite fragments or any chemical evidence of an impact.

When Gasperini and his team first visited more than 90 years after the event, much of the forest had regrown and trees obscured the damage. But signs remained, including trees uprooted by the explosion, submerged trunks in swamps and a few older trees that had survived the blast but showed abnormal growth rings dating to the event. Researchers examined resin from surviving trees and identified particles containing iron, calcium, aluminum, silicon and other elements that could be remnants of the object that caused the blast.

Scientific debate has offered up multiple theories about the cause of the event. One theory suggests that a comet, rather than an asteroid, caused the blast. The comet theory holds that the lack of a crater could be explained by the mostly-icy comet breaking up and vaporizing in the atmosphere.

But scientists in 2013 analyzed samples from peat bogs near the Tunguska epicenter and reported evidence of meteoritic material, supporting the idea that an asteroid rather than a comet was the source of the blast. Most scientists now think the Tunguska event was caused by an asteroid exploding several miles above the ground—an airburst like the Chelyabinsk meteor in 2013.

Estimates of the explosion’s energy vary, with some putting it at about 5 megatons of TNT—more than 300 times the explosive yield of the atomic bomb dropped on Hiroshima. Despite the ferocity of the blast, however, the remote location prevented many deaths. Fewer than five people are thought to have died in the blast, including two Indigenous reindeer herders who suffered injuries from the explosion.

An airburst would explain why no large impact crater has ever been found. But Gasperini and his colleagues have proposed that a large fragment from the exploding object did collide with the Earth, creating the small freshwater Lake Cheko about 5 miles from the blast’s epicenter.

“The lake is young, most likely no more than about a century old,” he explains. Several pieces of scientific evidence gathered during his expeditions support the idea that it was caused by the blast. In addition, the team’s seismic surveys had identified a “hard reflector” beneath the surface of the lake, but further investigation has not yet been possible, Gasperini says.

If Earth is threatened by a similar event, the consequences of an airburst over a populated area—rather than remote Siberia—could be catastrophic.

In 2022, NASA conducted its Double Asteroid Redirection Test, or DART, to determine whether a spacecraft could alter an asteroid’s orbit. The test successfully changed the orbit of Dimorphos, a small moonlet of the asteroid Didymos. But the asteroid was no threat, and the change in its trajectory was minor.

Meanwhile, other proposals for averting an imminent asteroid threat involve nuclear weapons: either by a "stand-off" burst that would vaporize the surface of incoming space rock and send it flying away, or by blasting it into pieces.

“If we have enough warning time, we could potentially launch a nuclear device, sending it millions of miles away to an asteroid that is headed toward Earth,” says physicist Mary Burkey of Lawrence Livermore National Laboratory in California. “We would then detonate the device and either deflect the asteroid, keeping it intact but providing a controlled push away from Earth. Or we could disrupt the asteroid, breaking it up into small, fast-moving fragments that would also miss the planet.”