Last month marked the 111th anniversary of the Tunguska event, a blast that flattened trees across half a million acres of Siberian forest on June 20, 1908. Scientists have been puzzling over the details ever since. We now have fresh evidence about what transpired back then, in the form of new data gleaned from a well-documented rare meteor burst near Chelyabinsk, Russia, in February 2013. That data shores up the hypothesis that the Tunguska event was most likely due to an asteroid impact. The findings are described in a series of scholarly papers commemorating the event, published in a special July 15 issue of the journal Icarus.

Seismometers all over the world recorded the Tunguska impact, which hit 5.0 on the Richter scale in some locations. But there weren't many human eyewitnesses, given its remote location—first-hand observations came mostly from a few Russian settlers and Evenki natives. They described a streak of light across the sky, followed by another flash of light and a loud sound with accompanying shock wave. "Suddenly the sky appeared like it was split in two, high above the forest, the whole northern sky appeared to be completely covered with blazing fire," a farmer named Sergei Semenov recalled; he'd been having breakfast just 40 miles (64km) from the impact. "At that moment, I felt a great wave of heat as if my shirt had caught fire." The shock wave was strong enough to knock him off his chair.

Sky fall

Still, the impact site was so remote that nobody investigated for more than a decade. It wasn't until 1927 that Russian mineralogist Leonid Kulik led a scientific expedition to the area. His Evenki guides believed the blast had been a punishment from their god of thunder, Agda. Kulik, on the other hand, believed it had been a meteor and was surprised to find no impact crater. But trees had been scorched over a five-mile radius, with all their branches blown off. Kulik made three more expeditions, during which he discovered small bogs resembling potholes. He thought those might be impact craters but found an old stump at the bottom of one when he drained it, effectively ruling out that hypothesis.

Over the ensuing decades, researchers continued to debate the likeliest source of the Tunguska event, ultimately narrowing it down to two options. In 1934, British astronomer F.J.W. Whipple suggested it was due to a comet exploding in the atmosphere above Siberia. He cited the glowing night skies over the region for several days after the impact as evidence of dust and particles found in a comet's tail. Critics countered that the comet would have been traveling at such a shallow trajectory, it would have disintegrated before it reached Earth's lower atmosphere.

The current consensus is that it was most likely an asteroid-like object. Orbital modeling of its atmospheric trajectories concluded an 83% probability that the Tunguska object followed an asteroid-like trajectory and may have come from the asteroid belt. Scientists have analyzed fragments from the site and resin from trees in the vicinity and found high levels of materials commonly found in rocky asteroids. There was no crater because the object disintegrated before it hit the ground.

Re-opening the books

And then came the Chelyabinsk meteor event in 2013. [corrected] This time there were advanced instruments monitoring the event and plenty of eyewitnesses. The shock wave from the blast shattered windows and damaged local buildings, injuring some 1,600 residents. NASA Planetary Defense Officer Lindley Johnson called it “a cosmic wake-up call,” highlighting the need for better systems to detect large asteroids before they strike Earth. So NASA sponsored a workshop at the Ames Research Center in Silicon Valley, "re-examining the astronomical cold case of the 1908 Tunguska impact event." The papers published in Icarus are the result of the workshop discussions.

"Tunguska is the largest cosmic impact witnessed by modern humans," said Ames planetary scientist David Morrison. "It also is characteristic of the sort of impact we are likely to have to protect against in the future."

Combining computer models with video footage of the fireball and maps of the area, scientists deduced that the Chelyabinsk object was most likely a stony asteroid the size of a five-story building that broke apart 15 miles (24km) above the ground. The resulting shock wave was as powerful as a 550 kiloton nuclear explosion—and the Tunguska object was probably much larger. Based on the Chelyabinsk models—augmented with survey records from the Tunguska region shortly after the event—the scientists concluded the Tunguska object was probably stony (rather than icy), and measured between 164 and 262 feet in diameter (about 50 to 80 meters). It whipped through the atmosphere at 34,000 miles per hour (about 54,700km/h), and produced an amount of energy equivalent to the 1980 eruption of Mount Saint Helens.

Those models, plus current data on the asteroid population, also enabled researchers to calculate how frequently such impact events are likely to occur. The good news is that this research suggests mid-size rocky bodies like the one that likely caused the damage at Tunguska occur less frequently than previously thought—on the order of millennia, rather than centuries.

NASA remains committed to improving its systems for detecting possible asteroid impacts. "Because there are so few observed cases, a lot of uncertainty remains about how large asteroids break up in the atmosphere and how much damage they could cause on the ground," said Lorien Wheeler, an Ames researcher with NASA’s Asteroid Threat Assessment Project. "However, recent advancements in computational models, along with analyses of the Chelyabinsk and other meteor events, are helping to improve our understanding of these factors so that we can better evaluate potential asteroid threats in the future."