June 30, 1908 In a remote part of Russia, a fireball was seen streaking across the daytime sky. Within moments, something exploded in the atmosphere above Siberia’s Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia.
This event – now widely known as the Tunguska event – is believed to have been caused by an incoming asteroid (or comet), which never actually struck Earth but instead exploded in the atmosphere, causing what is known as an air burst, three to six miles (5–10 kilometers) above Earth’s surface.
The explosion released enough energy to kill reindeer and flatten trees for many kilometers around the blast site. But no crater was ever found.
At the time, it was difficult to reach this remote part of Siberia. It wasn’t until 1927 that Leonid Kulik led the first Soviet research expedition to investigate the Tunguska event. He made a initial trip to the region, interviewed local witnesses and explored the region where the trees had been felled. He became convinced that they were all turned with their roots to the center. He did not find any meteorite fragments, and he did not find a meteorite crater.
Over the years, scientists and others concocted fabulous explanations for the Tunguska explosion. Some were pretty wild – such as the encounter of Earth with an alien spacecraft, or a mini-black-hole, or a particle of antimatter.
The truth is much more ordinary. In all likelihood, a small icy comet or stony asteroid collided with Earth’s atmosphere on June 30, 1908. If it were an asteroid, it might have been about a third as big as a football field – moving at about 15 kilometers (10 miles) per second.
Because the explosion took place so long ago, we might never know for certain whether it was an asteroid or comet. But in recent decades astronomers have come to take the possibility of comet and asteroid impacts more seriously. They now have regular observing programs to watch for Near-Earth Objects, as they’re called. They also meet regularly to discuss what might happen if we did find an object on a collision course with Earth.