Q15: The Tunguska Fireball On June 30, 1908

The Tunguska Fireball

On June 30, 1908, an enormous fireball shot across the sky and exploded high above the remote Tunguska River Valley in the Siberian region of Russia. Twenty miles away, huts were flattened and people were flung into the air. Villagers 40 miles away felt the heat and heard explosions, and tremors were recorded at a German seismic station more than 3,000 miles away.

What caused the intense light and the horrendous destruction of the Tunguska Fireball, as it came to be called? Scientists and laypeople have debated the question for over 100 years. Not until 1927 did a scientific expedition, led by Russian scientist Leonid Kulik, reach the remote site. Kulik discovered a vast region of scorched and downed trees, their trunks pointing away from the center of the blast. Convinced that a meteorite was responsible, Kulik searched for evidence—a crater, a buried meteorite, even fragments of the exploded mass. He never found it.

Other scientists, amateur astronomers, and the public suggested alternative theories of what had happened. Some thought it was an earthquake. Perhaps, others said, a black hole—a theoretical object in space where gravity is so strong that even light cannot get out—had collided with Earth or an alien spaceship carrying nuclear material had crashed.

For many years Russian scientists believed that the Tunguska Fireball was a comet—a cluster of dust and frozen gases—that had exploded a few hundred feet above the ground. They pointed to the lack of meteoric evidence and the June 30 date, a time of year when Earth was passing through cometary debris. Other scientists disagreed, arguing that a comet would have burned quickly in Earth’s atmosphere and could not have created such a blast.

Many Western scientists believed that a meteorite was responsible, despite the lack of evidence. The pattern of fallen trees suggested that they were knocked down by a blast about 4 miles above the ground with an energy of 15 megatons of TNT, consistent with a meteoric blast. Yet repeated searches of the surrounding forest and wetlands revealed no evidence of a meteorite.

During the 1990s an Italian physicist named Menotti Galli became interested. Galli had previously investigated whether particles from outer space can add carbon atoms to the cellulose in trees. Perhaps the trees at the Tunguska site hid evidence of such extraterrestrial material. Galli and his colleagues traveled to the site by helicopter to gather core samples from spruce trees that had lived through the blast. They analyzed the particles trapped in the trees’ resin, and the results were remarkable. Based on studying the trees’ rings during the time period when the Tunguska Fireball occurred, the researchers found that the resin contained high levels of high-proton elements, such as copper, gold, and nickel. In fact, it contained ten times more such particles than resin dating from either before or after that time period. Some previously discovered meteorites had also contained elevated levels of these elements, suggesting that the particles had an extra-earthly origin. But then where is the crater? Is it nearby Lake Cheka, as some claim? Or was the meteorite reduced to dust in the atmosphere? Can scientists rule out a comet carrying those elements? The mystery remains unsolved.