When Comets Attack: Solving the Mystery of the Biggest Natural Explosion in Modern History

http://www.popularmechanics.com/science/air_space/4316464.html

On the morning of June 30, 1908, the sky exploded over a remote region of central Siberia. A fireball as powerful as hundreds of Hiroshima atomic blasts scorched through the upper atmosphere, burning nearly 800 square miles of land. Scientists today think a small fragment of a comet or asteroid caused the "Tunguska event," so named for the Tunguska river nearby. Now, a controversial new scientific study suggests that a chunk of a comet caused the 5-10 megaton fireball, bouncing off the atmosphere and back into orbit around the sun. The scientists have even identified a candidate Tunguska object—now more than 100 million miles away—that will pass close to Earth again in 2045. Is there a hidden, but powerful, danger inside the seemingly harmless comet?

...Now, a controversial scientific study suggests that a chunk of a comet caused the 5 to 10 megaton fireball—what amounts to the largest non-nuclear explosion in modern history. Crucially, according to the new hypothesis, most of the comet bounced off the atmosphere and back into orbit around the sun. The scientists have even identified a candidate Tunguska object—now more than 100 million miles away—that was somewhere near Earth on June 30, 1908 and will be passing close to Earth again in 2045. But just how could a comet—basically a ball of water ice and cosmic dust—create such a massive explosion and leave no trace? The answer, the scientists believe, can be found in basic chemistry rather than complicated physics or evidence yet to be found.

One of the most peculiar leftovers from the Tunguska event is the pattern of scorched earth it left behind. No mere circular blast, Tunguska’s trail of charred trees fan out like a butterfly, with outer "wings" that spread both in a north-northeast direction and a south-southeast direction. When presented with this unusual pattern, top scientists, including Giuseppe Longo of the University of Bologna in Italy and Yuri Medvedev of the Russian Institute of Applied Astronomy, have circled around two main theories—that either two separate objects exploded in the skies over the region or one object skipped over the atmosphere, circled the earth and then re-entered over the Tunguska region on its second pass. Edward Drobyshevski, a research physicist at the Russian Academy of Sciences in St. Petersburg, thinks instead that the comet fragment packed its own explosives—in the form of hydrogen, the gas that turned the Hindenburg airship into a blazing inferno in seconds.

To come to his hydrogen theory, Drobyshevski focused on basic chemistry, namely electrolysis—the chemical splitting of water into its hydrogen and oxygen components using electricity. The hydrogen that caused the 1908 explosion, Drobyshevski says, most likely comes from the comet’s earlier incarnation—as a tiny part of an ice sheet on a moon of Jupiter or Saturn. Over time, the strong magnetic fields from the host planet split some of the water molecules in the ice sheet into little hydrogen and oxygen bubbles that remained trapped within it. Once enough hydrogen had accumulated in the ice sheet, a direct asteroid hit would have caused the sheet to explode, sending shards of hydrogen-bubble-filled ice into space. (As far back as 1981, Drobyshevski published a paper theorizing that hydrogen-rich ice on Saturn’s moon Titan exploded between 3000 and 10,000 years ago, enriching the structure of Saturn’s rings and perhaps also sending some volatile icy shards into the solar system.)...