Star Points for June, 2008; by Curtis Roelle Tunguska: One century ago This month marks the centennial of probably the largest non- volcanic above ground explosion during the first half of the 20th century. It detonated with energy one thousand times more powerful than the atomic bomb exploded over Hiroshima during World War II. And yet because the explosion occurred in a sparsely populated area of the world not a single fatality was reported. This was the Tunguska event, a massive earth-rattling explosion whose epicenter is in Siberia's Podkamenaia (stony) Tunguska River region, which occurred on the summer morning of June 30, 1908. A detailed description is found in Gerard P. Kuiper's influential 1963 book The Moon, Meteorites, and Comets, in a paper authored by E.L. Krinov (Meteorite Committee of the Academy of Sciences, USSR). According to Krinov, witnesses observed a rapidly moving "dazzlingly bright fireball" meteor in a clear daytime sky, trailed by a "thick dust train." When the meteor reached the end of its path they reported seeing a "fireball as a gigantic pillar" – a description reminiscent of the mushrooming fireballs commonly associated with very large explosions. The visual spectacle was followed by "deafening explosions" heard out to "a distance of over 1000 km" (621 miles) from ground zero. Ground and buildings shook, household items fell to the floor and hanging objects swung. Windowpanes cracked, and an "airwave" swept through through the region. In the vicinity of a factory located 60 km (37 miles) from the epicenter a man sitting on a house porch "was thrown several meters, felt a sensation of heat, and lost consciousness." The explosion was recorded by seismometers and "explosive airwaves" were picked up by meteorological barographs revealing that the blast traveled not once but twice around the world. For several months the night sky was bright enough that "at midnight it was possible to read a newspaper without artificial light." This was caused by dust particles from the meteor's dust train and explosion suspended high in the atmosphere scattering sunlight. I assume that the sky glowed with the same radiant crimson hues and glorious pastel pinks that graced the twilight skies here in Maryland following volcanic eruptions of Mexico's El Chichon in 1982 and the 1991 eruption of Mt. Pinatubo in the Philippines. The site of the explosion was so remote that no expeditions would reach the site for 19 years, in 1927. They were greeted with an incredible sight. On their way to the site, and still 25 miles from it, they found "blown over trees" many of which had been "torn up by their roots." The trees were all laying in the same direction, radiating away from the center of the blast. Trees were still standing on hillside slopes that served as blast shields. Trees closer to the explosion site were charred. In The Comet Book, comet expert John Brandt states that the most likely cause of the explosion was the nucleus of a small comet, 40 meters (130 feet) across, hitting the atmosphere while traveling 45 km/sec (28 miles per second). In another book, Introduction to Comets, Brandt adds that the comet exploded in mid-air at an altitude of 8.5 km (5.3 miles), which undoubtedly enhanced the explosive effect. Several alternative hypothesis have been floated, none of which has gained much traction. These include mechanisms such as antimatter, a small black hole, and even the crash of a nuclear powered alien spacecraft. Conspiracy theories, such as an ahead of its time Russian nuclear test or accident, have also been postulated. The late Eugene Shoemaker (U.S. Geological Survey) estimated the energy released by the Tunguska blast to be between 5 and 50 megatons of TNT, with 10-15 megatons the most likely range. The largest hydrogen thermonuclear bombs ever exploded by the United States are in this range. In 1952 the U.S. exploded the 10.4 megaton "Ivy Mike" bomb. Sixteen months later came the 15 megaton Castle Bravo bomb. According to Brandt, astronomers estimate that the earth will collide with a comet like Tunguska's once every 2,000 years. Even so the damage would be localized and more likely to occur over an ocean. For that reason Brandt recommends "no one should lose much sleep over this situation."