Close Encounters In Siberia

The Tunguska Event, sometimes called the Tunguska explosion, was a massive explosion that occurred near the Podkamennaya (Lower Stony) Tunguska River in what is now Krasnoyarsk Krai of Russia, at around 7:14 a.m. (0:14 UT, 7:02 a.m. local solar time) on June 30, 1908 (June 17 in the Julian calendar, in use locally at the time). The explosion was most likely caused by the air burst of a large meteoroid or comet fragment at an altitude of 5–10 kilometres (3–6 miles) above Earth's surface. Different studies have yielded varying estimates for the object's size, with general agreement that it was a few tens of metres across. Although the meteor or comet air burst rather than directly hitting the surface, this event is still referred to as an impact. Estimates of the energy of the blast range from 5 megatons to as high as 30 megatons[5] of TNT, with 10–15 megatons the most likely - about 1000 times as powerful as the bomb dropped on Hiroshima, Japan and about one third the power of Tsar Bomba. The explosion knocked over an estimated 80 million trees over 2,150 square kilometres (830 square miles). The earthquake from the blast measured 5.0 on the Richter scale. An explosion of this magnitude is capable of destroying a large metropolitan area. This possibility has helped to spark discussion of asteroid deflection strategies. The Tunguska event is believed to be the largest impact event on land in Earth's recent history; impacts of similar size in remote ocean areas would have gone unnoticed before the advent of global satellite monitoring in the 1960s and 1970s. The most recent major encounter between Earth and what may have been an asteroid was a 1908 explosion in the atmosphere above the Tunguska region of Siberia. The force of the blast flattened more than 200,000 hectares (500,000 acres) of pine forest and killed thousands of reindeer. The number of human casualties, if any, is unknown. The first scientific expedition went to the region two decades later. This expedition and several detailed studies following it found no evidence of an impact crater. This led scientists to believe that the heat generated by friction with the atmosphere as the object plunged toward Earth was great enough to make the object explode before it hit the ground. If the Tunguska object had exploded in a less remote area, the loss of human life and property could have been astounding. Military satellites—in orbit around Earth watching for explosions that could signal violations of weapons testing treaties—have detected dozens of smaller asteroid explosions in the atmosphere each year. In 1995 NASA, the Jet Propulsion Laboratory, and the U.S. Air Force began a project called Near-Earth Asteroid Tracking (NEAT). NEAT uses an observatory in Hawaii to search for asteroids with orbits that might pose a threat to Earth. By tracking these asteroids, scientists can calculate the asteroids’ precise orbits and project these orbits into the future to determine whether the asteroids will come close to Earth.

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