By now everyone has heard about the giant fireball that exploded in the sky above Chelyabinsk, Russia on February 15th, shattering glass, damaging buildings, and injuring about 1,500 people. What exactly caused all the rumpus, and how did it happen? The object in question has been variously referred to as a meteor, meteoroid, asteroid and bolide, so which is it? Asteroids are rocky objects orbiting the sun, usually between Mars and Jupiter, in what is known as the Asteroid Belt. Sometimes their orbits get disturbed, sending them closer to the earth and sun. They are not as big as planets and are not moons. Meteoroids are similar to asteroids, only smaller, and are often small pieces of rock from comets or asteroids orbiting the sun. When a meteoroid passes through the earth’s atmosphere and vaporizes, we call the resulting flash of light a meteor or shooting star. A meteoroid that survives passing through the atmosphere and lands on earth is called a meteorite.
At what point is a meteoroid large enough to be called an asteroid? From a search of the literature the distinction is not at all clear, so it is not surprising that news reports of the Chelyabinsk event use both terms interchangeably. We do know, from its orbital path, that the object was not a comet, a body comprised of ice and rock, originating in the outer solar system, whose ice can vaporize in sunlight, forming an atmosphere called a coma, often with a tail of dust and gas.
The Chelyabinsk meteoroid, as I have chosen to call it, had a diameter of about 56 feet and a mass of about 10,000 tons, making it the largest object to have entered earth’s atmosphere from space since the 1908 explosion of a comet or meteoroid over Tunguska in Siberia. Events of this size are estimated to occur about once a century. As it entered earth’s atmosphere at a speed of about 40,000 miles per hour the Chelyabinsk meteoroid exploded, releasing energy equal to about 500 kilotons of TNT, or about 30 times the power of the Hiroshima atomic bomb. The fireball created in the explosion, known as a bolide, was brighter than the sun. The air smelled like gunpowder, and eyewitnesses felt intense heat. Before the meteoroid disintegrated, air pressure accumulated in front of it, forming a shock wave which created a loud sonic boom. This was heard after the explosion, as the object was traveling faster than the speed of sound. Scientists believe that what shattered the glass was the explosion as the meteoroid broke apart, as well as the pressure waves that were created as it slowed down. These low frequency waves are known as infrasound waves and can resonate with glass, causing it to shatter.
The explosion also produced many meteorites which fell to the ground in the area around Chelyabinsk, and the search is still on for more. A 20 foot wide hole appeared in nearby Lake Chebarkul, and divers are currently searching its depths. The tiny meteorites found so far are about 10% iron, with some olivine and sulfites, and are of the stony chondrite type, the most common found on earth.
The surprise Chelyabinsk explosion stole the show from our much anticipated close encounter with asteroid 2012DA 14 which came within 17,200 miles of earth that same day but never posed a danger. At about twice the diameter of the Chelyabinsk meteoroid, it could have caused much greater damage had it entered our atmosphere. Scientists have, however, concluded that the two events were unrelated, as their trajectories were completely different. Their arrival on the same day was pure coincidence. Certainly a day to remember!
Join the Springfield Stars Club on Tuesday, March 26th at 7:30pm at the Springfield Science Museum for a talk by Dr. David Wexler on how stars are formed. A Stars Club board member, Dr. Wexler studied neuroscience and engineering at Case Western Reserve University, and completed medical studies at Stanford University. He is an ear, nose and throat specialist at Wing Memorial Hospital in Palmer. In 2011, Dr. Wexler received his Master’s degree in astronomy from James Cook University. Refreshments will be served, and the public is welcome free of charge.
Also, on Friday, April 5th at 7:30pm the Stars Club and Springfield Science Museum will host “Stars over Springfield,” an astronomy adventure for the whole family. Jack Megas will speak on “The Stars of Spring.” A fee of $3 for adults and $2 for children under 18 will be charged. Also, on Saturday, April 27th, join the Springfield Stars Club at the First Annual Pioneer Valley Outdoor Show from 9am to 3pm at the Scanlon Hall Living Room at Westfield State University. The public is welcome free of charge.
Copyright © Amanda Jermyn