Star Points for February, 1999; by Curtis Roelle Meteorite Primer In November and December we discussed two meteor showers that rounded out the year. Most meteors start out as meteoroids, small bits of debris in space traveling tens of miles per second with respect to the Earth. When they hit the atmosphere, friction from impacting air molecules will cause a meteoroid to heat and ionize the surrounding air making it visible for a brief period while briskly streaking across the sky. By definition a meteor burns up before touching the ground. Any part of the meteoroid that does reach the ground is known as a meteorite. Meteorites are typed according their composition. The thee basic types and the relative percentage of each are irons (6%), stony irons (1%), and stones (93%). Irons are metal alloys typically consisting of 85%-90% iron with the balance composed mostly of nickel. Stony irons are composed of roughly half iron and half silicates. As you probably guessed stones are primarily silicates containing just 10%-15% metallic material. The basic types are further divided into subtypes depending on the various materials and "inclusions" found therein. Although stones are the most common type of meteorite they are often difficult to identify because erosion and other effects makes it difficult to distinguish them from ordinary earth rocks. As a result most of the meteorites "in captivity" are irons. While meteorites themselves are rare in comparison to ordinary earth rocks (the chance of finding a meteorite laying in your back yard is remote) astronomers have estimated that up to 100,000 tons of meteoric material accretes at the Earth's surface daily. Most is in the form of microscopic micrometeorites measuring only a few microns across. Although meteorites are basically rocks their extraterrestrial origin makes them special. Meteorite collecting can be risky and prospectors and dealers conducting what they considered to be legitimate private transactions in foreign countries have been arrested when the local government decides that meteorites are national treasures thus making their export illegal and those caught doing so subject to prosecution. If a meteorite is large enough it will leave a visible crater. The best known crater in the United States is the Barringer Meteorite Crater near Winslow, Arizona. Astronomers estimate that a body 100 feet across impacted approximately 30,000 years ago, and is believed to be one of the youngest impact craters on Earth. Among meteorite collectors this crater is known as Canyon Diablo. Although collecting fragments from the crater is no longer permitted, there are pieces in circulation. This summer I purchased a 2 ounce piece of that meteorite from the University of Chicago while visiting the Yerkes Observatory in Wisconsin. To put this in perspective, consider that over the years some 25 tons of iron-nickel meteorite fragments have been collected from this one bountiful crater alone! Now perhaps you'd like to learn more about meteorites and see some real ones first hand? Westminster Astronomical Society (WAS) member Phil Schmitz will present a program and will display his personal collection of meteorites at the February 10 meeting of the group at 7:30 p.m. at Bear Branch Nature Center located off John Owings Road north of Westminster. If you have any meteorites or objects you suspect might be meteorites, Phil invites you to bring them along. A couple of years back a man brought an unusual small heavy chunk of rock that appeared to be made of iron to a WAS meeting. Its dark exterior led him to believe that it was charred during atmospheric entry. The object was eventually identified, but not by an astronomer. An amateur Civil War buff who was in attendance identified it as shrapnel from an exploding artillery shell. Your astronomy comments may be e-mailed to StarPointsUSA@yahoo.com.