Meteors come from larger objects caught revolving around the sun's gravity called meteoroids. Essentially, these could be any objects in space, including fragments of comets and asteroids. The term "meteor" is derived from the Greek word "meteoron," which means a phenomenon in the sky. Most of these fragments are burned up by the Earth's atmosphere and become phenomenal displays of speed and color. However, once a meteor falls through the Earth's atmosphere and impacts the surface, it becomes a meteorite.
There are three categories which are used to classify the different types of meteorites: stony, iron and stony iron. Chondrites are the most common type of meteorite, and they are classified as stony. Through a process called radiometric dating, these meteorites have been given an age of approximately 4.5 billion years.
Stony meteorites that have undergone intense melting and recrystallization are called achondites. Iron meteorites were formed from a combination of dense silicates and molten metal. They are often found with other metals like nickel and carbon alloys. There are so many iron meteorites that 13 subcategories are needed to group them.
Objects falling from space may often resemble other rocks and metals from Earth, but meteorites bear physical evidence of their harrowing journeys through space. Meteorites are identified and distinguished by these unique characteristics. For example, most meteorites are composed of iron alloys. Through chemical analysis and acid etchings, scientists are able to identify most meteorites by their trace metals. One of these trace metals is nickel and easily attracts magnets. Also, the outermost layer of most meteorites contains an ash-like coating called a fusion crust. This black coating forms when the outer layer of the meteor melts during atmospheric entry. However, the fusion crust is only an early identifier because the coating fades over time.
One of the driving forces for scientists and enthusiasts alike is determining the original source of meteorites. The melted metal fragments of meteorites contain trapped gases that can be traced back to planetary bodies and specific regions of space. In 1976, the Viking landers retrieved samples of the Martian atmosphere that match gases later found inside several meteorites found on Earth. Scientists also use comparisons from other extra-terrestrial objects to better identify meteorite samples. The Apollo space missions, for example, obtained lunar rock samples that were later linked to meteorite fragments found in Antarctica. Other sources of meteorites include comets and asteroids.
The word "meteor" is a general term encompassing a wide range of small particles and also the streaking phenomenon they produce across the Earth's atmosphere. That streak is actually a result of friction between the air and the surface of the meteor. Some observers have even observed sonic booms from the high-speed fireballs. Meteor storms and meteor showers mostly come from sporadic comets that shed particles and dust near the Earth's orbit.