Encyclopedic Entry meteor shooting star, falling star For the complete encyclopedic entry with media resources, visit: http://www.nationalgeographic.org/encyclopedia/meteor/ A meteor is a streak of light in the sky caused by a meteoroid crashing through Earth s atmosphere. Meteoroids are lumps of rock or iron that orbit the sun. Most meteoroids are small fragments of rock created by asteroid collisions. Comets also create meteoroids as they orbit the sun and shed dust and debris. When a meteoroid enters the Earth s upper atmosphere, it heats up due to friction from the air. The heat causes gases around the meteoroid to glow brightly, and a meteor appears. Meteors are often referred to as shooting stars or falling stars because of the bright tail of light they create as they pass through the sky. Most meteors occur in Earth s mesosphere, about 50-80 kilometers (31-50 miles) above the Earth's surface. Even the smallest meteors are visible from many kilometers away because of how fast they travel and how brightly they shine. The fastest meteors travel at speeds of 71 kilometers (44 miles) per second. The faster and larger the meteor, the brighter and longer it may glow. The smallest meteors only glow for about a second while larger and faster meteors can be visible for up to several minutes. Although thousands of meteors fall during the day, meteors are best observed at night, when the streaks of light are visible in the dark sky. Meteors appear in different colors, depending on the chemical composition of the 1 of 6
space rock and the air it is passing through. A meteor with high iron content, for instance, will appear yellow. A meteor with high calcium content may appear as a purple streak of light. Scientists think up to 50 metric tons of meteors fall on the Earth each day, but most are no bigger than a pebble. Meteors that don t burn up in the atmosphere strike Earth s surface. These meteors are called meteorites. Types of Meteors Meteors are described by their size, brightness and proximity to Earth. Earthgrazers are meteors that streak close to the horizon and are known for their long and colorful tails. Some earthgrazers bounce off Earth s upper atmosphere and re-enter outer space. Other earthgrazers break up in the atmosphere and streak through the sky as falling stars. The most famous earthgrazer is probably the 1972 Great Daylight Fireball, which entered the atmosphere over the U.S. state of Utah, streaking through the sky at 15 kilometers per second (9 miles per second). Thousands of people reported seeing the meteor. The earthgrazer exited the atmosphere over the Canadian province of Alberta. Fireballs are larger meteors, ranging in size from a basketball to a small car. Fireballs have brighter and longer-lasting light than earthgrazers. The International Astronomical Union describes a fireball as a meteor brighter than any of the planets. Fireballs are probably the most common type of meteor. Members of organizations such as the American Meteor Society report hundreds of sightings every year. As of July 2014, for instance, more than 1,500 fireballs were reported in the United States. Some were seen only in a small area, while others were reported by stargazers across several states. Bolides are even brighter and more massive than fireballs and often explode in the atmosphere. These explosions can be heard and even felt on the Earth s surface. Some astronomers classify bolides as fireballs that produce a sonic boom as they 2 of 6
streak through the atmosphere. Certain bolides, known as superbolides, are so bright and create such a large explosion that they become natural hazards, and dangerous to people and communities. The superbolide meteor that passed over Chelyabinsk, Russia, in 2013 exploded with the energy of around 500 kilotons of TNT. Its shock wave shattered windows in thousands of apartment buildings and sent more than 1,200 people to the hospital for injuries. The Chelyabinsk meteor was so bright 30 times brighter than the sun at its most intense that it left people with skin and retinal burns. Scientists are studying the Chelyabinsk event to better understand how vulnerable human life is to space object collisions, and to develop technologies that protect Earth from them. Meteor Showers Usually, just a few meteors are visible over the course of an hour, but sometimes the sky is filled with lights that look like heavenly fireworks. These meteor showers occur when the Earth passes through the orbit of a comet. Comets shed particles that appear as a dusty trail behind the dirty snowball of rock, ice, and gas that makes up the comet s nucleus. As the Earth passes through a comet s tail, the rocky debris collides with our atmosphere, creating the colorful streaks of a meteor shower. Meteor storms are even more intense than showers, defined as having at least 1,000 meteors per hour. All the meteors in a meteor shower seem to come from one spot in the sky. This spot is called the radiant point, or simply the radiant. Meteor showers are named after the constellation in which their radiant appears. The source of the meteors is not the constellation, of course, but rather the comet from which they have broken off. For example, the Leonid meteor shower appears to produce meteors falling from the constellation Leo, but are actually debris from Comet Tempel-Tuttle. Visible every November, the Leonids are considered some of the fastest and longest-lasting meteors. Other important meteor showers include the Perseids, the Orionids, and the Geminids. Like the Leonids, they are predictable events, occuring yearly at specific times. 3 of 6
Vocabulary Term Part of Speech Definition air noun layer of gases surrounding Earth. asteroid noun irregularly shaped planetary body, ranging from 6 meters (20 feet) to 933 kilometers (580 miles) in diameter, orbiting the sun between Mars and Jupiter. astronomer noun person who studies space and the universe beyond Earth's atmosphere. atmosphere noun layers of gases surrounding a planet or other celestial body. bolide noun unusually large, bright meteor. classify verb to identify or arrange by specific type or characteristic. collision noun crash. comet noun celestial object made up of ice, gas, and dust that orbits the sun and leaves a tail of debris. Comet Tempel- Tuttle noun comet that orbits the sun every 33 years. composition noun arrangement of the parts of a work or structure in relation to each other and to the whole. constellationnoun group of stars that form a recognizable shape. debris noun remains of something broken or destroyed; waste, or garbage. dust noun microscopic particles of rocks or minerals drifting in space. Also called cosmic dust or space dust. earthgrazer noun meteor that enters Earth's atmosphere and usually leaves again. explosion noun violent outburst; rejection, usually of gases or fuel fireball noun very bright meteor. fragment noun piece or part. friction noun force produced by rubbing one thing against another. gas noun state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion. horizon noun line where the Earth and the sky seem to meet. 4 of 6
Term Part of Speech Definition ice noun water in its solid form. intense adjectiveextreme or strong. Leonid annual event, usually in November, when debris from the tail meteor shower noun of Comet Tempel-Tuttle fall as meteors. Also called the Leonids. local adjectivehaving to do with the area around a specific place. Mars noun fourth planet from the sun, between Earth and Jupiter. massive adjectivevery large or heavy. region in Earth's atmosphere between the stratosphere and mesosphere noun the thermosphere, about 50-80 kilometers (31-50 miles) above the Earth's surface. meteor noun rocky debris from space that enters Earth's atmosphere. Also called a shooting star or falling star. meteorite noun type of rock that has crashed into Earth from outside the atmosphere. meteoroid noun small, rocky body traveling around the sun. large amount of rocky debris falling into Earth's meteor noun atmosphere, usually when Earth passes through the orbit of shower a comet. natural event in the physical environment that is destructive to noun hazard human activity. observe verb to watch. observer noun someone who watches, or observes. occur verb to happen or take place. orbit noun path of one object around a more massive object. outer space noun space beyond Earth's atmosphere. particle noun small piece of material. pebble noun very small, rounded rock. planet noun large, spherical celestial body that regularly rotates around a star. predictable adjectiveregular or able to be forecasted. protect verb to take action to prevent injury or attack. province noun division of a country larger than a town or county. proximity noun nearness. 5 of 6
Term Part of Speech Definition radiant pointnoun spot in the sky from which a meteor shower seems to originate. retina noun sensitive tissue at the back of the eye that receives images and sends signals to the brain about what is seen. rock noun natural substance composed of solid mineral matter. shed verb to release or cast off. shock wave noun moving, measurable change in pressure and density of a material. sonic boom noun loud noise caused by sound waves as an object travels at supersonic speed. specific adjectiveexact or precise. sun noun star at the center of our solar system. tail noun stream of gas or dust debris behind a comet. TNT noun (trinitrotoluene) chemical compound often used as a predictable, reliable explosive material with relatively safe handling properties. visible adjectiveable to be seen. vulnerable adjectivecapable of being hurt. Articles & Profiles National Geographic News: Leonid Meteor Shower Most Intense for Decades National Geographic News: Meteor Dust May Affect Weather, Study Says Video National Geographic Channel: Meteor Madness Websites NASA: Solar System Exploration Meteors and Meteorites NASA: Leonids American Meteor Society 1996 2017 National Geographic Society. All rights reserved. 6 of 6