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 air asteroid

Part of Speech

Definition

noun

layer of gases surrounding Earth.

noun

irregularly shaped planetary body, ranging from 6 meters (20 feet) to 933 kilometers (580 miles) in diameter, orbiting

astronomer noun

the sun between Mars and Jupiter. person who studies space and the universe beyond Earth's

atmosphere noun

atmosphere. layers of gases surrounding a planet or other celestial body.

bolide classify

noun verb

unusually large, bright meteor. to identify or arrange by specific type or characteristic.

collision

noun

comet

noun

crash. celestial object made up of ice, gas, and dust that orbits the sun and leaves a tail of debris.

Comet Tempel-

noun

comet that orbits the sun every 33 years.

Tuttle composition noun constellationnoun

arrangement of the parts of a work or structure in relation to each other and to the whole. 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 fragment

noun noun

very bright meteor. piece or part.

friction

noun

force produced by rubbing one thing against another.

gas

noun

horizon

noun

state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion. line where the Earth and the sky seem to meet. 4 of 6

Term ice intense

Part of

Definition

Speech noun water in its solid form. adjectiveextreme or strong.

Leonid meteor

noun

shower local

Leonids. adjectivehaving to do with the area around a specific place.

Mars massive

noun fourth planet from the sun, between Earth and Jupiter. adjectivevery large or heavy.

mesosphere noun

annual event, usually in November, when debris from the tail of Comet Tempel-Tuttle fall as meteors. Also called the

region in Earth's atmosphere between the stratosphere and 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

meteor shower

small, rocky body traveling around the sun. large amount of rocky debris falling into Earth's

noun

atmosphere, usually when Earth passes through the orbit of a comet.

natural hazard

noun

event in the physical environment that is destructive to human activity.

observe

verb

to watch.

observer occur

noun verb

someone who watches, or observes. to happen or take place.

orbit noun outer space noun

path of one object around a more massive object. 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 proximity

noun noun

division of a country larger than a town or county. 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. moving, measurable change in pressure and density of a

shock wave noun sonic boom noun

material. loud noise caused by sound waves as an object travels at

specific

supersonic speed. adjectiveexact or precise.

sun tail

noun noun

star at the center of our solar system. 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

visible

handling properties. 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

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