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 space rock and the air it is passing through. 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.
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 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 waveshattered 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.
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.
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 predictableevents, occuring yearly at specific times.