We all know how the Hollywood film goes; a giant comet is plummeting towards Earth and the hero has to stop it in order to save the world from destruction. But what exactly is a comet? And how does it differ from an asteroid? And what is a meteorite?
Asteroids and comets are both classified as near-Earth objects. Both asteroids and comets were formed early in the history of the solar system about 4.5 billion years ago.
Comets are composed of ice, rock, and organic (carbon-based) compounds and are believed to have formed in the cold outer solar system farther from the Sun where ices would not melt. Comets consist of substantial amounts of ice and volatile material, so as the comet approaches the Sun the ice vaporizes and creates a "fuzzy" appearance.
Comets follow highly elliptical paths, approaching the inner solar system and then retreating to a considerable distance from the Sun. If they stayed close in to the Sun all the ice would vaporize, and they would cease to be comets.
Asteroids are made of rock or metal and are thought to have been created in the warmer inner solar system closer to the Sun, where it was too warm for ice to remain solid. An asteroid can have a diameter anywhere from a few meters to a few hundred kilometres. There are only about 200 asteroids which have diameters exceeding 100 kilometres.
Most asteroids in the solar system are located in the 'asteroid belt', where they orbit the sun in the space between the orbits of Mars and Jupiter. Asteroids are different from planets and moons mainly because of their much smaller size, and they differ from comets because they have neither a coma nor a tail.
A meteoroid is a small rock or particle of debris in our solar system. When asteroids collide, meteoroids often result. They range in size from dust to around 10 metres in diameter (larger objects are usually referred to as asteroids). Meteoroids are also formed when a comet passes near the sun, and the heat releases dust particles from the comet's icy tail. They glow brightly after being heated by friction as they enter and pass through Earth's atmosphere.
When a meteoroid or asteroid enters the Earth's atmosphere it ignites creating a visible streak of light called a meteor. If you’ve ever looked up at the sky at night and seen a streak of light or ‘shooting star’ what you are actually seeing is a meteor.
A meteoroid that survives falling through the Earth’s atmosphere and collides with the Earth’s surface is known as a meteorite.
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