Asteroids, Comets and Meteors

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Asteroids are rocky or metallic remnants of planet formation, mostly in the belt between Mars and Jupiter. Comets are icy bodies from the outer Solar System: when they approach the Sun, ice vaporises into a glowing coma and tails pointing away from the Sun. A meteoroid is a small fragment; a meteor is the streak of light as it burns in the atmosphere; a meteorite is a piece that reaches the ground.

Impacts shaped planets' surfaces, and the one 66 million years ago contributed to the dinosaurs' extinction. Space agencies monitor near-Earth objects.

Think of the 2013 Chelyabinsk meteor, an object about 20 metres wide that exploded over Russia with the energy of several hundred kilotonnes of TNT; the shock wave shattered windows and injured over a thousand people. Most meteors you see on a clear night, like the Perseids each August, are grains of dust from a comet's trail no bigger than a grain of sand. Comet tails point away from the Sun because of the solar wind and radiation pressure, not because of the comet's direction of travel, so they can lead the comet when it is moving outward.

NASA's DART mission in 2022 deliberately hit the asteroid moonlet Dimorphos and shortened its orbit, proving that a planetary defence technique can work. Asteroids are also studied for resources and clues about early Solar System chemistry, with OSIRIS-REx returning a sample from Bennu. A common misconception is that the asteroid belt is a dense field of rocks like in films; the objects are millions of kilometres apart, and spacecraft pass through easily.