Comet, asteroid or meteor? | The Economist

The Rosetta space probe is currently investigating a comet called 67P. Dawn, another space probe, has visited an asteroid called Vesta, and is now on its way to another asteroid called Ceres. Throughout the year observers on Earth may witness showers of meteors. Sometimes fireballs appear in the sky, like that seen over Chelyabinsk in Russia in 2013. So what’s the difference between a comet, an asteroid and a meteor, and what is the relationship between them?

Start with comets. They formed billions of years ago around the far fringes of the solar system, and consist of lumps ice mixed with rock, dust and frozen gases. As a comet’s orbit brings it closer to the sun, it heats up and grows a “tail” of water and dust.

Asteroids, by contrast, formed closer to the sun: they are leftovers from the era of planetary formation, and are mostly found within the orbit of Jupiter. Rocky in nature, they are also sometimes called planetoids or minor planets.

Asteroids smaller than a few metres across are known as meteoroids, as are the small pieces of dust, rock and ice shed by comets.

When a meteoroid enters the Earth’s atmosphere it appears as a meteor or shooting star to observers -- or, if it is large enough, as a fireball. If any of it survives re-entry and reaches the ground, the resulting pieces are called meteorites.

Meteor showers occur when the Earth passes across the trail of debris left behind by a comet. Because the orbits of comets are known, the dates of meteor showers can be predicted: the Geminids in mid-December, for example, and the Perseids in mid-August.

In summary: comets are big and icy, asteroids are big and rocky, and meteors are small bits of debris burning up in the Earth’s atmosphere in a final blaze of glory. Happy skywatching.