Meteor showers and comets

Meteor showers are the most reliable shows in the night sky. They come from comets, and they teach us a lot about how meteors and meteorites differ.

Comet dust trails

A comet is a body of ice, dust and rock, typically a few kilometres across, that travels on a stretched orbit around the Sun. When it comes close to the Sun, its ices turn directly to gas and stream away, carrying dust with them. This creates the comet's glowing head and tails.

The dust released by a comet spreads out along its orbit, forming a stream of meteoroids. If Earth's orbit crosses that stream, we pass through it at the same time each year, and the result is a meteor shower.

Most shower particles are tiny, from dust grains to small pebbles. They are fragile, fluffy aggregates, and they burn up completely, high in the atmosphere. No meteorite has ever been reliably linked to a meteor shower.

Radiants and rates

Shower meteoroids travel along almost parallel paths. Because of perspective, their tracks in the sky seem to spread out from a single point called the radiant, just as parallel railway lines seem to meet in the distance. A shower is named after the constellation containing its radiant.

Astronomers measure a shower's strength using the zenithal hourly rate, or ZHR. This is the number of meteors one observer would see in an hour under a perfectly dark sky with the radiant directly overhead. Real observers usually see fewer.

Some major showers and their parents are: the Perseids from Comet Swift–Tuttle, the Leonids from Comet Tempel–Tuttle, and the Orionids and Eta Aquariids, both from Halley's Comet. The Geminids are unusual because their parent, 3200 Phaethon, looks like a rocky asteroid.

Showers versus meteorite falls

It is easy to assume that a spectacular meteor shower might drop meteorites, but the two phenomena are quite different.

Shower meteoroids are made of fragile comet dust and are mostly smaller than a grain of rice. Many shower meteoroids, like the Perseids and Leonids, also hit Earth very fast, at around 59 and 71 km/s respectively, so they are destroyed high up.

Meteorite-dropping meteoroids are usually solid pieces of asteroid, at least fist-sized, arriving more slowly and at random times. They come from the asteroid belt, not from comet trails.

So if you watch a meteor shower, enjoy the show, but you do not need to search for meteorites the next morning. A bright fireball on any ordinary night is far more likely to have dropped one.

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