What causes meteor showers?
Meteor showers happen when Earth passes through a trail of dust and small particles shed by a comet, or occasionally an asteroid, along its orbit. The particles hit the atmosphere at high speed and burn up as meteors.
Every time a comet swings close to the Sun, sunlight warms its icy nucleus and the escaping gas drags out dust and grit. These particles, called meteoroids, keep travelling along roughly the same orbit as their parent, gradually spreading out into a long stream or tube of debris. If that stream crosses Earth's orbit, our planet ploughs through it at the same time each year, and we see a meteor shower.
Each shower has a known parent. The Perseids come from Comet 109P/Swift–Tuttle, while the Orionids and Eta Aquariids both come from Halley's Comet, which Earth meets at two different points in its orbit. The Leonids come from Comet 55P/Tempel–Tuttle, and the Geminids are unusual because their parent, 3200 Phaethon, looks more like an asteroid; it may shed debris when intense solar heating cracks its surface. The Quadrantids are probably linked to the asteroid 2003 EH1, possibly an extinct comet.
The meteoroids strike the atmosphere at speeds from roughly 11 to 72 kilometres per second, depending on the direction they meet Earth. The Perseids arrive at about 59 kilometres per second. At that speed even a grain of sand heats the air around it so intensely that it glows, typically 80 to 120 kilometres above the ground, producing the familiar streak.
Showers vary over the years. When Earth hits a dense, recently released trail, rates can soar; the Leonids produced spectacular storms in 1833 and 1966, with thousands of meteors an hour, and strong displays around 1999 to 2002. As streams age, planetary gravity spreads and shifts them, so some showers fade while new ones occasionally appear. That is one reason peak dates and strengths vary slightly each year.