Meteor showers are often called shooting stars but they are nothing to do with stars at all. They are caused by streams of dust and rock called meteoroids. These meteoroids are tiny pieces of debris left behind (usually) by comets as they orbit the Sun. A comet is like a great big dirty snow ball or a snowy dirt ball! When it approaches the Sun the ice begins to turn from a solid straight into a gas. This is called sublimation. The ‘dirt’ in the comet then gets swept along behind the comet in a meteoroid stream also known as a dust trail. As Earth passes through this dust trail every year at the same time, meteoroids will enter the Earth's atmosphere and "burn up" leaving a bright visible streak called a meteor. If meteors occur only seconds or minutes apart then it is known as a meteor shower and it takes its name from the constellation from which the shower appears to originate (the radiant point). For example the Perseids appears to originate in the constellation Perseus and the Taurids appears to originate from the constellation Taurus.
Most meteoroids that cause meteors are the size of a grain of sand. Their bright streaks become visible between 40 and 75 miles (65-120 km) above the Earth's atmosphere and they disintegrate at altitudes of around 30-60 miles (50-95 km). At speeds of from 11 -72 km/s (25,000mph to 160,000mph) they are visible for between fractions of a second to seconds.
Meteoroids vary in size from a dust particle to a small boulder. The number of meteoroids appears inversely proportional to their size for instance there are more meteoroids the size of a dust particle than the size of a grain of sand and there are more meteoroids the size of a grain of sand than the size of a pebble etc. Millions of meteors "burn up" in the Earth's atmosphere every day but most of them are so small that they are invisible. Also many enter the atmosphere during the day.
So why do we see these streaks of light? As the meteoroid leaves the vacuum of space and enters the Earth's atmosphere at high speed there is a rapid compression (squashing) of the air in front of it, which becomes superheated so much so that it ionises. This is what causes it to glow and because it is travelling so fast it leaves a glowing trail behind it. It can also shed glowing material too adding to the bright trail behind the meteor. The larger and faster the meteor, the longer the trail may be and the easier it is to spot in a dark sky. The meteor itself disintegrates at such high temperatures. However, if the meteoroid is large enough and survives its traumatic entry through the atmosphere, it will fall to Earth. This is now called a meteorite. Scientists think that up to 50 metric tons of meteors fall on the Earth as meteorites each day, but most are no bigger than a pebble. Of course some fall through the atmosphere during the day and we will never see them.
|Name of Shower||Range||Peak||Rate||Origin|
|QUADRANTIDS||28 Dec - 12 Jan||3-4 Jan||>100/hr||Asteroid 2003 EH1 OR is it Comet C/1490 Y1?||Blue with fine trains|
|Lyrids||13 - 29 April||21-22 April||18||Comet Thatcher||Bright and fast|
|Eta Aquarids||18 April - 27 May||5-6 May||35||Comet Halley||Low in the sky|
|Perseids||16 July - 23 August||12-13 August||100||Comet Swift- Tuttle||Bright and fast with persistent trains|
|Draconids||5 - 9 October||8-9 October||10||Comet 21/P Giacobini-Zimmer||Slow moving. Best seen early evening|
|Orionids||10 October - 6 November||20-21 October||25||Comet Halley||Fast with persistent trains|
|Geminids||7-17 Dec||13-14 Dec||100/hr||Asteroid 3200 Phaeton||Most reliable with bright meteors & few trains|
|Ursids||16-25 Dec||21-22 dec||5-10/hr||8P/Tuttle||Sparse|
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