Meteors, or shooting stars, are pieces of rock and dust meeting a fiery end as they crash into the Earth's atmosphere and burn up. Sometimes they arrive as unexpected random "sporadic" meteors but most are associated with known comets and asteroids, which follow predictable orbits and so arrive at regular intervals.
See the Meteor Gallery for reports and pictures.
A comet can be thought of as a dirty snowball, a large lump of ice shot through with dust and organic particles. This nucleus is typically a few hundred metres to several kilometers across. This is tiny by astronomical standards, and is normally far too small to be seen directly from Earth - what puts on the show is the material driven off the nucleus and into space by the heat of the Sun.
Most comets travel on highly elliptical orbits and spend most of their time well away from the Sun, where the nucleus remains frozen. On their periodic visits to the inner solar system the the Sun evaporates its outer layers and drives water vapour and dust into space to form a fuzzy cloud, called the coma. This provides a source of material to be swept by the pressure of solar radiation into a long wispy tail, which can stretch for millions of kilometers when a comet is near the Sun.
The most well-known comet in a periodic orbit is Halley's Comet, and even this takes 76 years to go around the Sun. Most comets have orbital periods far longer than this - hundreds, thousands, perhaps even millions of years - and have thus most have only ever been seen once by mankind. It is the sudden and unexpected appearance of a comet in the night sky that captures the public's imagination, and in the past has sown panic and fear.
|This is Comet Hale-Bopp in April 1997 (not my picture!). It shows the blue ion tail, composed mainly of water, which points directly away from the sun, and the white dust tail offset to the right. It was a fine naked eye object for several months in 1997, far longer than any other comet, despite the fact that it never came closer than 120 million miles to the Earth. When the great distance is allowed for, Hale-Bopp may be the brightest comet ever recorded and it remains visible in big telescopes as it recedes into the outer reaches of the solar system on its 4,000 year orbit.|
|C/2007 N3 Lulin, a first time visitor to the inner solar system, 2009|
|17P Holmes, a normally faint comet explodes into life, 2008|
|C/2006 P1 McNaught, the brightest comet for over 40 years, 2007|
|C/2001 Q4 NEAT, 2004|
|S4 Linear, 1999|
|Hale-Bopp, spectacular in 1995|
|Shoemaker-Levy 9 (its impact on Jupiter), 1994|