The Perseids is one of several meteor showers that occur regularly at a particular time each year - others include the Leonids (16th-18th November) and the Geminids (12th-14th December) for example. They are all caused by comets that orbit the sun - as the comet gets nearer to the sun, the surface starts to vaporise and a trail of debris is left in the path of its orbit. This trail is replenished every time the comet completes another orbit. Then the Earth passes through this trail at the same time every year as its own orbital path crosses that of the comet - and voila! A meteor shower!
But you don't have to wait for a meteor shower to see shooting stars. Near space is full of debris from asteroids and disintegrated comets, and bits of this debris can enter the atmosphere at any time. It's estimated that around 44 tonnes of space debris enters the atmosphere on any one day. Usually these meteors are just tiny specks of dust that burn up high in the atmosphere – pretty, and harmless. But the brightness and energy of a meteor is directly related to its size, and sometimes they can be a lot larger and more threatening.
On February 15th 2013 a small asteroid about 20 meters across crashed into the earth's atmosphere, exploding about 30 km above the ground and fracturing into a number of smaller pieces, which streaked through the atmosphere above Chelyabinsk in Siberia, und with a huge explosive impact. The energy of this event was powerful enough to shatter glass throughout the city, injuring hundreds of people.
Fortunately such damaging meteoric events are rare - this was the largest known meteor since The Tungusha event in 1908, when a huge meteorite flattened over 2000 square kilometers of remote Siberian forest.
Birth of the Solar System
Many small fragments of the Chelyabinsk meteor survived and fell to the earth, and once the initial damage had been cleared up, the rush was on to recover these pieces of meteorite (a meteorite is a meteor which reaches the Earth's surface). Such objects are of great interest to scientists as they are pieces of comets and asteroids, which are ancient unchanged remnants from the birth of the solar system over 4.6 billion years ago. Examining such pieces of space rock is a rare and exciting chance to find clues about how the planets were formed, and what they were formed from.
So next time you see a shooting star, spare a thought for its eons-long journey through the lonely backwaters of the solar system before it finally meets its end in a brief but spectacular blaze of glory over the Earth.
Image credits:
1. Juraj Tóth - Astronomical and geophysical observatory, Comenius University
http://en.wikipedia.org/wiki/Meteor_shower#mediaviewer/File:AGOModra_Leonids98.jpg
2. Navicore - http://commons.wikimedia.org/wiki/File:Leonid_Meteor.jpg
3. Alex Alishevskikh (Creative Commons Attribution-Share Alike 2.0 Generic license)
http://frontierscientists.com/tag/chelyabinsk/
More about meteors / 'near earth objects':
http://neo.jpl.nasa.gov/neo/
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Meteors&Display=OverviewLong
http://neo.jpl.nasa.gov/news/fireball_130301.html - details about the Chelyabinsk event