Next meteorite in Observatory
What is a meteor shower?
Every meteor shower has a progenitor Comet. Comets which live a long way from the Sun sometimes tumble in towards the Sun and start to have a short periodic orbits of less than 200 years.
A good example is Halley’s Comet which orbits the Sun every 76 years and is the progenitor of the Orionids. As the Comet gets close to the Sun it heats up, and being a ball of mostly ice it starts to evaporate. Rather becoming a liquid it literally turns straight into a cloud of particles, just sublimes – a big cloud of debris.
As the comet goes around the Sun it is constantly filling its orbit with debris, and if its orbit coincides with the Earth’s orbit then every year you’ll go through that cloud of debris. Those little pieces then burn up in the Earth’s atmosphere as we smash into them and create shooting stars. Showers occur when we go through specific clouds of debris and then throughout the year we experience sporadic meteors which are just little pieces that are everywhere.
What is a meteor storm?
A meteor storm occurs when you go through a really unusually dense part of a comet’s debris cloud. It’s very unpredictable - you can’t really tell when it’s going to happen but when it does happen it’s possible to see thousands of meteors per hour for one or two hours.
I saw a meteor storm a number of years ago. It was extraordinary – I was seeing about 300 meteors in the space of about 20 minutes. And you could really see where the radiant was because the meteors were so regular that your eye could trace them all back to a common point where the radiant was.
What are meteorites?
When larger chunks of interplanetary matter enter the atmosphere it is unlikely that all of each one will be evaporated. The outer layers will disappear but the centre is likely to survive and will hit the ground as a meteorite. The speed with which small meteorites hit the ground can be around 500 km/h.
More than 2000 meteorites have been recovered. They are of different types, Stony meteorites, iron meteorites and the rare carbonaceous chondrites. The largest meteorite that has been found is the 60 tonne Hoba iron meteorite; the largest stony meteorite weighs about a tonne and the Allende carbonaceous chondrite was a series of chunks that totalled about 5 tonnes.
One of the best-known impact craters is the Arizona crater in the USA, which is 1280 metres across and 180 metres deep. It was formed several thousand years ago by a 250,000 tonne meteorite with a diameter of 70 metres hitting the Earth at a speed of nearly 60,000 km/h!
The Perseids are one of the best-known meteor showers and can be seen in August around 12 August. The radiant is in the constellation Perseus, just below the familiar 'W' of the constellation of Cassiopeia. At this time of year this can be seen reasonably high in the north-eastern sky at nightfall.
If no prominent shower is active then most of the meteors that are seen will come from random directions in space. These meteors are called sporadic meteors and about one every ten minutes is the normal rate for them to be seen.
Most fire-balls and meteorites are sporadic meteors. The material in these meteors is associated with the material in the asteroids and it is likely that they represent material that has come from fragmented asteroids.