METEOROIDS AND METEORITES
Geminid meteor shower December 2009
December every year is a good time to look out for the Geminid meteor shower. And for December 2009 its visibility will peak between 11pm and 4am from Monday 14 to Tuesday 15 December. Gemini, which becomes visible at about 11pm in December, is next in line along the ecliptic after Taurus, low in the north-east.
Usually the best time to see meteor showers is early in the morning. But these meteors, coming from the constellation of Gemini the twins should provide good viewing from about 11pm onwards which is relatively early in the night as far as meteors are concerned. Locate Gemini very late in the evening, low in the north-east and watch it until about an hour or so before sunrise and you’re almost guaranteed to see what look like small shooting stars moving at up to 35 kilometres per second. Please remember: 'shooting stars' are not stars. They are small pieces of dust or rock entering the Earth's atmosphere, heating up and burning up.
The meteors can appear all over the sky, but if they are traced back they will all appear to come from a point near Castor (as illustrated on the diagram), one of the twin bright stars of Gemini. The higher the constellation is in the sky the better the chance of seeing meteors. Of course, seeing faint meteors in a city with bright lights is very difficult. The darker the sky you can find for observing the better and more meteors are likely to be seen. Note that meteor showers vary from year to year and some years there are less meteors than other years and sometimes more. Let's hope 2009 will be a year with more Geminid meteors!
The Geminids is a meteor shower caused by an object known as 3200 Phaethon which is thought to be an extinct comet or an Apollo asteroid.
The Sydney Observatory monthly sky guide for December - which includes a sky map, audio tour of the night sky and transcript, will be available online from 1 December. (If you click the link above, you will receive the sky guide for November - until 1 December when the December sky guide is published.) The star map shows what you can see in the night sky at about 8.30pm summer time - so Gemini, which doesn't become visible until about 11pm, is not on the map. But if you follow the line of the ecliptic in the night sky past Taurus towards the north-east, you will see Gemini in the night sky from about 11pm.
Meteoroids and meteorites
Moving around the Sun with the asteroids, comets and planets are billions of tiny pieces of rock and ice called meteoroids. They range in size from a grain of sand to a boulder. When a piece enters the Earth's upper atmosphere at speeds between 11 and 72 kilometres per second (40,000 - 260,000 km/hr), friction with the air heats the outside to more than 1000 degrees Celsius. This happens at altitudes between 80 and 100 kilometres. To an observer on Earth a streak of light is observed in the night sky and is commonly called a shooting star or a falling star. The correct name is meteor. A meteor is called a meteorite if it's large enough to survive the trip through the Earth's atmosphere and land on the ground.
Dating of meteorites collected by astronomers shows that they are 4,500 million years old, which is approximately the age of the solar system. The most common types of meteorites are stony, stony-iron and iron.
Meteoroids orbit around the Sun in either random chaotic orbits or in streams. Most streams are associated with the orbits of comets and are likely left over material from comet tails or when comets break up. When the Earth's orbit intersects a meteor stream you see an increased number of meteors called a meteor shower. Of meteors in random orbits, typically about 12 per hour can be seen at night. For meteor showers the number observed varies, but can be as high as thousands per hour. Famous meteor streams are named after the constellation that they appear in. For example the Leonids are named after the constellation of Leo and are visible around mid-November.