Meteorites are rocks from outer space that have crash-landed on Earth. Most meteorites come from the asteroid belt between the orbits of Mars and Jupiter, about 400 million km from the Sun. Most are fragments formed when larger asteroids collide and break up, and other less common meteorites originate from the Moon, Mars or comets. About 500 meteorites fall to the Earth's surface each year, with most falling into the oceans. Of these annual falls, fewer than 10 per year are recovered and reported, although many meteorites are discovered each year.
This meteorite, named Cranbourne No 2, weighs 1,525 kg, and is one of two large iron meteorites discovered near Cranbourne, Victoria, in 1854. The two fragments are from a single meteorite fall and at the time of discovery were among the largest meteorites known. These Cranbourne masses are believed to have formed from a larger single mass breaking up in the Earth's atmosphere. So far 12 fragments have been recovered around the Cranbourne district.
The Cranbourne No 2 mass was purchased by the Natural History Museum in London but was subsequently returned to Australia and is now held at Museum Victoria. The largest piece was named Cranbourne No 1, weighs 3,500 kg and was donated to the British Museum (Natural History), London. It continues to be the centrepiece of the British Natural History Museum's meteorite display, where it has been exhibited since 1865.
There are three broad types of meteorites: iron, stony-iron and stony. These meteorite types differ in the amount of metal or silicate minerals that they contain. The Cranbourne meteorites are examples of iron meteorites, which consist almost entirely of metal, mainly an alloy of iron and nickel. Stony-iron meteorites contain almost equal amounts of metal and silicate minerals, and stony meteorites consist mostly of silicate minerals. Stony meteorites are the most common type recovered.
While in space, a meteorite is called a meteoroid, and a meteor if it produces light or a vapour trail as it passes through the Earth's atmosphere. Although small meteors tend to burn up before reaching the Earth's surface, the larger meteors speed through the atmosphere at up to 70 km per s, and transmit a fiery glow that is sometimes visible from Earth. They may land either intact or, if they break up while travelling through the atmosphere, fall as numerous fragments spread across a wide area.
The study of meteorites provides us with clues to invaluable information about the composition and the formation of the Solar System, which is thought to have formed about 4,500 million years ago. Asteroids also formed around the same time, and meteorites originating from asteroids contain material from this early period. Iron meteorites consist of material similar to that of the Earth's core and probably represent fragments of the core of a large asteroid that has been ripped apart during a collision with another asteroid.