The nucleus of a comet is comprised of ice and dust particles and is often referred to as a 'dirty snowball'. Occasionally, when disturbed by the gravitational pull of a star, one comet among billions is ejected into orbit from a zone surrounding the solar system called the Öpik-Oort Cloud. When comets pass close to the Sun, the ice evaporates to form a glowing gas.

An asteroid is a large chunk of stony or metallic material. Many are found in the asteroid belt between the orbits of Jupiter and Mars, and some are tugged out of this region by the gravitational pull of Jupiter. The biggest known asteroid is Ceres, which has a diameter of about 940 kilometres (585 miles).

A meteorite is a small lump of stone or metal from space that actually hits a planet's surface. The vast majority originate from asteroids and comets, which are debris left over from the formation of the solar system 4.5 billion years ago, although a tiny proportion of meteorites come from the Moon or Mars and are blasted into space during huge impacts.

The term meteor refers to the streak of light — or 'shooting star' — that is produced when the pieces of rock first enter the Earth's atmosphere and are heated up by friction so they glow. As they pass through the atmosphere, the rock pieces are called meteoroids.

Could Ripert's rock have landed intact?

In general, meteoroids whose mass is less than 100 tonnes are slowed down by friction in the Earth's atmosphere. When they land, they tend to make a big hole and end up just below the surface. Depending on the trajectory and what terrain the cosmic rock strikes on landing, the crater will either be bowl-shaped with debris all around (if the angle was steep) or oval with the debris at one end (if the angle was shallow).

Larger meteorites usually explode on impact, having held on to some of their original velocity while penetrating the atmosphere. The resulting craters are usually round because they have effectively been formed from an underground eruption as the fast-moving mass bored through the planet's crust and heated up the rock to bursting point.

However, according to a study in the 1970s by meteoricists Robert Fudali and Dean Chapman, there is an outside chance — put at 0.1-1.0% — that a Chinguetti-sized rock could land without disintegrating: 'Contrary to popular belief, very large meteorites can be sufficiently slowed by aerodynamic drag [the gaseous particles that make up the Earth's atmosphere] to survive impact … provided that they enter the Earth's atmosphere at very low angles.'

Meteorite make-up

The three main types of meteorites are often described as irons, stones and stony irons.

Irons contain mostly iron and a smaller percentage of nickel. Almost all irons have a distinctive criss-cross impression called the Widmanstätten pattern, which has taken millions of years to form.

Stones often resemble Earth rocks. Of these, chondrites are composed of round crystallised minerals (chondrules) containing nickel, iron, feldspars, olivine and pyroxene, and achondrites have no free iron or nickel and don't contain chondrules.

Stony irons contain a nickel-iron alloy, which is combined with silicates such as olivine and pyroxene.

Spotting a meteorite

The following are some clues that may help you decide whether a rock you have found is a meteorite. If it is, its surface will usually be smooth, and if it has fallen from the sky relatively recently, it will be covered in a black 'fusion crust'. Exposure to the air weathers this to a browner colour. Meteorites are generally irregularly shaped and heavier than ordinary rocks: irons by 3.5 times and stones by 1.5 times. Irons ring like bells when struck.

Most meteorites contain some metal and are therefore magnetic. However, not all are, and some Earth rocks are magnetic. The nickel content of meteorites is also within a set range, whereas normal rocks have either a lot or not much.

The distinctive chequered Widmanstätten pattern in irons can be seen most clearly by slicing the meteorite, polishing the flat surface and dripping acid on it.

Meteorites for sale

Trade in meteorites is big business. The internet is awash with sites selling chunks of space rock. Some mundane examples can be bought for around $1 (60p) a gram, but one firm - called catchafallingstar.com - is offering pieces of the Dar Al Gani 400 Mars meteorite for $2,750 per gram. A 122g lump of the Australian Murchison chondrite that fell in 1969 will set you back $12,000.

According to a report in the Independent on Sunday in January 2000, a New York-based company called meteorites, inc. sold more than $1 million worth of space rock in the previous year. The outfit has recently sold a 'gorgeous' 825g polished slice of a stony iron found in Chile's Atacama desert in 1822 for $24,750.