Science: Pennies From Heaven

Dusk is falling in the Draa Valley in Algeria and a battered old Toyota pick-up truck is winding its way along a narrow dirt track, a billowing dust cloud trailing in its wake.

Every now and then the truck stops and a young man jumps out, waves a metal detector over the sandy ground for a few moments, then shakes his head at the driver and clambers back on board.

The father and son team have spent most of their day, and yesterday and the day before that, criss-crossing the parched valley, searching for likely sites for a new kind of local black gold. The right find need only be the size of a fist to bring them more than they could earn in a lifetime tending the goats and tilling the barley on their arid farm. Just 1 gram of what they are looking for would sell in New York or Bonn for more than pounds 1,000.

But it is not oil, or precious metal, or ancient artefacts, or any other traditional buried treasure that the two are searching for. To the untrained eye, their quarry may seem to be an ordinary-looking rock, but what makes it special to them and, more importantly, to prospective buyers in America, Germany and elsewhere is not so much its appearance as the fact that is has come from outer space.

These people are meteorite hunters, a disparate and growing band of entrepreneurs, adventurers, scientists and Third World farmers who comb the least populated areas of the world, the deserts of Africa and the Middle East, the plains of South America, and the wildernesses of Greenland and Antarctica, looking for the rocks that can bring instant fame or fortune.

For the commercially motivated, there is big money waiting to be made in meteorite hunting. Michael Casper, who runs Meteorites Inc in New York (www.meteorites.com), is the Mr Big of the meteorite buying and selling business, and is among more than 20 entrepreneurs advertising their wares on the Internet. He estimates that he has sold more than $1m worth in the last 12 months.

"I am the biggest in the business. I buy and I sell everywhere and I have the best stuff you can get," he says. "All sorts of people want to buy because they are taken by the idea of owning a meteorite, their own bit of space. A lot of them never realised it was possible to buy anything like this, and having a piece of rock in your house that has been millions of miles out in space, circling the sun, has a certain amount of fascination."

The latest finds from the Draa valley, 20g and 90g slices of the 100kg meteorite which is thought to have hit Earth in January 1995, are currently advertised on the World Wide Web at $1.50 a gram. Similar size objects found in Chile, Libya, Australia and Morocco are also on offer.

The majority of meteorites are fragments of asteroids, most of which circle the sun between the orbits of Mars and Jupiter, that break away and eventually plunge to earth at speeds of up to 30,000km an hour - 30 times the speed of a hand-gun bullet. A dozen or so meteorites have come from Mars, and are believed to be some of the debris that was scattered through the solar system after a giant impact on the planet. A similar number are rocks that come from the moon, which has a lesser gravity than Earth.

"Meteorites are attractive purchases because they are the only way the average person is going to get their hands on a bit of the moon," says Dr Jeffrey Grossman, a geo-chemist and expert on meteorites with the US Geological Survey (USGS) in Virginia. "That's why pieces of these particular meteorites are selling for thousands of dollars a gram. Now we have private collectors going into the northern Sahara, where they find meteorites on the desert surface. More than 1,000 have been found in recent years and within the last year they have found Martian and lunar meteorites in Libya. Right now they are very valuable commodities."

Michel Protain, a Swiss meteorite recovery specialist based in Algeria, has found, sliced up and sold more than a dozen meteorites.

"If you know what you are doing, they are easy to find," he says. "The newest ones are the best because they are more interesting to look at. Once they have been on Earth for a long time and blasted by the sand they lose a lot of their edge. People buy them to have as features in their homes or to make jewellery out of."

For geo-scientists like Dr Jeff Wynn, a meteorite expert with the USGS, the goal of meteorite hunting is to add to the tally of the 180 known craters that mark the impact of the bigger meteorites.

New research shows that meteorites and their small cousins, the pebble- size meteoroids that burn up in the Earth's atmosphere, are far more common than was once thought. The latest suggestions are that every day Earth is showered by up to 3,000 tons of space dust, the fall-out from meteoroids burning up in the atmosphere. Between 100 and 2,000 meteorites land on Earth each year.

With so much meteorite activity, it comes as no surprise that people have been hit. "We only know of a few people who have ever been hit," says Dr Grossman. "There was Anne Hodges, who suffered a bruised leg when her home in Sylacauga, Alabama was struck by a 20kg meteorite in 1954 and, in the early 1990s, a young African boy survived being hit on the head by a walnut-sized object after it had first struck a banana tree." In 1992 a 12kg meteorite struck Michelle Knapp's car while it was parked in her drive in up-state New York. The car was written off but, with meteorite included, its value soared to $69,000. "It seems likely that there must have been other people in history who have been hit, but there are no records," says Dr Grossman.

Of the 24,000 meteorites that have been found on Earth, about 1,000 were actually seen to land. Atmospheric friction between the falling debris and atmospheric gases heats it to a point at which it glows brighter than a full moon. Most meteorites glow like this for only a few seconds before hitting the surface of the Earth. Once inside the Earth's atmosphere, a sonic boom can be heard.

Meteorites have always held a fascination - parts of Coleridge's "Rime of the Ancient Mariner" are thought to have been inspired by the Leonid meteor shower in 1792.

The upper air burst into life

And a hundred fre-flags sheen

To and fro they were hurried about

And to and fro, and in and out

The wan stars dance between

And the coming wind did roar more cold

And the sails did sigh like sedge

And the rain poured down from one black cloud

And the moon was at its edge

Interest has soared in recent years, fuelled by greater knowledge about impact craters. It was only in 1978 that the site of the impact that marked the demise of the dinosaurs was discovered in Chicxulub in the Yucatan Peninsula, Mexico. We now know that it was caused by a 10km wide meteorite which hit 65 million years ago.

Craters are difficult to spot, particularly the older structures which tend to have been covered over time with layers of sediment, and those that are submerged. They can also be confused with volcanic eruption sites, but a geologist can detect clues such as rock which looks like black glass, often found around the rim, which is the molten rock that splattered after the impact. Quartz and other materials such as sand can also be structurally changed by the colossal forces involved in the impacts.

Fears, both real and fictional, about new giant projectiles on their way to collide with Earth have added to the interest in meteorites. Earlier this year it was reported that the asteroid XF11 was on course to hit Earth in 30 years, while big-budget Hollywood films like Deep Impact and Armageddon perpetuate the idea of the imminent destruction of life as we know it.

Virtually all of the 24,000 known meteorites on earth are classed as either rock or iron, or a combination of both. While some meteorites can be traced to specific asteroids, the origin of most is unknown. About 85 per cent of meteorites are recognised as stony chondrites, which contain silicates and, of these, the carbonaceous types are the most prized by collectors. Iron meteorites, which can be 94 per cent iron, make up to five to 10 per cent of the total, while the remaining percentage is composed of both stony chondrites, which show evidence of cooling and solidification at high temperatures, and the hybrid stony-iron type, considered the prettiest.

Black iron meteorites are more easily distinguished from their surroundings. Stony meteorites are more difficult to spot, but a number of specialist laboratories, including the University of New Mexico, offer analytical services. These are often used by dealers to provide certificates for the meteorites they sell. Signs of the huge impact speeds involved, evidence of the heat the rock experienced travelling through the atmosphere, and the presence of iridium in quantities not usually found in surface rocks on Earth are tell-tale signs that the rock is a meteorite.

Dr Wynn spends much of his spare time searching for craters; this field work is a personal back-up to his work for the USGS. He has looked at a number of impact sites and says that in order to create world-wide damage and threaten civilisation, a meteorite needs to be greater than 2km in diameter.

The good news is that Dr Wynn calculates that such an event occurs on average only once every 100 million years. The bad news is that two asteroids - 1627 Ivar and QS52 - are already in earth- crossing orbits.

Major Meteorite Impact Sites Around The World

Barringer Crater near Winslow, Arizona is a believed to have been formed 49,000 years ago by the impact of a 300,000 ton meteorite, 50m in diameter. Considered to be the finest example of a crater on earth, it is 1.2km in diameter and 200m deep.

Chicxulub in Mexico marks the impact of a 10km wide meteorite that killed off the dinosaurs 65 million years ago. More than 70 per cent of other life on the planet was also destroyed. The 170km wide crater lies under 2,000m of more modern material and a number of holy wells used by Mexican indians run along one section of the rim.

The Tunguska explosion over Siberia 90 years ago, which destroyed more than 2,000 square kilometres of forest, is thought to have been caused by a meteoroid.

The Hoba iron meteorite in Namibia is the largest single meteorite in existence and probably weighed around 100 tons when it hit Earth.

The Williametter meteorite in Oregon which was found in 1902 and which weighs 15 tons is the largest to fall on America.

Major iron meteorite craters are in Odessa, Texas; Henbury, Australia; and Sikhote-Alin, Siberia.