Rocks on your head

Meteorite collectors are up in arms, and all over a dead cow. Duncan Steel explains why

Each day about 100 tons of cosmic detritus arrives from space. Most burns up high in the atmosphere. But every so often a rock makes it through.

Asteroid impacts are dangerous because such behemoths hit the ground at phenomenal speed, releasing a vast amount of energy when they do so: an explosion that could kill millions, even billions. Just ask the dinosaurs.

Smaller rocks from space are decelerated by the atmosphere and reach the ground at the same terminal velocity as a heavy object dropped from an aeroplane. These we call meteorites. And the community of those who collect and study meteorites has been much perturbed of late by a particular lump of space rock.

Meteorites could be hazardous, if you were unfortunate enough to be hit. Only a handful of meteorites reach the ground each day, and human bodies cover a tiny fraction of the planet's surface. A decade ago, a boy in Uganda was struck but not badly hurt. In 1954 one punched through the roof of a house in Alabama and severely bruised a woman's arm. The casualty rate is low.

There are many records of buildings and cars being struck. This is no bad thing for the owners: the value of the meteorite may be greater than that of the damaged car. A car that was damaged by a fall near New York in 1992 was sold for a considerable sum as a museum exhibit.

Stories of meteorites hitting animals are legion, attaining mythological status. One story is that a dog was killed by the Nakhla meteorite, which fell in Egypt in 1911 (and is one of the 16 meteorites known to have come from Mars). Recent investigations seem to show that the story is apocryphal, rather than apocalyptic. A meteorite that fell at Ohio in 1860 reportedly killed a colt or a pony. Again the story has never been validated.

But now a "death by meteorite" rumour has been confirmed, a Venezuelan cow having been the victim in 1972. The meteorite fell on a farm to the east of the town of Valera, and in consequence takes that as its name, as is the norm. It is almost three decades ago but the object has only just been listed in the Meteoritical Bulletin, the standard publication, because the eyewitnesses thought little of it at the time. Only through later detective work were the circumstances pieced together.

On the evening of October 15 that year a bright light was seen in the sky, and a loud noise heard. The next morning three people found a dead cow, one of its forequarters crushed by the impact, with three fragments of the broken meteorite next to it on the ground. These weighed about 50kg altogether: enough to kill any animal, if dropped from a great height. One of the three - a physician - believed the rock had fallen from the sky and caused the cow's death. The two smaller fragments were taken indoors, the larger left outside, the people having no idea of the potential value of their find. The cow was eaten.

Recently, Dr Ignacio Ferrin, an astronomer at the University of the Andes, traced the witnesses and the pieces of the meteorite. Bits have been distributed to professional meteoriticists. Much is now in private hands, with collectors clamouring for good samples. You could buy a slice (meteorites are often sawn into thin samples to be displayed), a typical cost being about £5 per gram. That gives the original 50 kg a value of £250,000.

The value of the Valera meteorite comes from its new reputation as a cow-killer, and samples come with a copy of an affidavit vouching for its validity, signed by the medic involved, Dr Arginiro Gonzales. Some meteorite enthusiasts observe that Valera's value has been boosted 10-fold by the connection with the cow, and suggest that a different type of killing might be being made.

There is another twist. Many meteorite collectors possess samples of a large fall found in the Atacama Desert in Chile, in 1861. More than 80 fragments have been identified, many in recent years. These weigh about four tons, so there has been plenty to go around. These pieces were strewn around a dry riverbed called Vaca Muerta, giving this meteorite its name.

Vaca Muerta means "dead cow" in Spanish. So the Venezuelan meteorite is being called Vaca Muerta II by some. The cynics see a more invidious connection, thinking the name of the famous meteorite from the Atacama may have provided the inspiration for a bit of money-making further north in Latin America.

· Duncan Steel teaches space and astronomy subjects at the University of Salford.

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