Philips Atlas of the Universe

Meteorites

A

meteorite is a solid particle which comes from space

and lands on the Earth, sometimes making a crater. It is

not simply a large meteor, and there is no connection

between the two types of objects. Meteors, as we have

seen, are the debris of comets. Meteorites come from the

asteroid zone, and are associated neither with shooting-star

meteors nor with comets. It is probably true to say that

there is no difference between a large meteorite and a

small asteroid.

Meteorites are divided into three main classes: irons

(siderites), stony-irons (siderolites) and stones (aerolites).

Irons are composed almost entirely of iron and nickel.

Aerolites are of two sorts, chondrites and achondrites.

Chondrites contain small spherical particles known as

chondrules, which may be from one to ten millimetres (less

than half an inch) across and are fragments of minerals,

often metallic; achondrites lack these chondrules. Of spe-

cial interest are the carbonaceous chondrites which contain

not only carbon compounds but also organic materials. It

was even suggested that one famous carbonaceous chon-

drite, the Orgueil Meteorite which fell in France on 14 May

1864, contained ‘organized elements’ which could have

come from living material, though it seems much more

likely that the meteorite was contaminated after it landed.

Most museums have meteorite collections; irons are

more often on display than stones, because they are more

durable and are more likely to be recovered in recogniz-

able form. Areas such as Western Australia and, particular-

ly, Antarctica are fruitful grounds for meteorite-hunters,

because there has been relatively little human activity

there. All known meteorites weighing more than 10 tonnes

are irons (the largest aerolite, which fell in Manchuria in

1976, has a weight of only 1766 kilograms), but it is not

always easy to identify a meteorite simply by its appear-

ance, and often it takes a geologist to tell what is meteoritic

and what is not. One test for an iron meteorite is to cut it

and etch with dilute acid. Some irons show the geometrical

‘Widmanstätten patterns’ not found in ordinary minerals.

Meteorites have been known since very early times,

though it was not until 1803 that a shower of stones,

at L’Aigle in France, gave conclusive proof that they

come from the sky. Some interesting specimens are found

here and there. The Sacred Stone at Mecca is certainly a

meteorite, and it is on record that as recently as the 19th

century part of a South African meteorite was used to

make a sword for the Emperor Alexander of Russia.

The largest known meteorite is still lying where it fell,

in prehistoric times, at Grootfontein near Hoba West in

Namibia. It weighs at least 60 tonnes. There are no plans

to shift it, but not so long ago action had to be taken to

protect it from being vandalized by troops of the United

Nations peacekeeping force. Second in order of size is

the Ahnighito (‘Tent’), which was found in Greenland

by the explorer Robert Peary in 1897, and is now in the

Hayden Planetarium in New York.

Over 20 meteorites have been known to fall over the

British Isles, and most have been recovered. The most

celebrated of them shot over England on Christmas Eve in

1965 and broke up, showering fragments around the

Leicestershire village of Barwell. The latest British

meteorite – a small chondrite – fell at Glatton, in

Cambridgeshire, on 5 May 1991, landing 20 metres from a

retired civil servant who was doing some casual garden-

ing. Incidentally, there is no known case of serious injury

caused by a tumbling meteorite, though admittedly a few

people have had narrow escapes.

Both the greatest falls during the 20th century were in

Siberia. On 30 June 1908 an object struck the Tunguska

region, blowing pine trees flat over a wide area which was,

mercifully, uninhabited. Owing to the disturbed state of

Russia at that time no expedition reached the site until 1927,

and though the pine trees were still flat there was no crater

and no evidence of meteoritic material. It is possible that the

impactor was icy, in which case it may have been a frag-

ment of a comet, but we do not really know. There is no

mystery about the second Siberian fall, in the Sikhote–Alin

area on 12 February 1947; many small craters were found,

and many pieces of the meteorite were salvaged.

There has been much discussion about the eight SNC

meteorites, named after the regions in which they were

found (Shergotty in India, Nakhla in Egypt and Chassigny

in France). They seem to be much younger than most

meteorites, and to be different in composition; it has been

suggested that they have come from the Moon or even

Mars. This is highly speculative, but is at least an intrigu-

ing possibility, though it is not easy to see how they could

ATLAS OF THE UNIVERSE

 Nickel-iron meteorites

found at the site of the

Meteor Crater in Arizona,

and now on display in the

museum there.

▼ 
 Tektites are of terrestrial

origin.

 The Glatton Meteorite

which fell in Cambridgeshire

on 5 May 1991. It weighed

767 grams.

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