Air Namibia, Flamingo April 2005 Text and photos by Tim Osborne
Beware of Falling Meteors!
Is Namibia a place where your chances of being hit by rocks falling from heaven are high? You might well think so from the large number of meteorites that have been found here. No one knows for sure, but there are probably thousands of meteorites lying all over the country.
Most people know that meteorites come from space, but only a few people understand what they are. There are many types of meteorites, and they come from several different sources in outer space. Before they enter the earth’s atmosphere they are called meteoroids. Once they have entered and are burning brightly, they are referred to as meteors, and once on the ground, as meteorites.
If you sit back and stare at the sky on a clear night, especially between the hours of midnight and sunrise, you can see hundreds of meteors, or shooting stars, as they are popularly called. These mainly microscopic space-dust particles ranging in size from 0.5 micron to 2 mm slowly float down to earth over days or weeks after their fiery entry. Their source is the dust left by comets and at certain times of the year, when the earth’s orbit around the sun takes it through the path of comet tails such as Haley’s, there can be spectacular showers of thousands of shooting stars. It is estimated that 100 000 tons of space dust falls to earth each year.
The most visible and interesting meteorites are the sporadic ones that arrive as fireballs of light, sound and smoke. These are parts of the asteroid belt orbiting between Mars and Jupiter. There are thousands of asteroids and they frequently collide. From time to time pieces are forced out of orbit and occasionally
head towards earth. On entry, a process called ablation occurs. The front edge of the meteor heats to melting point and burns, giving off light and smoke, consuming about 90% of the meteor. Ablation is what the heat tiles on the space shuttle are supposed to prevent. When tiles were knocked off the Columbia space shuttle on takeoff, they led to the re-entry disaster.
If people see the entry of a meteor, the chance of someone finding the meteorite is high. When on the ground they are usually black and heavy. The shape of the meteorite is dictated by the amount of frag- mentation and the ablative forces. Of the estimated 24 000 meteorites that arrive each year, 75% fall into the oceans.
The science of meteorology is still very recent. Up until 1985 only 2 611 authentic meteorites had been recorded. By 2001 scientists had recorded over 24 000 meteorites and the three basic types of meteorite had grown to 15 major groups including very rare ones that originate from the moon and Mars.
Scientists classify meteorites by their mineral com- position. The three basic types are stones, stony–irons and irons. The stony meteorites are called chondrites and comprise 82% of all meteors. They have little spherical inclusions called chrondrules, which give the meteor its name. No earth rocks contain these structures, so they are important in identifying meteorites as originating in space. They are composed mainly of silicate minerals with a 20%–25% iron-nickel alloy. Due to their low iron content they heat up faster in the atmosphere but then cool off faster. People who have picked up freshly fallen chrondrites report that they are warm to the touch. The stony-irons do not have chrondrules and contain about 50% of an iron-nickel alloy. Only 3% of the meteorites found are this variety. The irons are usually 98% iron with varying amounts of nickel. Nickel is a rare element on earth and is not naturally found alloyed to iron. Because of their high iron content, irons are very hot when they strike the earth.
Iron meteorites are classified by the internal crystalline structure of the iron-nickel alloys, which are called kamacite and taenite. These crystals can be seen when the meteorites are cut, polished smooth and treated with a nitric acid/ethanol solution. In photographs the alloys are arranged in 60-degree angles. These structures are called Widmanstätten after Count Alois von Widmanstätten of Vienna, who noticed the pattern while heating some meteorites.
The Gibeon shower occurred in 1836 when thou- sands of meteorites fell to earth. They are classed as octahedrites, defined as iron meteorites composed of low nickel kamacite bundled by high nickel taenite arranged in plates on the faces of an octahedron (eight-sided) crystal. The best place to see examples of the Gibeon shower is at the Post Street Mall in downtown Windhoek where 21 of the meteorites are on display.
The Hoba Meteorite is the largest known meteorite in the world. It is named after the farm on which it was found in 1920 and was proclaimed a national monument in 1955. It is a rough rectangular block, 2.95 x 2.84 metres on the flat upper surface with a thickness varying from 55 cm to 122 cm and estimated to weigh 60 tonnes. When it was found, only a small portion of the top was exposed. It was excavated in 1929 by Samuel Gordon on the Fourth Philadelphia Academy-Vaux Mineralogical Expedition. Gordon dug through a large amount of ‘iron shale’, the oxidised or rusted remains of the meteorite. Based on the amount of the iron shale present, the original meteor is thought to have weighed close to 100 tonnes when it fell approximately 80 000 years ago.
Between 1920 and 1929 collectors using oxyacetylene torches damaged the edges of the meteorite. They took two scientific samples weighing 150 g and 15 g for analysis. The specific gravity of the sample was 7.971, while the specific gravity of most earth rock rarely exceeds 2.0. Analysis of the meteorite found that it was composed of 82.4% iron, 16.7% nick- el, 0.74% cobalt, 0.02% sulphur and traces of carbon, chromium, zinc, gallium, germanium and iridium. The meteor is classified as an iron ataxite, with no crystal- line structures composed of nickel-rich taenite. There are only 13 known ataxites in the world.
By studying space dust and meteorites found on earth, scientists discover clues to how our solar sys- tem and the universe were formed. Meteorites from the asteroid belt have been dated to 4.5 billion years ago. This makes them the same age as the oldest Martian, earth and lunar rocks, indicating a similar time span of formation. Meteorites contain water, organic chemicals and minerals produced under extreme conditions, information that contributes towards our understanding of life on earth. Unfortunately, after surviving for billions of year in space, once they arrive on earth meteorites are immediately subject to the forces of erosion, so that most have a lifespan that rarely exceeds a hundred thousand years.
Are you safe from falling rocks in Namibia? More meteorites have been found in the United States and Europe than in Namibia, so your best chance to avoid being struck by a meteor is to simply stay put. On the other hand, there has never been a recorded fatality from falling rocks, although in 1954 a woman was slightly injured by a meteorite that bounced around in her Alabama house before hitting her on the leg.
So relax, sit back and enjoy the night sky.