The Hoba meteorite is an ataxite iron-nickel meteorite that landed in northern Namibia at some point in the last 80 000 years. Because of its shape and trajectory (it’s thought possible that it “skipped” across the atmosphere like a stone on a pond) it landed at subsonic speeds, and didn’t bury itself particularly deep, leaving no crater behind.
The Hoba meteorite was discovered by a farmer as he was ploughing a field, and has never been moved due to its large size; it has a mass of around 60 000 kilograms. (It was estimated to be about 66 000 kilograms when it fell, but its mass has been reduced by erosion, sampling and vandalism.) The Hoba meteorite is the world’s largest meteorite, and the most massive naturally occuring piece of iron on Earth’s surface.
Meteorites can be separated into four main groups:
Chondrites are grainy stony meteorites composed mainly of silicate minerals like olivine and pyroxene. They make up about 80% of the meteorites found on Earth.
Achrondrites are similar to chondrites, but at some point in their lifetime they have been melted (like magma) and therefore they do not demonstrate the same graininess that chondrites do.
Iron meteorites are composed mainly of meteoric iron. Despite its name meteoric iron is actually an iron-nickel alloy, and most iron meteorites are composed of either kamacite or taenite.
The fourth and final group are the siderolites, stony-iron meteorites, and they are beautiful. I am particularly keen on pallasites, which contain centimetre-sized grains of peridot olivine embedded within a matrix of meteoric iron.
The origin of pallasites is uncertain, but it’s thought possible that they may be formed when two asteroids collide, or when a meteorite collides with the surface of a planet.