Hoba

The heaviest known meteorite on Earth has never been moved from the spot where it landed.

Key facts

Type

Iron, IVB (nickel-rich ataxite)

Fell or found

Found

Year

1920

Country

Namibia

Mass

about 60 tonnes (estimated)

Official name

Hoba

In 1920 a farmer named Jacobus Hermanus Brits was ploughing a field on the Hoba West farm, not far from the town of Grootfontein in northern Namibia, when his plough struck something hard and jolted to a stop. Digging around the obstacle, he uncovered a flat, dark slab of metal. It turned out to be the heaviest single meteorite ever found, and more than a century later it still lies in the very spot where it came to rest.

A slab of space iron

Hoba is roughly 2.7 metres long, 2.7 metres wide and about 0.9 metres thick: picture a giant square tabletop made of solid metal. When it was first studied its mass was estimated at around 66 tonnes. Weathering, scientific sampling and, sadly, souvenir hunters chipping off pieces have reduced it, and today it is usually quoted at about 60 tonnes. Nobody has ever weighed it directly, because no practical scale could do the job; the figure comes from its size and the known density of iron.

An iron meteorite is a piece of metal, mostly iron with some nickel, that once sat deep inside an asteroid large enough to melt and separate into layers. Heavy metal sank to the centre to form a core, while lighter rock floated above it as a mantle and crust. Much later, violent collisions smashed such asteroids apart and scattered fragments of their cores through space. Some of those fragments eventually crossed Earth's path.

Hoba belongs to a rare chemical group called IVB. These irons are unusually rich in nickel, about 16 per cent in Hoba's case, and they are classed as ataxites, from a Greek word meaning 'without structure'. Most iron meteorites, when cut, polished and etched by specialists in a laboratory, reveal a criss-cross of metal crystals called the Widmanstätten pattern. Ataxites show almost nothing: their high nickel content means the crystals that formed as the metal slowly cooled are far too fine to see with the naked eye.

The puzzle of the missing crater

A 60-tonne lump of iron arriving at full cosmic speed ought to have blasted out a sizeable crater, yet there is no sign of one at Hoba. The meteorite was found only partly buried in limestone. A leading explanation is that its flat shape, and perhaps a shallow angle of entry, allowed the atmosphere to brake it hard, so that it reached the ground at a relatively modest speed rather than exploding on impact. Its fall was not witnessed and no one knows exactly when it happened, though it was clearly a very long time ago: a thick crust of iron oxide, a kind of rust, had formed around it.

Protecting a giant

Because it is too heavy to steal whole, Hoba's main threat has been people breaking pieces off. To protect it, the authorities declared it a national monument in 1955. In the 1980s the site passed into public ownership and a small visitor centre was built, with stone terraces arranged around the meteorite like a little amphitheatre. Visitors can walk right up to it, but damaging it or taking anything from it is illegal. That is a good general lesson: meteorites on protected sites, or on someone else's land, are never free for the taking.

Why Hoba matters

Hoba is more than a record-breaker. It is a giant sample of the metallic heart of a long-destroyed small world, and studying meteorites like it helps scientists understand how planets form cores. Its survival without a crater shows that the atmosphere can do surprising things to incoming bodies, especially those with unusual shapes. And because it has never been moved, Hoba gives visitors something no museum can: the chance to stand where a fragment of an asteroid core came down, in the same red earth that has held it ever since.

Did you know?

See more

Hoba in the catalogue