A large metallic ball has fallen out of the sky on a remote grassland in Namibia, prompting baffled authorities to contact NASA and the European space agency.

The hollow ball with a circumference of 1.1 metres was found near a village in the north of the country some 750 kilometres from the capital Windhoek, according to police forensics director Paul Ludik.

Locals had heard several small explosions a few days beforehand, he said.

With a diameter of 35 centimetres, the ball has a rough surface and appears to consist of "two halves welded together".

It was made of a "metal alloy known to man" and weighed six kilograms, Mr Ludik said.

The ball was found 18 metres from its landing spot, a hole 33 centimetres deep and 3.8 metres wide.

Several such balls have dropped in southern Africa, Australia and Latin America in the past 20 years.

Media reports say the ball is likely to be a "composite overwrapped pressure vessel", used by NASA to store gases under pressure in space.

The strength needed to maintain pressure within the balls means they are manufactured from tough material including Kevlar, and therefore likely to remain intact when they re-enter the earth's atmosphere.

The sphere was discovered in mid-November, but authorities first did tests before announcing the find.

Police deputy inspector general Vilho Hifindaka concluded the sphere did not pose any danger.

"It is not an explosive device, but rather hollow - we had to investigate all this first," he said.

AFP