NIGER has opened up another front against the theft of its natural wealth — investigating how the biggest chunk of the planet Mars known to have landed on Earth got from the Sahara desert to the Sotheby’s auction house in New York.

The 24.6-kilogram meteorite is believed to have been knocked off the red planet by an asteroid and hurled 140 million miles before landing in the desert in the Agadez region of northern Niger.

Meteorite hunters often scour the Sahara, where the climate encourages preservation and a lack of human disturbance leaves extraterrestrial matter untouched. NWA 16788, as the rock was labelled, sold at Sotheby’s last month for $5.3 million (£3.9m), going from an anonymous seller to an anonymous buyer.

Niger's authorities want to know how such a unique item was spirited out of the country without their knowledge, though Sotheby’s says it complied with all relevant laws.

A number of developing countries have sought to assert ownership of meteorites and fossils found on their territory, given a growing tendency for traders to unearth material of scientific value and auction it off to private buyers.