Iranian forces on Thursday downed a US Navy Global Hawk, the first time the high-end spy drone has ever been shot down.

Costing nearly $150m each, the Global Hawk drone is prohibitively expensive – which helps explain why the US military only has 35 of them.

The surveillance aircraft are designed to fly at a maximum altitude of 19.8km (65,000 feet), hoovering up the electromagnetic spectrum, searching for targets and being able to scan an area larger than the size of Greece in 24 hours.

Global Hawk drones, made by Northrop Grumman Corp, are semi-autonomous. This means that once they are given their orders, they can take off to make their way to the target site, complete their mission and return – all without the aid of human intervention.

They are extremely well-suited for wide-area surveillance, making them a very useful maritime deployment – they can fly across the Pacific Ocean without the need for refueling.

The US Navy has been trialing them and has its own variant, the Triton. Last year, Japan agreed to buy three Global Hawk drones to monitor its expansive territorial waters – an ideal mission for such an aircraft.