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 refuelling.

The US Navy has been trialling 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.

Valuable tool

Global Hawks are considered a strategic intelligence-gathering drone.

In one recent operation in Iraq, Global Hawk flights accounted for five percent of high-altitude reconnaissance flights, but generated 55 percent of the targeting information – a powerful indication of just how valuable they can be.

However, they are unarmed and fairly slow. Their main defence is their operational height, which allows them to fly above the reach of most air defence systems, although the Iranian military was able to shoot one down.

Drone vs manned reconnaissance aircraft

The speciality of drones lies in their extended loitering time, which can often be more than a day.

The fact that they are unmanned though means that a country can send its drones into a contested airspace without the danger of the loss of human life and, similarly, the targeted country can destroy the drone without the potential political risk of killing a pilot.

However, the destruction of a high-value US military asset is a sharp escalation in an already highly charged situation, increasing the likelihood of a major military confrontation.