Airbus’s Zephyr solar-powered drone flew for 25 days straight during a test-flight over Yuma, Arizona beginning on July 11, 2018. The flight represented a...

Airbus’s Zephyr solar-powered drone flew for 25 days straight during a test-flight over Yuma, Arizona beginning on July 11, 2018. The flight represented a record for aircraft endurance, breaking the previous 14-day record also set by a Zephyr back in 2015.

The long flight has big implications for military surveillance. Drones like Zephyr could loiter over a low-intensity battlefield far longer than current drones can do. The latest high-endurance Reaper drone maxes out at 40 hours in the air.

The propeller-driven Zephyr belongs to a class of aircraft known as “high-altitude pseudo-satellites,” or HAPs. Flying as high as 70,000 feet for weeks or even months at a time, HAPs perform many of the same missions that low-orbiting satellites do.

“The main HAP applications are in telecommunications and remote sensing, both civilian and military,” Flavio Araripe d’Oliveira, Francisco Cristovão Lourenço de Melo and Tessaleno Campos Devezas wrote in a 2016 paper.

Compared to comms satellites, HAPs have the advantages of lower latency and the ability to land for maintenance or reconfiguration, d’Oliveira, de Melo and Devezas explained. For surveillance missions, HAPs unlike satellites can linger over a particular area and could produce images with better resolution, since they fly lower than satellites do.

HAPs could be more vulnerable to enemy defenses, however. Where satellites orbit many hundreds of miles over Earth, beyond the reach of most conventional weaponry, Zephyr — so far the only HAP undergoing realistic testing — attained a maximum altitude of 70,000 feet, well under the ceiling for modern air-defense missile systems such as the Russian S-300.

Also, the drone is slow, with a cruising speed of just 20 miles per hour.

Zephyr and similar pseudo-satellite drones could be best-suited for operations over lightly-defended territory. In 2016, the U.K. ministry of defense bought three Zephyrs for around $6 million apiece in order to evaluate them for potential use by the military and other government agencies.

“Zephyr is a cutting edge, record-breaking piece of kit that will be capable of gathering constant, reliable information over vast geographical areas at a much greater level of detail than ever before,” then-defense secretary Michael Fallon said in a statement.

Airbus is still refining Zephyr, in particular its power-consumption. During daytime, the lightly-built solar-powered drone — which features an 82-foot wingspan and yet weighs just 165 pounds — can fly as high as 70,000 feet while also charging its batteries.

After the sun goes down, Zephyr runs on batteries … and slowly loses altitude. During the record-setting Yuma flight, the drone dipped as low as 50,000 feet at night.

The challenge for Airbus is to balance weight and power-consumption to produce the optimal flight profile for a particular task. “You have to find the right equation between flying altitude plus battery life, maintaining this or that power,” said Alain Dupiech, an Airbus spokesperson.

It’s unclear just how long Zephyr could stay aloft under the right conditions. The drone’s lithium-ion battery eventually dies, forcing it to land for maintenance. But battery technology is advancing rapidly, driven in part by consumer demand for electric cars, d’Oliveira, de Melo and Devezas wrote.

In the short term, a maximum endurance of several months is not inconceivable. But longer flights might not be particularly useful for surveillance and comms missions, Dupiech said. “At this stage, most of those missions are not calling for a year and half up there.”

Airbus has scheduled Zephyr’s next test flight for October in western Australia.