The US and Chinese militaries are both working on solar-powered drones that can spend weeks flying at the edge of space

• The US and China are both testing drones that can spend weeks or months flying at the edge of space.
• In a US-China war, high-altitude, long-endurance drones would be vital to monitoring the Pacific.

What is the most valuable attribute of a drone? Depending on the task, it might be speed or maximum altitude or how many weapons or sensors it carries.

In truth, the most valuable attribute of a drone may be simple endurance. Prized indeed would be an unmanned aircraft that could continuously orbit the battlefield without needing to refuel and without a pilot who needs to eat and sleep.

Hence, the US and China are both testing long-endurance, solar-powered UAVs that can stay airborne not for hours or days but for months — and do so while flying nearly to the edge of space.

In August, a Zephyr 8 drone crashed in Arizona during a US Army test. It's not uncommon for drones to crash, but most don't do so after flying for 64 straight days.

The Zephyr 8 cruised at an altitude of 60,000 to 70,000 feet over South America, Central America and the southwestern US, nearly breaking a world record for the world's longest continuous flight.

Meanwhile, in September China unveiled the Qimingxing ("Morning Star") 50.

"The drone is able to fly at altitudes of up to over 20 kilometers [12.4 miles] and can stay airborne overnight," claimed the state-owned newspaper China Daily. "The solar panels are placed on the wings that have a combined length of 50 meters [164 feet]."

Though the drone only remained aloft for 26 minutes on its maiden flight, China has much more ambitious plans. The US and China both aim to fly drones in "near space," a region sandwiched between the upper atmosphere and where outer space begins, or about 12 to 62 miles above the Earth's surface. One Chinese scientist even referred to these type of drones as "quasi-satellites."

The stakes here are immense. In an era of precision-guided weapons, ISR (intelligence, surveillance, and reconnaissance) systems that can locate targets — and relay their coordinates to hypersonic missiles and other long-range munitions — are vital.

But current ISR platforms have their limitations. Manned reconnaissance aircraft, such as the U-2, are expensive, scarce, and vulnerable to interception. Spy satellites orbit the Earth at 18,000 mph, so they can't continuously scan the same patch of land or ocean.

An inexpensive reconnaissance platform that could stay aloft for months using solar power and scan vast areas from a vantage point near space would be invaluable.

For example, drones have been suggested as a means to destroy North Korean missiles during their vulnerable boost phase. Such a scheme would almost certainly require long-endurance UAVs.

In a US-China conflict, long-endurance drones would be crucial to span the vast distances of the Pacific.

Should the US and its allies provide military support to Taiwan in response to a Chinese invasion, China would quite likely try to pulverize key airbases, like those in Guam or Japan, with salvoes of ballistic missiles.

If those attacks prevent the use of regular drones or manned reconnaissance aircraft, a UAV that could be airborne for long periods, or that could operate from more distant, undamaged airfields, could provide ISR data instead.

Still, challenges remain. As with solar power for homes, weather can affect energy collection by solar-powered drones.

Solar arrays must be lightweight so a UAV can carry enough of them to generate adequate power but sturdy enough to withstand the rigors of flight. It must also have an airframe, sensors, and computers that can function for extended periods without returning to base for maintenance.

Nonetheless, just as manned aircraft developed longer range over time, it would be natural for drones to do the same. That eye in the sky may eventually never sleep.

Michael Peck is a defense writer whose work has appeared in Forbes, Defense News, Foreign Policy magazine, and other publications. He holds a master's in political science. Follow him on Twitter and LinkedIn.