At this altitude, thin air makes it hard to generate lift for drones while extremely low temperature means electronic components like batteries can fail.
However, a new type of drone developed by China is being tested to overcome such difficulties, marking a significant step toward China's ambitions of exploiting near space for military intelligence, Hong Kong-based South China Morning Post reported.
Near space has long been seen as a promising frontier for intelligence services, but has remained relatively untapped because it is too high for most aircraft to fly, and too low for satellites. The goal of scientists is to develop a durable near space vehicle capable of observing large areas for weeks, months or even years. Drones, which cost just a fraction of satellites with comparable abilities in optics such as gathering intelligence with cameras, are seen as one of the best ways of reaching that goal.
Until now, the Northrop Grumman RQ-4 Global Hawk, which can reach to an altitude of about 19 km, has been the highest-flying drone in use.
Last month, a research facility in Inner Mongolia successfully tested an experimental drone at an altitude of 25 km, the report said. The test involved two experimental unmanned aerial vehicles being sent up on a high-pressure balloon before being deployed at different altitudes. The second drone was deployed at an altitude of 9 km, it said.
Each of the drones, about the size of a bat, was launched using an electromagnetic pulse that accelerated them from zero to 100 km per hour instantly.
"It shot out like a bullet," said Yang Yanchu, lead scientist of the project with the Academy of Optoelectronics at the Chinese Academy of Sciences in Beijing. The drones then glided toward their targets more than 100 km away, adjusting course and altitude in flight without human intervention. On-board sensors beamed data back to a ground station.
"The goal of our research is to launch hundreds of these drones in one shot, like letting loose a bee or ant colony," Mr Yang said.
Similar test flights had been conducted by the US Navy and the NASA in recent years as the US searched for a new weapon to penetrate air defence systems and gather intelligence behind enemy lines, he said.
Yang Chunxin, a professor at the school of aeronautic science and engineering at Beihang University in Beijing, said there were still many challenges in developing high-altitude drones.
"One of the biggest headaches is the near-vacuum environment, where electric currents can produce a spark. This can lead to shortages and damage electronic equipment," he said. "This is why high-altitude drones are more difficult... Whether they can play a practical role in military operations remains an open question," he was quoted as saying by the South China Morning Post.