A prototype solar plane that could one day lead to cheap "terrestrial satellites" has been destroyed in an accident in the Pacific.
The Helios unmanned aerial vehicle (UAV) was lost on a checkout flight from the US Navy's Pacific Missile Range Facility on the Hawaiian island of Kauai.
Helios was testing a power storage system based on a fuel cell
Helios set an altitude record in 2001 for a non-rocket-powered winged aircraft when it climbed to 96,863 feet (29.5 kilometres).
Its design was intended to show that squadrons of "eternal planes" could remain aloft for months as low-orbit communications and environmental monitoring platforms.
These UAVs would be launched and maintained at a fraction of the cost of putting satellites into space.
The US space agency (Nasa), which sponsored the Helios research, also sees a role for the technology in future drones to fly over the surface of Mars.
The atmospheric conditions Helios encountered on its record-breaking adventures in 2001 are thought to resemble closely those found in the thin air of the Red Planet.
Helios crashed about 30 minutes into Thursday's test flight.
"We were flying at about the 8,000-foot altitude west of Kauai over the ocean and the aircraft simply broke up," said Alan Brown, a spokesman for Nasa's Dryden Flight Research Center.
The cause of the crash was unknown, Brown said. Nasa is forming an accident investigation team.
With a wingspan of 247 feet (75 metres), Helios was wider than a Boeing 747. It looked more like a flying wing than a conventional plane and it was extremely light, weighing 2,400 pounds (1,080 kilograms).
Current to power its 14 propeller motors and onboard electronics was generated by high-efficiency solar cells spread across the upper surface of the long wing.
When it crashed, Helios was testing an experimental, renewable fuel-cell system that would have enabled the vehicle to store daylight power to keep it aloft during the night.
The British Zephyr can go higher because it is smaller
Finding the right power storage system is seen as critical to the development of these UAVs. Without it, the planes of the future would have to be brought down during dark hours.
Helios had been flying under the guidance of ground-based mission controllers for AeroVironment of Monrovia, California, the plane's builder and operator.
Its people, along with those of Nasa and the US Navy, will make up the investigation team.
The Helios altitude mark could be beaten in the next few months by a British UAV that plans to fly to 123,000 ft (40 km). The Zephyr 3, which is much smaller (12-m wingspan; 14 kg), will not set an official record, however.
The UK UAV is being carried aloft for the first 30,000 ft (9 km) by a balloon and therefore does not qualify for an official record.