Helios, an unmanned solar-powered flying wing created by AeroVironment, is now being prepared for another NASA-sponsored major milestone -- the world's first multi-day fuel-cell-powered flight in the stratosphere. The aircraft in 2001 shattered the world altitude record for non-rocket-powered aircraft by flying to 96,863 feet, powered solely by silicon solar cells mounted on its wing. A 12-hour test flight that will take place this month sets the stage for the multi-day flight test, planned for Hawaii in July. "The Helios prototype has proven its capabilities to conquer the day on solar power," said John Del Frate, Helios project manager at NASA's Dryden Flight Research Center. "Now to fulfill the long-term vision for it to fly routinely for extreme duration, the next challenge is to conquer the night. The key to that is development of the fuel-cell system. It's the necessary next step to extreme endurance." This year's mission for Helios will be powered with high-efficiency solar arrays by day and a newly developed fuel-cell-based electrical energy system at night. Developed by Helios' manufacturer, AeroVironment, of Monrovia, Calif., the system combines advanced automotive fuel-cell components with proprietary control technology designed for the harsh environment above 50,000 feet. The AeroVironment system consumes no fossil fuels, emits no atmospheric pollutants, and has a power-to-weight ratio about twice that of the best battery systems.