Most scientists today recognize global warming as a reality. According to researchers from Cessna Aircraft Co. and Wichita State University, world energy use is doubling every 30 years, and carbon dioxide generated by fossil fuel combustion has been identified as a major "greenhouse gas" contributor to global warming. While aircraft account for only 10% of transportation petroleum-fuel usage, their emissions occur at high altitudes, where the impact is greater than with surface transportation. Global-warming concerns are stimulating accelerated research and development of alternative fuels and propulsion systems throughout the transportation industry. On the aerospace side, Helios, a solar cell high-altitude research aircraft designed by Aero Vironment for NASA, conducted successful flight operations at extreme altitudes on purely electric solar cell power. It had been scheduled as a high-altitude fuel-cell demonstrator before it was lost during tests in December 2003.
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