It took Solar Impulse 18 hours and 23 minutes to fly the final leg of its trans-America route from Washington, D.C., to New York -- slow enough for someone to drive the distance three times. While the aircraft was applauded for not using a single drop fuel, the 11,628 solar cells embedded on the Impulse's wings only function at 12% efficiency. For the time being, the future of green aviation is likely in biofuels and more efficient engines.

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