The idea, ultimately, is to build a drone that will stay in the sky for five years or more. But in the meantime, Pentagon wild research arm Darpa will just have to settle for a solar-powered robotic aircraft that flies for a month a time. Not bad, considering that's nearly 10 times the current drone endurance record of 3 days, 10 hours.

Darpa just launched the $155 million second phase of the project, dubbed "Vulture." The idea is to build a drone that can carry a thousand-pound payload for at least thirty days.

If phase one is any guide, ideas for the long-fliers will be pretty wild. Lockheed's proposal included a robo-plane with rotating tails that swiveled to collect sunlight. Aurora Aerospace's Odysseus concept (see illo above) counts on sending three vehicles into the air, and then have them dock together in the stratosphere, Voltron-style. "This avoids having to fly that huge wing through the turbulent lower atmosphere, and allows an individual section to undock and fly home for repair if needed," Aviation Week noted. If that idea sounds too crazy even for the military's mad scientist bureau, consider this: former Darpa director just joined Aurora's board.

[Illo: Aurora]