Imagine a '57 Chevy cruising through the air, and you get an idea of what single-engine, propeller-driven airplanes do to the environment. The average private plane, such as the popular two-seat Cessna 172, is 30 years old. It carries a four-cylinder piston engine designed in the 1940s that burns leaded gasoline, has no catalytic converter, and gets as little as 12 miles per gallon. “It’s fair to say that small aircraft are gross polluters,” says Mark Moore, an engineer who has led personal-aircraft projects for NASA.

The highest-endurance aircraft currently flying is Northrop Grummans Global Hawk UAV, which can stay aloft for up to 40 hours. Now Darpa—which, to its credit, is never short on outlandish ideas—wants to beat that endurance record more than 1,000 times. The goal of Darpa's recently launched Vulture Program is to build a kind of atmospheric satellite that can stay aloft for five years at a time with little or no maintenance.

The future of secure travel hinges on seamless, instant communication-and 24/7 autonomous surveillance. For a look at the technologies that will soon safeguard your travel plans, launch the photo gallery.

For years, the U.S. military has wanted a plane that could loiter just outside enemy territory for more than a dozen hours and, on command, hurtle toward a target faster than the speed of sound. And then level it. But aircraft that excel at subsonic flight are inefficient at Mach speeds, and vice versa. The answer is Switchblade, an unmanned, shape-changing plane concept under development by Northrop Grumman.