To better track potential threats around the globe, the Pentagon is developing two unique unmanned surveillance aircraft systems: the Rapid Eye and the Vulture.

Adapted from NASA concepts for aircraft that might one day explore Mars, the Rapid Eye is envisioned as an unmanned aerial vehicle carrying a 500-pound payload that would be deployed in a ballistic missile to arrive at any target on Earth within two hours of launch.

If the threat level is deemed sufficiently high, the Vulture might be launched to continue long-term surveillance of a region. The Vulture is expected to be able to carry a 1,000-pound payload into the stratosphere, flying continuously at an altitude of 90,000 feet for up to five years.

The Rapid Eye and Vulture programs are intended to be complementary, with the former expected to provide virtually immediate short-term information about a region or theater of interest and the latter generating continuous surveillance and intelligence data for as long as necessary.

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Surveillance satellites typically are locked into fixed orbits and must fly quickly over a region, providing only snapshots of activities at ground level. In contrast, the Vulture would act as a pseudo-satellite, offering the opportunity for persistent surveillance of a region by repositioning itself as needed. It could loiter for days, weeks or months above a war zone, a riot or a smuggling operation.

The final design is expected to rely upon up to 5 kilowatts of electrical power, perhaps generated from solar energy panels or fuel cells.

The Vulture program is divided into three phases during which conceptual designs first are developed, followed by the fabrication and testing of one-sixteenth-scale prototypes capable of carrying 150-pound payloads and finally the launch of full-scale prototypes. The Defense Advanced Research Projects Agency recently awarded contracts to Boeing, Lockheed Martin and Aurora Flight Services to develop competing prototypes.

The Vulture prototypes might include modular designs in which portions of the craft return to Earth for refueling and maintenance while the primary section continues to remain above a region of interest.