A British experimental UAV, the
Zephyr, recently set an endurance record by staying in the air, at about 60,000
feet, for over three days (82 hours, 37 minutes). The 70 pound aircraft uses
solar panels on its 59 foot wingspan to power the propeller, and batteries for keeping the aircraft going at night.
All the flying was done under software control. On the downside, the aircraft
only has a payload of about four pounds. Any useful payload (camera, or
communications transponder so the UAV could act as a local space satellite
substitute) would need power, which would reduce endurance.
was partly funded by the U.S. Department of Defense, and the basic design could
be scaled up, and make use of low power sensors and communications devices in
development. Eventually, this type of aircraft would have a practical use.