A BBC article offers an overview of eternal UAVs; flying robots designed to fly perpetually. The goal is to build a solar powered UAV that can fly at an altitude of 18-20km. During the day, the robot stores excess power from the solar panels and uses it to continue flying at night. One of the earliest attempts to build an eternal UAV was NASA's Helios, which we reported on several times in the past. Helios set numerous world records but crashed in 2003. Right now, the Mercator UAV is being developed by the Belgian Flemish Institute for Technological Research as part of the Pegasus project. Mercator has a 16m wingspan and weighs just 27kg. More details and an artist conception of Mercator can be found in a press release from one of the contractors.