The U.S. is testing a larger version of the Zephyr solar powered UAV, to see if it can stay in the air continuously for at least two weeks. Apparently, it can. This version of the Zephyr has a 22.5 meter (70 foot) wingspan, weighs 50.1 kg (111 pounds) and has a payload of about three kilograms (7 pounds).
Two years ago, a slightly smaller version of Zephyr stayed up for 82 hours. Three years ago, Zephyr made its first long endurance flight. Several more, larger and larger, versions of this UAV are in the works, before it is ready for regular use by the troops.
Zephyr is by powered electric motors, using batteries recharged during the day by solar panels on the wings. During the day, the solar energy powers the aircraft's two propellers, and charges a battery. At night, the battery provides all the power needed to keep the aircraft aloft, although losing about a third of its altitude (which is normally about 18,000 meters/60,000 feet.)
To save weight, there is no landing gear. The current version is launched by having five men pick it up and walk it into the wind until its lifts off. It crash lands, but because of its light weight, there is not a lot of damage to be repaired.
Also called the HALE (High Altitude Long Endurance) aircraft, the Zephyr is basically a powered glider. The ultimate goal is to have the Zephyr fly for months at a time, at high altitude (over 16,000 meters/50,000 feet, way above the weather). Built of lightweight materials, earlier Zephyrs weighed only 30 kg/ 66 pounds. The most likely job for Zephyr would be for communications relay, substituting for a communications satellite. There is growing demand for this, as commsats are expensive and take a long time to build and get in orbit.
But as the years go by, sensors (cameras and electronic detectors of all sorts) get smaller, lighter and require less power. Thus while Zephyr is currently seen as a cost-effective communications satellite replacements (for troops beneath it, for several hundred kilometers in all directions), it could also take still pictures regularly, which would be very effective for a UAV circling around up there for months at a time.