The solar-electric Helios Prototype flying wing bisects the volcanic atoll of Lehua off the coast of Kaua'i, Hawaii, during a checkout flight on June 7, 2003. Helios is now equipped with an experimental fuel cell system to allow nighttime flight.

Helios is an all-electric airplane. In addition to being non-polluting, Helios can fly above storms, and use the power of the sun to stay aloft during daylight. Key to the success of this type of aircraft is the ability to fly in darkness, using fuel cells when sunlight cannot furnish energy.

Helios flew over the Navy's Pacific Missile Range Facility where favorable sun exposure and test ranges closed to other air traffic benefited the NASA research effort. In 2003 the aircraft was lost to a crash.

Developed by AeroVironment, Inc., of Monrovia, Calif., under NASA's Environmental Research Aircraft and Technology (ERAST) project, the unique craft was designed to demonstrate two key missions: the ability to reach and sustain horizontal flight near 100,000 feet altitude on a single-day flight, and to maintain flight above 50,000 feet altitude for almost two days, the latter mission with the aid of an experimental fuel cell-based supplemental electrical system now in development.

The Helios Prototype is an enlarged version of the Centurion flying wing that flew a series of test flights at Dryden in late 1998. The craft has a wingspan of 247 feet, 41 feet greater than the Centurion, 2 1/2 times that of the Pathfinder flying wing, and longer than the wingspans of either the Boeing 747 jetliner or Lockheed C-5 transport aircraft.

The remotely piloted Helios Prototype first flew during a series of low-altitude checkout and development flights on battery power in late 1999 over Rogers Dry Lake adjacent to NASA's Dryden Flight Research Center in the Southern California desert. In upgrading the Centurion to the Helios Prototype configuration, AeroVironment added a sixth wing section, a fifth landing gear pod and a differential Global Positioning Satellite (GPS) system to improve navigation, among other improvements. The additional wingspan increased the area available for installation of solar cells and improved aerodynamic efficiency, allowing the Helios Prototype to fly higher, longer and with a larger payload than the smaller craft. During 2000, more than 62,000 bi-facial silicon solar cells were mounted on the upper surface of Helios' wing. Produced by SunPower, Inc., these solar arrays convert about 19 percent of the solar energy they receive into electrical current and can produce up to 35 kw at high noon on a summer day. The second milestone established by NASA for its development a long-endurance demonstration flight of almost two days and nights required development of a supplemental electrical power system to provide power at night when the solar arrays are unable to produce electricity. AeroVironment developed an experimental fuel cell-based electrical energy system combining advanced automotive fuel cell components proprietary control technology designed for the harsh environment above 50,000 feet altitude. The first version of this system combines gaseous hydrogen from two pressurized tanks mounted on Helios' outboard wing sections with compressed oxygen from the atmosphere via a series of proton-exchange membrane fuel cell "stacks" mounted in the central landing gear pod. The system produces more than 15 kW of direct-current electricity to power Helios' motors and operating systems, with the only by-product being water vapor and heat. The system will increase the Helios Prototype's flight weight by about 800 lb to about 2,400 lb. Two other versions of the system are contemplated: One, employing liquid hydrogen, would enable the Helios to fly for up to two weeks in the stratosphere anywhere around the Earth, not limited to temperate or equatorial latitudes. Another version, a closed or "regenerative" system, uses water, a fuel cell, and an electrolyzer to form a system similar in function to a rechargeable or "secondary" battery, but with much greater efficiency than the best rechargeable battery systems. A production version of the Helios with the regenerative fuel cell system is of interest to NASA for environmental science, the military and AeroVironment for various roles, primarily as a stratospheric telecommunications relay platform. With other system reliability improvements, production versions of the Helios are expected to fly missions lasting months at a time, becoming true "atmospheric satellites."