The Helios Prototype was a unique electrically powered experimental lightweight flying wing developed by AeroVironment, Inc., under NASA's Environmental Research Aircraft and Sensor Technology (ERAST) program. Using energy derived from the sun by day and from fuel cells at night, the Helios Prototype was designed as the forerunner of high-altitude unmanned aerial vehicles that could fly on ultra-long duration environmental science or telecommunications relay missions lasting for weeks or months without using consumable fuels or emitting airborne pollutants.

Unfortunately, the program suffered a major setback when the Helios experienced control difficulties while on a checkout flight on June 26, 2003, and the aircraft suffered some structural failures and was subsequently destroyed when it fell into the Pacific Ocean about 10 miles west of the Hawaiian island of Kauai. The experimental fuel cell system carried aboard the Helios Prototype on that flight was lost. An investigation to determine the cause(s) of the control problems that led to the loss of the craft is still in progress as of this writing.

NASA established two major goals for the Helios Prototype uninhabited aerial vehicle (UAV). The first milestone was to reach an altitude at or near 100,000 ft. on a single-day flight with a small payload. This mission was to demonstrate that an aircraft could carry a science instrument to extreme altitudes. The power required to lift a small (100 lb.) payload to 100,000 ft. also enables the aircraft to carry much larger (600 lb.) payloads to altitudes up to 70,000 ft., making the Helios platform a versatile UAV.

In diminishing sunlight on August 13, 2001, Helios effectively demonstrated this capability when it reached an unofficial record altitude for non-rocket-powered aircraft of 96,863 feet.

Flight at 100,000 ft. is quite similar to that expected in the Martian atmosphere, so data obtained from the record altitude flight will also help to build NASA's technical and operational data base for future Mars aircraft designs and missions.

The second milestone for the Helios Prototype was to achieve extreme duration. Initially, plans called for this aircraft to maintain flight day and night for 96 hours, most of which would have been above 50,000 feet altitude, by 2003. Technical considerations led to those plans being modified, with a new goal of maintaining flight for 40 hours with at least 14 hours above 50,000 feet altitude by September, 2003, with the original milestone reset for 2006. Both of these goals would be dependent upon the development of advanced lightweight electrical energy systems for high-altitude airborne applications, based on fuel cell technology.

A NASA-industry planning team is now reviewing a variety of options to continue further development of the solar and fuel cell system technology for airborne applications, within the framework of a "technology development roadmap" that will set the course for further development of critical HALE UAV technologies and capabilities. That roadmap, which will take into account the findings and recommendations of the Helios Mishap Investigation Board, is expected to be complete by the end of September.

The roadmap will address many technology areas, such as hydrogen fuel, regenerative and primary fuel cell systems, emissionless electric propulsion, multi-function airframe structures and autonomous mission management. Based on the roadmap, a revised set of research projects under NASA's Vehicle Systems Program is expected to get underway beginning in October. These new projects are expected to lead to even more revolutionary HALE UAV aircraft capable of flying routinely as reliable "atmospheric satellites" on critical scientific and civil missions by the 2010—2015 time period.

The fuel cell systems designed for the Helios were of two types: regenerative and non-regenerative. The non-regenerative system would combine hydrogen from storage tanks carried on board with oxygen obtained from the atmosphere via a permeable proton-exchange membrane fuel cell unit. The experimental version developed by Helios manufacturer AeroVironment, Inc., employed gaseous hydrogen and was designed to maintain flight for up to two nights. A later version of this system using liquid hydrogen would be intended to maintain nighttime flight for up to two weeks anywhere on the globe.

The second "regenerative" version would add a device called an electrolyzer to the fuel cell system. This system would store excess electrical energy generated by the solar cells to disassociate water molecules. Oxygen and hydrogen gases would then be accumulated in separate tanks under pressure. At night, when the solar cells stop producing electricity, the process would be reversed. The oxygen and hydrogen gases would be fed into a fuel cell unit that produces water and electricity. The electricity is used to power the Helios Prototype and the water is returned to a storage tank until the morning, when the sun comes up and the cycle starts all over again.

If this concept is proven out, flights that last for several months are just around the corner. This is expected to spawn a new generation of aircraft that are being called "atmospheric satellites".

There are a number of potential applications for an "atmospheric satellites". These aircraft may be able to do work such as telecommunications more efficiently and at much lower cost than our current space-based satellites. They could also monitor weather, track hurricanes, and provide coverage of disaster sites such as fires, mud slides, flooding and earthquakes in order to more precisely direct emergency resources.

Project Milestones

Helios Prototype Links