By Richard Black
BBC environment correspondent
A US company says it has successfully completed test flights of a potentially environment-friendly aircraft powered by liquid hydrogen.
The plane has 50ft (15m) wings (picture courtesy of AeroVironment)
Liquid hydrogen stored on board and oxygen extracted from the air are combined in fuel cells. The electricity generated drives the propellers.
California-based AeroVironment says a full tank of hydrogen would keep the unmanned plane in the air for 24 hours.
Planes using fuel cells might help curb greenhouse gas emissions from aviation.
The aircraft, called Global Observer, looks more like a glider than a conventionally powered plane, with its wingspan of over 15m, small fuselage slung underneath and extended, "dragon-fly" tail.
Along the front edge of the wing is a line of eight propellers. What is innovative is their power source.
"We're carrying liquid hydrogen on board at very low temperatures," AeroVironment's Director of Washington Operations, Alex Hill, told BBC News. "So the insulation on the tank is crucial."
The precise design of the tank is being kept under wraps, as is the nature of the fuel cells which combine hydrogen and oxygen, the latter extracted from the air as the plane flies.
"These flights were a proving ground, first of all to ensure we could handle the liquid hydrogen - we had lots of tests of that on the ground," said Mr Hill.
"Then we wanted to fly the technology, but we didn't fuel the plane all the way up - if we had done, we could have flown for about 24 hours. But as it is, we had two flights of just over an hour each."
The plane, which looks like a glider, was first flown on 26 May at the US Army's Yuma Proving Grounds in Arizona. A second test flight took place on 2 June.
The company previously fitted a fuel cell propulsion system to its mainly solar-powered plane Helios, which set an altitude record for a non-rocket-powered winged aircraft in 2001 when it climbed to 29.5km (96,863ft).
However, Helios broke apart on a subsequent flight in June 2003 before the fuel cell system could be evaluated.
The company believes aircraft along the lines of Global Observer could be used as telecommunications platforms, replacing or complementing the role of satellites.
Of more general interest is the principle of hydrogen-powered craft to reduce the growing impact of aviation on climate change.
Greenhouse gas emissions from air travel are growing faster than those from any other economic sector.
In 1999, the Intergovernmental Panel on Climate Change (IPCC) - the body which collates and evaluates scientific evidence on climate and distils it for policymakers - calculated that aviation contributed around 3.5% to human-induced global warming.
Passenger travel is now growing by 5% per year, with air freight expanding even faster, at 6% per year. Even with improvements in efficiency, the IPCC believes aviation's global warming contribution will be between 2.6 and 11 times greater by 2050.
New technology which avoids hydrocarbon fuels and their carbon dioxide emissions could be an important way of curbing this trend.
But the advantages of hydrogen power have not been established definitively. Fuel cells produce water vapour, which is itself a greenhouse gas.
Released several kilometres up in the atmosphere, it is not clear how the global warming impact from this technology would compare with emissions from conventional engines.
As the fuel cell aircraft becomes a reality, practical research to answer this question will become more feasible.