British engineers are preparing to push the limits of aeroplane technology.
By Jonathan Amos
BBC News Online science staff
Zephyr 3, a solar-powered propeller-driven vehicle, is set to fly to 132,000 feet (40 kilometres) in the next few months.
Boxed and set: Zephyr sits in its packing case ready for despatch
Only experimental rocket planes and the space shuttle will have gone higher.
It will rise into the stratosphere to take pictures of a giant helium balloon that will attempt to break the world altitude record for a manned envelope.
But Zephyr - built by QinetiQ, a commercial offshoot of the UK's Ministry of Defence - is more than just a flying camera gantry. The prototype vehicle, and others like it, may soon lead to a cheap alternative to space satellites.
Squadrons of these high-flying solar planes could be stationed permanently in the sky, for use in environmental monitoring or to supply immediate mobile phone coverage in remote areas, perhaps in a disaster zone.
The military, too, believes such platforms will have applications above the battlefield.
ZEPHYR 3 - STRATO-PLANE
The all-up mass is just 14 kg
Its five motors consume 1 kW
It will beam still photos to Earth
Zephyr's chief designer, Chris Kelleher, told BBC News Online: "You can communicate over long distances; at the altitude we are going to, you can see the whole UK, so you can see a very long way.
"You are operating in the sort of region that currently relies on low-Earth orbit satellites but these make perhaps two passes in a 24-hour period, one of which is in the dark.
"These aircraft, if we meet the night-storage of power issues, would operate continuously and indefinitely over a city or battle theatre."
The US space agency (Nasa) has developed a solar-powered plane, Helios, which has already gone to 96,000 feet (29 km). But Zephyr's smaller size will allow it to go higher still.
The British vehicle has a 12-metre wingspan and weighs little more than 12 kilograms. Its Mylar skin covers a carbon composite frame; the solar cells on the top of the wing provide 1 kW of power to five motors.
Its intended operating altitude has presented engineers with unique problems.
The model plane motors have been adapted for use at high altitude
Zephyr will have to cope with extremes of temperature. Sun-facing surfaces will get very hot; shaded regions of the airframe will experience cooling down to about minus 50 Celsius.
A special "space grease" is needed to protect motor bearings. Consideration has also been given to the performance of batteries and electronics to ensure they continue to perform efficiently throughout the flight.
Zephyr is actually quite fragile; it has to be picked up in several places at once or it will snap.
For the ballooning record attempt, Zephyr will be tethered to the envelope's gondola on a 450-metre line. The tether will attach to the end of the wing.
At launch, the plane will be raised on a special beam with its wing in a vertical position. For the first 30,000 feet (9 km), Zephyr will be a passenger under the gondola.
Then its motors will be opened up by one of the balloon's pilots, Colin Prescot, and the vehicle will slowly circle and raise itself into a position to take pictures of the giant balloon.
Big buttons are easier to use in a space suit
A high-resolution camera will feed images direct to Earth; a broadband video camera will send moving pictures via microwave link, first to the gondola and then down to Earth.
"Zephyr is stunning - very ambitious," said Prescot, who will control the plane through most of the balloon journey.
"I have a control box with large buttons to direct the trim and yaw of Zephyr by radio link. The buttons have to be big because my fingers inside the space suit I'll be wearing will be very podgy.
"I have a video screen in front of me to keep track of it and I will be able to see it out of the corner of my visor."
At the peak altitude, Zephyr should be making three circuits every two minutes, travelling at a speed of 70 metres per second (155 mph).
Even if Zephyr reaches the intended altitude it will not actually set any official records. The rules for its particular class of aircraft demand vehicles take off from the ground.
"This flight is about validating the assumptions we have made about operating unmanned aerial vehicles (UAV) at high altitude," Paul Brooks, the QinetiQ manager in charge of the Zephyr project, said.
All testing points to an attempt on the balloon record in the next three months
"For future applications, we don't think you'll need to go so high but we're going there because it will be very demanding and will prove the concept."
Colin Prescot and co-pilot Andy Elson plan to make their balloon journey some time in the next three months. As tall as the Empire State Building, their manned envelope will be the biggest ever flown.
The balloon will be launched from a ship off southwest England and will travel out into the Atlantic, taking perhaps 11 hours to get up and down.
British Airways has agreed to move the flight path of high-flying supersonic Concorde further to the south to take it away from the balloon.