By Jo Twist
BBC News science and technology reporter
Unmanned surveillance vehicles are increasingly evident in a world that relies on knowing what people and places are doing.
Unmanned Aerial Vehicles (UAVs) patrol innocuous-looking skies and silently report back streams of strategically important data, video, and images from locations around the world.
They are the ultimate Earth watchers.
It is believed that up to 800 remotely piloted aircraft are in operation in Iraq and Afghanistan.
But observing Earth from afar is not just about battlefields and spy missions, the type usually done by expensive and heavy craft like the US's Predator drone. UAVs are increasingly being recruited to carry out more humanitarian missions, from the stratosphere.
Called high-altitude long-endurance (Hale) craft, they can be crucial information-gatherers for disaster management, crop management, coastal management, and mapping; they can be more flexible and cheaper options than low-orbit satellites, particularly for developing nations.
But one of the biggest issues for such craft is finding the power to fly continuously at altitudes of 18 to 20km, for the very long periods of time often required for such missions.
Once this barrier is overcome, they can rightfully be called "eternal planes". It is hoped that a new Hale UAV vehicle, built by former UK defence research lab QinetiQ, will push the boundaries for eternal planes a bit further.
It is called Mercator. It has a wingspan of 16m and a light weight of 27kg. Together with its hi-tech ground station, it makes up the equally grand-sounding project known as Pegasus.
"No one has produced an eternal airplane yet," Andrew Rogoyski, head of QinetiQ's space division explained to the BBC News website.
The Nasa-backed Helios craft crashed in 2003
"The US has spent lots of money on the Helios platform which is aimed at demo-ing eternal flight." QinetiQ hope to bring its space technology expertise to Mercator.
In 2001, the Nasa-backed Helios reached a record altitude for a non-rocket-powered winged aircraft, climbing to 96,863ft (29.5km), but it crashed in 2003 on a flight from the US Navy's Pacific Missile Range Facility in Hawaii.
Earth observation is crucial for environmental monitoring and management. Usually, it is done by either airborne or space-borne platforms.
Satellites do a splendid job in most circumstances of keeping an eye on what is happening on Earth. But they are expensive to run and not as flexible as remotely controlled aircraft.
A solar powered Hale UAV could also be a viable option for nations that perhaps aspire to a space programme, but are economically restricted, thinks Paul Davey, QinetiQ's Mercator project manager.
In 2000, the Belgian Flemish Institute for Technological Research (Vito) set up the Pegasus project.
Pegasus (Policy support for European Governments by Acquisition of information from Satellite and UAV-borne Sensors) was designed to spearhead Europe's development of solar-powered UAVs that can stay airborne for long periods.
"One of these craft could get anywhere in the world in 24 hours," explains Dr Rogoyski.
Mercator: The craft has a wingspan of 16m
"A UAV will be able to keep up over a particular part of the world for months at a time, whereas satellites have to continue travelling in orbit."
Precise, high-quality images and data can be sent back in under 30 minutes to a mobile ground station which can then disseminate images online.
But for a solar eternal plane to be exactly that it needs an efficient energy harvesting, conversion and storage system that will work when the Sun has set.
"We use a lightweight solar array coupled to lithium-based rechargeable batteries. They are incorporated into the carbon fibre airframe which gives us a low-mass structure capable of flying at high altitude," says Dr Davey.
The challenge is to make the craft as light as possible, carrying payloads of 2kg, while making it robust enough to withstand the extreme conditions it has to navigate. Temperatures at the altitudes UAVs fly drop to 50 degrees below zero.
Based on computer modelling, the craft QinetiQ has developed thus far could complete a day-night cycle. It would use solar power during the day, while recharging the batteries, which would then provide enough "juice" to fly at night.
It seems straightforward, but it is only the improved efficiencies in solar cell and battery technologies, even over the last year, that has made this at all possible.
"The key technologies are solar cells and batteries. Battery technology is receiving billions worldwide and it is a very rapidly evolving technology," says Dr Rogoyski.
The light weight is achieved by using paper-thin solar panels
The solar panels on the craft are paper thin and deformable so they hug the surface of the plane itself, cutting out excess material.
Initial tests on a 40% scale model of the QinetiQ Mercator UAV have proved successful.
"The technology we have right now will enable us to stay up in the air for 60 hours, but we will not be trying that tomorrow," says Dr Davey. "Trials are planned for next year."
By then, Dr Davey expects improvements in battery efficiency of about 20%.