An aeroplane has begun the first leg of a journey across the United States with no fuel other than the sun's energy.
The plane, dubbed the Solar Impulse, departed shortly after 6am on Friday from a joint civil-military airport at San Francisco Bay, California. It was heading first to Phoenix on a flight expected to take 15 to 20 hours.
The plane made its first intercontinental flight, from Spain to Morocco, last June and its developers hope to eventually pilot it around the world, Reuters reports.
After additional stops in Dallas, St Louis and Washington DC, with pauses at each destination to wait for favourable weather, the flight team hopes to conclude the flight in about two months' time at John F Kennedy International Airport in New York.
Swiss pilots and project co-founders Bertrand Piccard and Andre Borschberg are taking turns flying the plane, which has a single-seat cockpit.
The project, which began in 2003 with a 10-year budget of €90 million, has involved engineers from Swiss escalator maker Schindler and research aid from Belgian chemicals group Solvay. The backers want to test new materials and technologies while also gaining brand recognition.
Project organisers say the flight is also intended to boost worldwide support for the adoption of clean-energy technologies.
With the wingspan of a jumbo jet and the weight of a small car, the Solar Impulse is a test model for a more advanced aircraft the team plans to build to circumnavigate the globe in 2015.