An experimental airplane powered solely by energy from the sun departed from Tulsa, Oklahoma early Saturday, 21 May, on the 12th leg of a historic bid by its pilots and developers to fly around the globe without using a drop of fuel. The single-seat Solar Impulse 2 aircraft took off from Tulsa International Airport in Oklahoma at 4:22am local time (9.22am GMT) en route to Dayton, Ohio.

The long hours required for covering relatively short distances, like its previous 18-hour flight from Phoenix, shows how slowly the plane flies compared with conventional aircraft.

With a wingspan exceeding that of a Boeing 747 but an ultra-light carbon fibre skin and overall weight of a car, the Solar Impulse cruises at speeds ranging from 34-62 miles per hour (55-100 km/h).

The four engines of the propeller-driven aircraft are powered exclusively by energy collected from more than 17,000 solar cells built into its wings. Excess energy is stored in four batteries during daylight hours to keep the plane flying after dark.

The plane can climb to 28,000 ft (8,500 m) but generally flies at lower altitudes at night to conserve energy.

Piccard and Andre Borschberg have been taking turns piloting the plane on each leg of the journey. Both men have trained to stay alert for long stretches of time by practising meditation and hypnosis.

The Swiss team's ultimate goal is to achieve the first round-the-world solar-powered flight, part of its campaign to bolster support for clean-energy technologies.