Sun shines on historic solar-powered flight

The plane with its 262ft 6in wingspan left Payerne airfield shortly before 7am, the Solar Impulse team said.

Clear blue skies allowed the prototype to soak up plenty of solar energy as it flew over the Jura mountains west of the Swiss Alps. The big question, however, was whether its 12,000 solar cells could fill up its batteries with enough energy so the plane could fly through the night.

The aircraft has 12,000 solar cells in its wings.

The flight was going "extremely well," said team co-founder Bertrand Piccard, a record-breaking balloonist whose father and grandfather were air and submarine pioneers.

"The goal of the project is to have a solar-powered plane flying day and night without fuel," Piccard said. "This flight is crucial for the credibility of the project."

By early afternoon, pilot Andre Borschberg had his oxygen mask on and was cruising at 19,680ft, after trying to dodge low-level turbulence and thermal winds.

"Andre now reached high altitude in a cloudless sky … this situation will last the entire afternoon," according to the team, adding later "batteries are nearly fully loaded. Sun rays now for climbing only."

The plane later attained an altitude of 27,900ft. It's average flying speed was 44 mph.

Piccard said he was confident the carbon-fibre plane would collect enough solar energy to theoretically fly through the night but was not exactly sure how energy efficient the plane would prove.

"We will most probably take the decision to go through the night," he said.

"We'll have to be very careful, because then we have to see if the energy that we have in the batteries will be enough."

The plane was flying in loops in Swiss airspace and within gliding range of Payerne airport so it could land if it runs out of energy, he said.

The solar plane needs a wide, concrete runway because of its wingspan.

Every aspect of the aircraft is monitored by engineers on the ground. Claude Nicollier, a four-times astronaut and head of the craft's test flight programme, said the revolutionary project excited him as much as the space flights in which he had taken part.

All went according to plan, the aircraft would slowly descend to 4,920ft before midnight and stay there until Borschberg attempted a landing. The 57-year old Swiss, a former fighter jet pilot, is wearing a parachute.

Piccard said Wednesday's the test flight - the third major step after its first "flea hop" and an extended flight earlier this year - would demonstrate whether the ultimate plan is feasible: to fly the plane around the world.

Piccard, who achieved the first non-stop circumnavigation of the globe in a balloon, the Breitling Orbiter III in 1999, said if successful, the next step would be a solar Atlantic crossing. That will be done in a second, lighter prototype, because it will involve new challenges and dangers, he said.

Although the goal is to show that emissions-free air travel is possible, the team has said it doesn't see solar technology replacing conventional jet propulsion any time soon. Instead, the project is designed to test and promote new energy-efficient technologies.

Belgian chemicals company Solvay, Swiss watchmaker Omega, part of the Swatch group, and German banking giant Deutsche Bank, are the project's three main sponsors.