The Solar Impulse 2 will take off from Payerne airfield in Switzerland.

It is a larger, upgraded version of the aircraft that flew across America last year with adventurers Bertrand Piccard and Andre Borschberg at the controls.

The two men want to push the boundaries for alternative energy and believe their plane can be a standard bearer.

The SI2 has completed a series of ground and taxi tests in recent weeks that saw it almost reach take-off speed on the runway.

Project leaders are now confident all systems are ready to go airborne, and the Swiss Federal Office of Civil Aviation is satisfied the vehicle is airworthy.

Assuming the weather is compliant, the plane should take to the skies above Switzerland at about 07:30 Central European Summer Time (05:30 GMT; 06:30 BST).

The carbon-fibre aircraft has a huge wingspan, which at 72m is wider than a Boeing 747 jet. And yet, the vehicle weighs only 2.3 tonnes.

The tops of the wings are covered by 17,000 solar cells, which drive four brushless electric motors at speeds of up to 140km/h (90mph).

During the day, the solar cells will recharge lithium batteries, which can then be used to keep the SI2’s propellers turning through the night.

The first Solar Impulse plane set a number of world records, including the longest manned solar-powered flight at 26 hours, the first inter-continental flight in a solar-powered plane, and the greatest distance covered on a piloted solar-powered flight. (Autonomous solar-powered drones can stay aloft for weeks).

That last record was set during Piccard’s and Borschberg’s epic TransAmerica journey in May, June and July last year.

But as challenging as that effort was, it will be dwarfed by the difficulty and complexity of completing a global flight.

This is because it will have to include passage across the Atlantic and Pacific Oceans. The latter could take all of five days and nights to complete.

Only one pilot can fit in the cockpit. It has a reclining seat to make room for exercising and to permit Piccard and Borschberg, whoever is at the controls, to take short catnaps.

BBC