Prototype plane Solar Impulse, with pilot André Borschberg onboard, flies at sunrise above Payerne's Swiss airbase on July 8, 2010.
For the prototype's grand finale, Borschberg and project partner Bertrand Piccard traded off legs on a five-part journey across the U.S. starting in May 2013.
Since then the team has focused on building the plane's successor, Solar Impulse 2, which has been 10 years and about $150 million in the making.
André Borschberg, left, and Bertrand Piccard are the intrepid pilots behind the Solar Impulse project.
Building on their history-making 2010 flight, they next plan to take an updated version of the plane, shown here, on the first round-the-world solar flight.
Even an eight-hour flight can cause blood clots; five days of sitting motionless could be a killer. Here, Piccard practices modified yoga and pilates exercises during his three-day simulator flight.
After three days in the simulator, Piccard emerges with a grin — ready to do it all for real in 2015.
Solar Impulse 2 test flights are due to take place in May 2014, followed by training flights over Switzerland.
The attempt to make the first round-the-world solar-powered flight is scheduled to start in March 2015 from the Gulf area.
Solar Impulse 2 will fly, in order, over the Arabian Sea, India, Burma, China, the Pacific Ocean, the United States, the Atlantic Ocean and Southern Europe or North Africa, before closing the loop by returning to the departure point.