An experimental plane trying to fly around the world on solar energy was about a third of the way between Hawaii and California Friday on its quiet quest to make history.

Solar Impulse 2 pilot Bertrand Piccard, 58, who is spending 62 hours by himself in the tiny cockpit, did the closest thing he could to stretching his legs -- after spending a day and a night sitting in that cramped space.

To wit, he raised and lowered his arms to do exercise, according to a photo on the mission web site.

It shows him wearing sun glasses and a long sleeved orange shirt as he sat at the controls.

"It is high time for Bertrand to make some physical exercises to fight muscle atrophy, not easy in the 3.8 m3 cockpit, but something he has trained for intensely over the past years," the photo caption read. The cockpit measures 134 cubic feet.

The plane's wingspan is wider than that of a jumbo jet but its weight is roughly the same as a car's, thanks to its light construction. The aircraft is powered by 17,000 solar cells.

The plane is due to land in Mountain View, not far from San Francisco in the heart of California's Silicon Valley, on Saturday evening. It took off from Hawaii Thursday.

This is the ninth of 13 legs in a journey that began last year in the United Arab Emirates.

The goal of the flight is to promote the use of renewable energy.

The SI2 was grounded in July last year when its batteries suffered problems halfway through its 21,700-mile (35,000-kilometer) circumnavigation.

The crew took several months to repair the damage from high tropical temperatures during the flight's final Pacific stage, a 4,000-mile flight between Japan and Hawaii.

The aircraft was flown on that stage by Piccard's teammate Andre Borschberg, whose 118-hour journey smashed the previous record of 76 hours and 45 minutes set by US adventurer Steve Fossett in 2006.

The plane is expected to cross the United States, stopping in New York before a trans-Atlantic flight to Europe, from where the pilots plan to make their way back to the point of departure in Abu Dhabi.