Solar airplane begins journey across the continental US

The aircraft departed Moffett Field in Silicon Valley at 5:30 a.m. local time bound for Phoenix, where it is expected to land at around 9 p.m. Solar Impulse 2 had been in California for just over a week after arriving from Hawaii.

The goal of the flight is to highlight and promote clean energy.

The aircraft is powered by solar energy, gathered from thousands of solar panels that blanket the top of the aircraft. They charge up four batteries with enough power to keep the aircraft in the air day or night.

Theoretically, it could stay aloft for months at a time, but the needs of the human pilot mean flights are limited to no more than a couple of days. On the journey from Hawaii to California, pilot Bertrand Piccard flew Solar Impulse 2 for almost three straight days, catching 20-minute naps in the cramped cabin during the flight.

The journey began in Abu Dhabi last summer and has seen the aircraft traverse Asia and across the Pacific to Hawaii. A problem with the battery charging system delayed its departure from Hawaii and, by the time it was fixed, the team decided to wait until the better spring weather to resume the flight.

From Phoenix, the aircraft will fly across the U.S. to New York, making stops at cities on a route yet to be determined. From New York, it will cross the Atlantic to either southern Europe or northern Africa before arriving back in Abu Dhabi sometime in the middle of 2016.