Solar Impulse Plane Begins First Leg of US Flight Path The solar-powered plane took off in darkness this morning bound for Phoenix.

 -- The Solar Impulse plane took off in darkness from San Francisco this morning for the first leg of its journey across the United States.

The solar-powered plane is expected to finish the 720-mile journey tonight when it lands at Phoenix Goodyear Airport. While the early-morning darkness may seem less than ideal for a plane that is powered by the sun, Solar Impulse is equipped to fly day and night because of solar-energy stored in batteries on the aircraft.

The plane weighs over 5,000 pounds; about the same size of a truck. Solar Impulse arrived in San Francisco last week, marking the completion of its Pacific Ocean crossing.

The trip began in March 2015 from Abu Dhabi, with stops in Oman, Myanmar, China and Japan, before flying over the Pacific Ocean and reaching Hawaii in July 2015.

Solar Impulse was grounded in Hawaii after the plane's battery system sustained damage during the Japan-to-Hawaii leg of the trip. After at least two planned stops in the United States, Solar Impulse will fly from New York to Europe, according to the project's website. The final leg, which will be from either Europe or Northern Africa to Abu Dhabi, is expected to take 120 hours and be completed this summer.

Pilots Andre Borschberg and Bertrand Piccard began the pioneering project with the goal of highlighting clean energy.

"Maybe it will be boring in 20 years when all the airplanes will be electric and people will say, 'Oh, it's routine.' But now, today, an airplane that is electric, with electric engines, that produces its own energy with the sun, it can never be boring," Piccard told The Associated Press.