Sun-powered plane continues U.S. journey

The sun-powered Solar Impulse 2 plane took off from Dayton, Ohio, early Wednesday on its fourth flight across the U.S. mainland as it continues its around-the-world tour.

The aircraft will make its way to Allentown, Pa., where it will land at Lehigh Valley Airport late Wednesday after a journey that's expected to last about 17 hours.

Pilots and Swiss adventurers André Borschberg and Bertrand Piccard set out to circumnavigate the globe last year in the plane without using fuel or spewing polluting emissions.

The two pilots alternate legs of the journey. Piccard is piloting the Dayton to Allentown trip.

The plane has already traveled from San Francisco to Phoenix, Phoenix to Tulsa, and Tulsa to Dayton. New York City is the final U.S. destination.

The first leg began in March 2015 in Abu Dhabi. It continued with several more legs across Asia before Borschberg completed the world's longest non-stop solo flight, a four-day, 21-hour and 52-minute excursion from Japan to Hawaii.

But that record-breaking flight damaged the plane's battery. Because of the time needed to repair it and the decreasing sunlight as a result of the changing seasons, the pilots called off their attempt to complete the trip in a single year.

The plane took off again in late April from Hawaii to San Francisco. After flights across the U.S., it's scheduled to undertake two final legs over the Atlantic Ocean and the Mediterranean Sea before landing back in Abu Dhabi.