Solar plane soaring across U.S.

An airplane entirely powered by the sun landed in Washington on Sunday after a flight from St. Louis, the next-to-last leg of a journey across the United States intended to boost support for clean energy technologies.

The Solar Impulse landed at Dulles International Airport outside Washington at 12:15 a.m. ET, organizers said in a statement. It will remain in the U.S. capital until it takes off for New York in early July for the last leg of its historic trip.

If the spindly experimental aircraft completes the journey as planned, it will be the first solar-powered plane capable of operating day and night to fly across the United States.

"It proves the reliability and potential of clean technologies, and this is crucial in pushing our message forward," Swiss pilot Bertrand Piccard said in the statement. Piccard founded the Solar Impulse project with fellow pilot Andre Borschberg.

With the wingspan of a jumbo jet and the weight of a small car, the Solar Impulse is a test model for a more advanced aircraft the team plans to build to fly around the world in 2015.

The plane completed the first leg of the journey from San Francisco to Phoenix in early May and flew later that month from Phoenix to Dallas.

In early June, the Solar Impulse made the trip from Dallas to St. Louis. It flew to Washington in nearly 30 hours of flying over two days.

The project began in 2003 with a 10-year budget of $112 million US. It has involved engineers from Swiss escalator maker Schindler and research aid from Belgian chemicals group Solvay.

The plane made its first intercontinental flight, from Spain to Morocco, in June 2012.

The aircraft is propelled by energy collected from 12,000 solar cells in its wings that simultaneously recharge batteries for night use.