The first plane that can fly day and night powered only by the sun is set to begin a transcontinental journey that will reach Washington by mid-June.

Solar Impulse, scheduled to lift off from this World War II-era airfield on Friday, has room for only one person and an average cruising speed of about 43 miles per hour. But its Swiss developers say the technology suggests the possibilities of clean-energy flight.

The plane has an ultra-light, carbon-fiber frame that allows it to weigh 3,500 pounds — about the same as a mid-size car. It has the wingspan of a 747 and a slender fuselage, giving it the look of a giant, high-tech dragonfly.

The plane’s power is drawn from the sun by 12,000 photovoltaic cells that form the top of its wings. The juice is collected in a series of batteries arrayed behind the craft’s four electric engines. It routinely reaches altitudes of up to 28,000 feet, about a mile below the thin air traversed by big commercial airliners zipping around at close to 500 miles per hour. On-board instruments alert the pilot if the plane banks even a degree too far.

For all of its innovations, at this stage of development, Solar Impulse is no more practical for commercial flight than was the single-engine Spirit of St. Louis that Charles Lindbergh piloted across the Atlantic Ocean in 1927.