Solar Impulse Pilot Shares Most Sublime Moment of Journey So Far

— -- Solar Impulse, the lightweight solar-powered airplane being flown by two pilots on a journey around the world, hit a milestone this afternoon after successfully crossing the United States.

Swiss adventurers Bertrand Piccard and Andre Borschberg take turns flying the one-seater airplane, which is solely powered by the sun's energy. Piccard spoke to ABC News from the cockpit today as he made the 400-mile journey from Dayton, Ohio, to Lehigh Valley, Pennsylvania, where he said he expects to land around 9 p.m. ET.

"When I heard the air traffic controller from New York, I thought, 'Wow, we have crossed the United States. We are arriving now on the East Coast, coming from Hawaii through San Francisco, Phoenix, Tulsa, Dayton,'" he told ABC News. "That is so great. We are very happy."

The final American leg will include Borschberg flying from Pennsylvania and past the Statue of Liberty before landing in New York ahead of a grueling transatlantic flight. Compared to what is ahead, Piccard said today's flight "is not very technical because it is short."

"I took off in the night at 4 in the morning to avoid the bad weather coming through Dayton," he said. "I will arrive a little bit too early over Lehigh Valley and I will start now holding. That means to wait for several hours until the wind decreases and then I will land after 9 p.m. tonight."

Solar Impulse is able to fly day and night because of the solar energy is stored in batteries on the aircraft. The duo expect to complete their around-the-world journey this summer in Abu Dhabi. While the project is designed to raise awareness about clean energy, it's also provided some memorable personal moments.

"As the promoter of the message of Solar Impulse, it was the most fantastic moment for me when I could speak live to the General Assembly of the United Nations from the cockpit of Solar Impulse and speak about clean technologies. That was fantastic for me," Piccard said.

As an explorer, Piccard said his most memorable moment thus far was when he was crossing the Pacific Ocean between Hawaii and San Francisco.

"I was in the middle of the ocean, in the middle of the night, alone in the plane and I was just really happy because this is the world I love," he said. "This is the world of exploration. You get out of your comfort zone, you explore the unknown. You are pushing the limits and you discover what you have inside of yourself."