The Swiss pilot of a solar plane has embarked on the longest leg of the first attempt to fly around the world without a drop of fuel.

André Borschberg took off from Nanjing, China, at 2:39 a.m. Sunday in the Solar Impulse 2 for a flight across the Pacific Ocean expected to last six days and five nights, or at least 130 hours.

The journey started in March in Abu Dhabi, and the solar plane has made stops in Oman, India, Myanmar and China. The 8,175-kilometer flight to Hawaii from Nanjing is the seventh of 12 legs, and the longest and most dangerous.

Borschberg and another Swiss pilot, Bertrand Piccard, have been taking turns flying the single-seater Swiss plane during a five-month journey to promote renewable energy use.

"This is the moment of truth," Borschberg, 62, said before takeoff.

He said that if successful, the flight to Hawaii will demonstrate the credibility of the vision he and Piccard embraced 16 years ago "to change our mindset regarding the enormous potential of clean technologies and renewable energies."

After Hawaii, the plan is for Piccard to pilot the aircraft on to Phoenix, Ariz.