The Swiss pilot of a solar plane embarked on the longest leg of the first attempt to fly around the world without a drop of fuel.
Andre Borschberg took off from Nanjing, China, at 2:39am local time 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 stopped in Oman, India, Burma and China.
The 5,079 mile flight from Nanjing to Hawaii is the seventh of 12 flights and the longest and most dangerous.
Mr 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," Mr Borschberg, 62, said before take-off.
He said that if successful, the flight to Hawaii will demonstrate the credibility of the vision he and Mr 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 Mr Piccard to fly the aircraft on to Phoenix, Arizona.