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Swiss pilot Andre Borschberg has begun his bid to cross the Pacific, from China to Hawaii, in the zero-fuel Solar Impulse aeroplane.
The experimental aircraft, which has a wingspan bigger than a jumbo but weighs little more than a large car, left Nanjing at 02:39 (18:39 GMT).
It is likely to take Mr Borschberg five to six days of continuous flight to reach his central Pacific destination.
He will try to stay awake for much of that time, taking only short catnaps.
His progress will be monitored the entire way from a control room in Monaco.
Meteorologists and flight strategists will constantly update him on the best route to follow.
If, early on in the flight, the weather turns bad or he encounters a major technical problem, Mr Borschberg can always choose to turn around and head back to China or Japan.
But there will come a point where that option is denied to him, and Mr Borschberg and his support team have had to prepare for the possibility of ditching in the Pacific if something goes seriously wrong.
The pilot himself would not go down with the plane because of the risk of electrocution once in the water. Instead, he would bail out with a dinghy and wait for a ship to come and pick him up.
If he succeeds in reaching Kalaeloa airport, he will set several aviation records - not least the longest-duration journey for a single-seater plane.
The purpose of the Solar Impulse project is not really to showcase a particular kind of future for aviation, but rather to demonstrate the potential of clean technologies more generally.