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By Alan Boyle

After more than a month of preparation and waiting, the Swiss-built Solar Impulse 2 airplane took off on Saturday from Nanjing, China, for the most grueling leg of its unprecedented round-the-world odyssey: a six-day, nonstop flight across the Pacific Ocean to Hawaii.

Pilot Andre Borschberg, co-founder and CEO of the $150 million project, will eat, sleep and do everything else that needs to be done in the single-seat cockpit during the flight. He's been practicing yoga and meditation to deal with the journey's psychological and physical challenges.

Just before takeoff, Borschberg said he had no doubts about taking on the flight. "I wouldn't leave if I had doubts," he told reporters.

The point of the months-long series of flights is to demonstrate environmentally friendly technologies, ranging from the plane's ultra-light building materials to its fuel-free power source. Companies from Switzerland and other countries are sponsoring the project to highlight their products and services.

Solar Impulse 2 is an upgraded version of a solar-powered plane that flew across America in 2013. It gets all its energy from more than 17,000 solar cells installed on its fuselage, wings and tail — and can fly through the night, thanks to the power stored in more than 1,300 pounds (633 kilograms) of batteries. Its wings are as wide as those of a Boeing 747, but it weighs only about as much as a family car (2.6 tons).

The drawback to all that efficiency is that the plane flies as fast as the family car, not the 747. That poses big challenges for the 5,000-mile (8,172-kilometer) passage across the Pacific. The cargo includes energy bars, drinks and other supplies to sustain Borschberg for six days and nights. The cockpit seat is built to recline into a couch or convert into a toilet. An autopilot system can take control while Borschberg naps.

Just in case something goes terribly wrong, Borschberg is equipped with a parachute, a survival raft stocked with extra supplies and an emergency beacon to alert rescuers in the Pacific.

Related: Solar Impulse Poised for Pacific Crossing

Solar Impulse's team had to wait for a weather report that promised calm weather for the duration of the flight: Dark, stormy skies would not be good for a solar-powered plane. The original plan called for a five-day transit, but mission controllers added an extra day as a cushion.

"It will be one more day of dream," joked Bertrand Piccard, Solar Impulse's chairman and co-founder.

Piccard and Borschberg have been trading places in the pilot's seat during the Solar Impulse trek, which began March 8 in Abu Dhabi. The plane touched down in Oman, India and Myanmar, then flew to the Chinese cities of Chongqing and Nanjing.

After Hawaii, Solar Impulse will make another long flight to Phoenix. Other stops on the 22,000-mile-long (35,000-kilometer) circuit include a spot to be determined in the U.S. Midwest, then New York, then someplace in southern Europe or northern Africa, and finally a return to Abu Dhabi. Closing the circuit would make Solar Impulse 2 the first solar-powered plane to fly around the world.

"It's not just a normal mission of Solar Impulse. ... We're trying to achieve the impossible," Piccard said.