NANJING, CHINA — (CNN) The Solar Impulse 2, the experimental plane that pilots are trying to fly around the world without using a drop of fuel, had been set to take off from China early Tuesday morning on the most challenging and dangerous part of its journey.

But late Monday, the flight was canceled because of deteriorating weather over the Pacific Ocean.

The pilots and their team had even given the leg a name: “The moment of truth.” Andre Borschberg, a Swiss engineer and former fighter pilot, was going to be alone in the cockpit during the nonstop flight from Nanjing to Hawaii.

“I am feeling a bit high, actually, in the sense that we have been working hard to find a window for many weeks,” Borschberg told CNN by phone from Nanjing on Monday, before the flight was aborted.

“I knew it would take time … but it wasn’t easy to wait.”

But the moment of truth came in the form of a weather forecast, extending the team’s stay in China even longer and making the wait more agonizing.

It is not the first time weather has delayed the trip. The plane has been in China since the end of March after a planned overnight pit stop in Chongqing extended into three weeks, and then in Nanjing since April 21 as poor weather has repeatedly delayed the flight.

The project team had forecast that the flight, across the Pacific Ocean to Hawaii, would take up to six days and six nights because of weather along the route. But the new weather forecast raised the possibility that the flight would be extended to a grueling seven days, and that was one of the reasons for the decision to delay the attempt.

Even the six-day estimate was up from the 120 hours the team had initially projected it would take to fly the 8,000 kilometers (4,971 miles) — by far the longest leg of the trip.

Borschberg and his business partner, Bertrand Piccard, a Swiss psychiatrist and explorer, have been taking turns flying the plane since departing Abu Dhabi, United Arab Emirates, in March.

There is much at stake.

Their mission, which the pilots say is aimed at proving the power of renewable energy, and inspiring innovation, has been 12 years in the making.

Both pilots have made it clear that there are no guarantees.

When CNN caught up with Piccard in China in April, he was blunt about the challenges of flying solo for days over the open ocean.

“Maybe it will fail. Andre and I are very clear with ourselves, that maybe we’ll bail out,” Piccard said.

Both pilots have been trained extensively on how to ditch the plane, to remove their parachutes in the ocean and to inflate a lifeboat that is attached to their flight suits.

Their support team of more than 150 people has also prepared for everything from a communications problem to the worst-case scenario.

“On one hand, you have a tremendous amount of excitement, but there’s a certain sense of nervousness,” said Gregory Blatt, the team’s managing director.

“What we’re doing, no one else has done it before. It’s the first ever.”

The flight will also present tremendous physical and mental challenges for the pilot.

The cockpit isn’t pressurized, which means Borschberg will have to wear an oxygen mask when the plane flies at higher altitudes — about half of the six-day journey.

At night, if the weather permits, Borschberg will be able activate the autopilot and nap, but only in 20-minute increments.

He will also practice yoga and use specialized breathing techniques in order to relax.

Once the Solar Impulse reaches Hawaii, Piccard will take his turn at the controls for the flight to Phoenix.

Their round-the-world journey, covering some 35,000 kilometers (21,750 miles), is expected to take at least five months to complete.

Blatt did not say when the next window for the plane to take off might appear.

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