How much do you trust the weather forecast? Is it enough to attempt to fly an unwieldy solar-powered airplane for five days and five nights across the Pacific Ocean, when even a single stray storm could be enough to destroy the craft? There is no alternative airport, so any mishap means ditching the electric airplane in the sea.

Andre Borschberg and Betrand Piccard, the two pilots of the Solar Impulse 2, think forecasts are good enough for Borschberg to take off from Nanjing in China to Honolulu on May 31. Borschberg will take this first leg to Hawaii and then, assuming all goes well, Piccard will take over for the final Pacific crossing from Honolulu to Phoenix in coming weeks.  

"Once in the air, you're stuck," Piccard explains. "I had this vision 16 years ago of an airplane flying around the world without fuel. Now it's the moment of truth to see if this vision is realistic or completely impossible."

The primary challenge will be energy. The airplane has to generate enough electricity from sunlight to recharge the batteries during the day to permit another flight through the long night. The two pilots are energy-challenged as well. They each have to get enough rest—in 20-minute spurts—while the autopilot takes over as many as 12 times on a good night, but perhaps much less. "Sustainability is what matters," Borschberg says. "If I am exhausted on the third day I will not be able to cope. Every day, every cycle I must make sure I get my energy, the airplane gets its energy and so together we go forward."

The point of this flight is not just to prove that that a five-day solar flight over water can be done. It is to raise awareness for the "Future Is Clean" initiative to push clean energy alternatives and energy efficiency, like the solar panels that power Solar Impulse 2 and the efficiency of its electric systems that allows it to achieve powered flight. "The world needs these clean technologies," Piccard says. "It will protect the environment but will also create jobs, make profits and stimulate the growth of industry and economic growth for countries."

Solar Impulse 2 is complicated to fly, given its inability to bank, low power and the lack of a pressurized, climate-controlled or even oxygenated cockpit. The pilots have to wear oxygen masks whenever flying above 3,600 meters. But one thing the solar plane does have in abundance is backup safety systems to go with back-up batteries and even four sets of oxygen masks in case of failure. On the flight from Mandalay in Myanmar to Chongqing in China over the Tibetan Plateau, known colloquially as "the roof of the world," alarms started blaring because the original oxygen mask was improperly assembled. Fortunately, there were those backups.

The only preparation for this epic endurance flight has been previous experiences, like Piccard's past balloon flights around the world and Borschberg's 40-years of flying—as well as 72 hours in a flight simulator. "That was completely different," Borschberg says. After all, the pilots were being watched by at least 40 people during that experiment, and they were on the ground.

The pilots will enjoy a precisely apportioned 2.4-kilograms of food per day—Piccard's favorite meal is a breakfast at dawn of flakes, powdered milk and water plus scenery while Borschberg avoids eating pasta because it tires him out. But the meal plan does not include caffeine or other stimulants, except a small supply in case the landing comes in the middle of a cat-nap cycle. Their comfort is total, according to both. "I feel completely at home," Piccard says. "You tame the cockpit."

The unheated, unpressurized cockpit of just 3.8 cubic meters may be home, but it is a tiny one, akin to living in something a bit larger than a Japanese-style coffin hotel room while alternately freezing and boiling and reaching altitudes of 8,500 meters.

Borschberg's plan to cope is yoga, aided by the cockpit seat that allows a pilot to recline to postures akin to sitting or lying down such as "shoulder bridge," "spinal twist," and "knees to chest." "Yoga [is] to keep the body fit but also to keep the mind alive and in the right mindset," he says. "I can do all the breathing techniques, which is helpful to re-energize, to cool down, to calm down, to get to sleep or to wake up if necessary." He also plans to listen to classical music, citing plans to play Vivaldi during moments of joy. "I will enjoy or appreciate every minute of the flight," he adds. "You don't have this opportunity more than once, once in a lifetime."

There is also the particularly delicate matter of the toilet, which is built into the cockpit's single seat. "Until now I am the first person in the world to have used the toilet in the solar powered airplane," Piccard notes, though Borschberg will no doubt employ it during the estimated 5-day flight over the western Pacific Ocean. "The team is always teasing me with this, yeah, it's a big first." Up until now such waste has been kept onboard but during the Pacific crossing it will be jettisoned over the ocean in biodegradable bags.

Success depends mostly on the weather, which will determine how long the flight takes, or even if it succeeds. "Sure, it's unreliable," Borschberg admits of the all-important weather forecast. "We try to identify a pattern which we know unfolds in such a way." The unofficial motto of the flight team is: "cogito ergo circumvolat," or "we think therefore he flies around."

That places the focus squarely on the reliability of the forecast. The two-man forecast team lead by Luc Trullemans must make a 5-day weather projection using the same data from the U.S. and E.U. employed by your local forecaster. Piccard says he needs a more personal touch. "I need to hear the voice," Piccard admits of even previous, much shorter flights. "I call them on the satellite phone and I say: 'I need to hear your voice and your degree of confidence in what you have written.'"

That's because after the plane passes the west coast of Japan at the end of the first day of this flight, there will be no airports—or even land—between it and its next destination. "You are committed," Borschberg explains. "You need to go to Hawaii. There is no other way or you end up in the water." And with a top speed of 45 kilometers per hour, the plane cannot escape unpredicted atmospheric disturbances.

On the bright side, turbulence is at a minimum over the ocean, thanks to the lack of mountains to fly over—and this airplane was not built to handle turbulence. On the other hand, clouds are a constant concern. "If you cannot recharge your battery then you cannot make it through the night to the next sunrise then you have to ditch and bail out," Piccard notes. The risks of a bailout range from parachute or life raft failure to something else unique to the Solar Impulse 2: electrocution if the pilot fails to get far enough clear of the entirely electric aircraft.

Flying over the ocean also means that sea and sky often appear to merge, and the lack of a sharp horizon forces pilots to navigate solely on instruments to tell up from down. Piccard, who flew over the Arabian Sea from Muscat in Oman to Ahmedabad in India, recalls: "I saw nothing outside. Where is the sky and where is the water?"

And if the flight takes much longer than expected, the pilots only have eight days worth of oxygen on board.

Despite these hardships, flying for days at a time is never boring, unlike spending nearly a month waiting for just the right weather conditions to finally take off. "We have the most beautiful scenery of the world, it's just the Earth below your ankles," Piccard says. "It's a pity we don't have two seats."