Solar Impulse 2 is one of the most ambitious aircraft ever built, because it’s powered by sunlight alone; there’s not a drop of gasoline spinning its four propeller engines. It’s a sprawling mass of carbon fiber and silicon with a wingspan 11 and a half feet longer than a Boeing 747-8 jumbo jet. It is legitimately enormous, projecting the unwieldy awkwardness of a growth-spurting teenager whose limbs aren’t quite the right size for their body. (It also looks a bit like a UFO when it’s airborne.) Accomplished endurance pilots André Borschberg and Bertrand Piccard are currently flying it around the world in 12 segments in a bid to raise global awareness about renewable energy.

Early next week — assuming the weather holds — a lone man will be drifting across the Pacific Ocean for days on end, floating slowly and deliberately from the eastern edge of China all the way to Hawaii. It’s a journey that will take him across the most featureless, unforgiving expanse of saltwater on Earth.

That’s where this unprecedented flight from Nanjing, China to Hawaii comes into play — a flight that’s expected to last 120 hours, or five full days. If it’s successful, it would shatter the record for the duration of a solo aircraft flight: 67 hours and one minute, set by the late adventurer Steve Fossett in his Virgin Atlantic GlobalFlyer while circumnavigating the globe. The Solar Impulse 2 is much, much slower, which is why they will likely break the duration record over a much shorter distance. "The plane itself flies between 25 and 30 knots, which is quite slow," Piccard says. "And you have to add the speed of the wind. When I went to China, I started to fly backward, which is quite strange in an airplane." To put that in perspective, a Boeing 737 cruises at over 440 knots. "When I went to China, I started to fly backward, which is quite strange in an airplane." The Nanjing-to-Hawaii leg will be piloted by Borschberg, while Piccard monitors the aircraft and encourages him via satellite phone from mission control in Switzerland. "We are supporting each other with our personal experience," says Piccard, who is scheduled to fly the aircraft’s subsequent leg from Hawaii to Arizona. "So when I fly, he can give me some advice, support me, and so on, and that will be the case when I fly to Phoenix." But with or without verbal encouragement, how does a single human being physically pilot an aircraft for that long? It’s not a simple matter of taking off, switching on the autopilot, and reading a book, as might be the case with an advanced airliner. "Sleeping, we don’t exceed 20 minutes at a time because we can’t let the aircraft go longer without really taking care of it," says Borschberg. The plane’s gigantic wingspan and light weight make it far more subject to the whims of the atmosphere than a typical aircraft, and its autopilot system only works in reasonably calm conditions. Piccard notes that it’s not uncommon in turbulence for the pilot to need to take the ailerons, the devices on the wings that allow the aircraft to bank and turn, all the way to their limits until the pilot’s control stick won’t go any further.

Sleeping in 20-minute spurts sounds unpleasant, but there’s precedent for it: polyphasic sleep is the fairly well-established practice of disregarding the body’s natural desire to sleep once per day, and some have managed to keep it up for years on end. Borschberg will augment that meager sleep schedule with a regimen of stretching and yoga poses in his cramped cabin to stay alive, awake, and alert. "In between [naps], we use all the techniques… yoga and meditation, and I’ve adapted these to the conditions of the airplane. So I will use these breathing techniques that you get from yoga to stimulate the mind, to keep the right mindset, to stimulate the body," he tells me. There’s no room to stand up, but the pilots can change positions, sit up, and lay down. "If the night is too long, we don’t have enough energy storage for the entire night." He notes that I’m probably suffering the same fate in my line of work. "If you work eight hours in front of a computer, you are in terrible shape. It’s the same here — if you fly all the time, you end up in terrible shape, so we need to find ways to get rid of these strains that you have on the body." Furthermore, flying the Solar Impulse 2 long distances requires a daily routine that involves climbing to 28,000 feet in the morning to capture sunlight and descending to just 3,000 feet at night to conserve energy. There’s not much margin for error in keeping the batteries charged enough for flight, either. "When you fly east, you have shorter nights. This is important for us, because if the night is too long, we don’t have enough energy storage for the entire night," Piccard says. It’s not just staying comfortable, awake, and fully charged that Borschberg needs to worry about, though. At Solar Impulse 2’s peak altitudes, the human body needs supplemental oxygen to survive, and the aircraft isn’t pressurized like an airliner cabin would be. That means oxygen masks, yet another complexity and inconvenience that the pilots must endure. What’s more, the deep-freeze temperatures at 28,000 feet require layered clothing in the unheated cockpit. (Pressurization and heating are energy-sapping luxuries that a solar aircraft simply can’t afford.)