Solar Impulse 2 passed the "point of no return" early Monday, about 16 hours into the solar-powered plane’s audacious attempt to cross the Pacific from Japan.

No immediate landing is now available to pilot Andre Borschberg as he attempts to reach Hawaii, a 4,908-mile journey across a vast expanse of ocean, which is expected to last 120 hours. Solar Impulse 2 took off from Nagoya, Japan at 2:03 p.m. ET Sunday, embarking on the most difficult leg of its round-the-world trip.

“The real moment of truth still lies ahead,” said Borschberg, in a statement. “We are now at the point in the Round-the-World Solar Flight where everything comes together.”

Solar Impulse 2, a larger version of a single-seat prototype that first flew five years ago, is made of carbon fiber and has 17,248 solar cells built into the wing that supply the plane with renewable energy, via four motors. The solar cells recharge four lithium polymer batteries, which provide power for night flying.

The plane has a 236-foot wingspan, larger than that of the Boeing 747, but, thanks to the lightweight carbon fiber, weighs about as much as a car at around 5,070 pounds.

Related: Solar Impulse 2’s epic journey in pictures

The aircraft will climb to the altitude of Mount Everest, almost 29,500 feet, during the day to get more sunlight, recharge the batteries and store more energy. At nighttime, the plane will fly lower, at a minimum of 3,000 feet. Borschberg will experience temperatures ranging from 95 degrees Fahrenheit in the morning to minus 4 degrees Fahrenheit early in the evening while the plane is still high up.

Solar Impulse 2 is expected to fly between 30 mph and 60 mph during its journey.

The flight to Hawaii also presents huge mental and physical challenges to Borschberg. Solar Impulse 2’s cockpit is too small to stand in, although the seat can recline into a horizontal position to allow him to lie down. Borschberg plans to use meditation and breathing techniques to calm down and sleep, and expects to sleep for 20 minutes at a time, up to eight times a day.

Borschberg and his fellow Swiss pilot Bertrand Piccard are taking it in turns to fly the solar-powered aircraft on its five-month journey across the globe. Solar Impulse 2 set off from Abu Dhabi in March, and has stopped in Oman, India, Myanmar and Nanjing, China, en route to its unscheduled stop in Nagoya.

The Solar Impulse originally left Nanjing, China, for Hawaii, but diverted to Japan on June 1 because of unfavorable weather ahead. It had been waiting for the right conditions to depart before Sunday’s takeoff.

Related: Solar Impulse 2 plane takes renewable energy and clean tech to new heights, says pilot

After Hawaii, the second Pacific leg of the journey will take the plane to Phoenix. Solar Impulse 2 is slated to stop in New York before flying over the Atlantic Ocean. It will then stop in southern Europe or North Africa, depending on weather conditions.

Piccard will pilot the plane from Hawaii to Phoenix, a journey of 3,106 miles, which is expected to take between three and four days. “An airplane flying day and night without fuel is more than a spectacular milestone in aviation, it's the living proof that clean technologies and renewable energies can achieve incredible feats; and that all these energy efficient technologies should now be used globally in order to have a cleaner world,” he said in a statement Monday.

Follow James Rogers on Twitter @jamesjrogers