The Solar Impulse 2 took off from an airport in Nagoya early Monday, and is in the midst of a 120-hour flight to Hawaii over 8,340 kilometres.
After 16 hours of flight, Solar Impulse 2 "passed the non-return point [and] is entering the night," a tweet from the project's account reads.
Pilot André Borschberg is manning the one-pilot plane for this leg of the journey, which will last five days and five nights.
"It is a feat never accomplished before in the world of aviation," according to the project website.
Borschberg is equipped with 25 litres of water and 18 rations of food. Organizers estimated the pilot would need 2.4 kilograms of food, 2.5 litres of water and one litre of a sports drink during each day of the solar flight.
The plane is powered by more than 17,000 solar cells on its wings that recharge its batteries, enabling it to fly. During the night, when the plane's solar cells no longer collect energy and the plane flies on battery power alone, the plane will fly at 2,438 metres instead of its daytime level of 8,534 metres.
The sun is expected to rise and start powering the plane again around 4:30 p.m. ET.
The plane requires the right weather conditions. Organizers withheld officially announcing the takeoff until Borschberg passed the point of no return.
The plane originally left Nanjing, China, for Hawaii. However, it was diverted to Japan in early June because of unfavourable weather conditions ahead. It has been waiting for the right conditions to depart.
The Solar Impulse 2 has already completed seven legs, spanning from Abu Dhabi to Nagoya and covering nearly 9,000 kilometres.
After landing in Hawaii, the plane will fly through the U.S. and Europe before arriving back in Abu Dhabi.
The project is meant to demonstrate the potential of improved energy efficiency and clean power, though solar-powered air travel is not yet commercially practical.