A solar-powered plane attempting to circumnavigate the globe without fuel took off from Japan for Hawaii early Monday after an unscheduled, monthlong stop.

The Solar Impulse 2 departed at about 3 a.m. from an airport in Nagoya in central Japan. The plane, with one pilot, is attempting a 120-hour flight to Hawaii, the longest leg of its journey so far.

The plane requires the right weather conditions, and organizers were withholding an official announcement of the takeoff until they were sure the flight could continue.

"A formal communique will be issued once Solar Impulse has passed the Point of No Return and that we know that the Pilot Andre Borschberg is on a safe track to reach Hawaii," the press team said in an email to journalists.

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.

The flight over the Pacific to Hawaii is risky because there is no place to land in an emergency.

The Solar Impulse 2 is powered by more than 17,000 solar cells on its wings that recharge its batteries, enabling it to fly.

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.