A single-seat plane left the John F Kennedy International Airport on Monday on one of the longest legs of its ambitious round-the-world journey. Here’s the catch, the plane does not have a single drop of fuel.

The sun-powered Solar Impulse 2 left at around 2:30 a.m. EDT on a 90-hour trip described as one of the most difficult legs in the whole journey. Swiss adventurers Bertrand Piccard and Andre Borschberg have been taking turns to fly the plane, reports say.

“Goodbye #NewYork! Now looking forward to the vast expanse of the #Atlantic Ocean and reaching Europe in a few days,” Piccard tweeted as they were setting off.

The flight is expected to land Thursday either in Spain or France depending on weather conditions, spokeswoman for the Solar Impulse 2 team Elizabeth Banta told Reuters.

The plane weighs no heavier than a car but has the same wing span as that of a Boeing 747. It is being flown at a speed of 30 miles per hour but can go double that speed when exposed to full sunlight. The plane runs on solar power during the day, and at night, on battery-stored power.  It has more than 17,000 solar cells built into wings.

The plane, which set out on March 9, 2015, from Abu Dhabi, has travelled across Asia and the Pacific to the U.S. purely on solar power. The Solar Impulse team is part of a campaign to build support for clean energy technologies. The team hopes to complete the circumnavigation in Abu Dhabi. A major chunk of the journey still remains with the plane having to fly across Europe to reach Abu Dhabi.

Borschberg piloted the plane's 4,000-mile flight between Nagoya, Japan, and Hawaii, which broke the record for the longest uninterrupted journey in aviation history. The trans-Pacific flight lasted 118 hours.

A patron of the project, Prince Albert of Monaco gave the flight the green signal from its mission control center in Monaco, telling Piccard “you are released to proceed.”