The all-electric Solar Impulse 2 plane left America’s shores tonight and began what’s expected to be a 90-hour trip across the Atlantic Ocean to Spain.

This 3,600-mile leg of the solar-powered, round-the-world flight ranks as the longest single stretch since last summer’s Japan-to-Hawaii trip. During that earlier flight, Solar Impulse’s batteries overheated – forcing a months-long delay to make repairs and wait for the return of temperate weather.

The Swiss-led team says it has upgraded the batteries and added a cooling system to guard against a repeat. Nevertheless, this week’s over-ocean trip is likely to pose the biggest challenge left for the 15-month odyssey.

The fuel-free plane took off just after 2:30 a.m. ET Monday (11:30 p.m. PT Sunday) from New York’s John F. Kennedy International Airport, with Solar Impulse co-founder Bertrand Piccard in the cockpit. His destination is Seville, which is near Spain’s Atlantic coast and the Strait of Gibraltar.

Just before beginning the flight, Piccard paid tribute to Solar Impulse’s other co-founder, Andre Borschberg. “Thanks to our partnership together, your dedication with your engineers, I have the first solar airplane now to cross the Atlantic. Can you imagine?” he said.

Takeoff came only a week and a day after Solar Impulse arrived in New York, the last stop on the plane’s route across America.

https://www.youtube.com/watch?v=fIqxWhB_OvI

Piccard, a Swiss psychiatrist/adventurer, has been alternating the piloting duties with Borschberg. In advance of tonight’s takeoff, Borschberg noted that Monday marked the summer solstice. Every day from now on will provide less sunlight to keep Solar Impulse’s batteries charged. That’s why the team went for the first opportunity presented by weather forecasters.

“Waiting could be an option – but of course, time is against us from today on,” Borschberg said.

Although Solar Impulse has been an aviation adventure, there’s a bigger purpose behind the $150 million, sponsor-funded effort. The plane is designed to demonstrate environmentally friendly, energy-efficient technologies ranging from ultra-light composite materials to the more than 17,000 solar cells covering the plane.

Solar Impulse’s 236-foot wingspan that’s wider than that of a Boeing 747 jet, but it weighs only as much as minivan (5,000 pounds). It can fly around the clock, thanks to 800 pounds of batteries.

However, the plane can fly only as fast as a minivan, with a cruising speed of about 40 mph. And it can accommodate only one person in a cramped cockpit.

That’s why Piccard has to eat, sleep and do everything else that needs to be done over the next few days in his seat – which can be reclined to serve as a couch, or opened up to provide a toilet. Solar Impulse’s autopilot is programmed to keep the plane on track while Piccard takes a break.

Solar Impulse began its 22,000-mile odyssey in March 2015 in Abu Dhabi. Stopovers were made in Oman, India, Myanmar, China, Japan, Hawaii, California, Arizona, Oklahoma, Ohio, Pennsylvania and New York. From Seville, the plane is expected to head eastward and finish the round-the-world journey in Abu Dhabi by the end of July.