Solar Impulse 2 has successfully crossed the Atlantic — the first journey of its kind made by a solar-powered plane. The experimental aircraft set off from New York on Monday and landed this morning in Seville, Spain.

The four-day crossing is one of the most difficult sections in the aircraft's round-the-world flight. The journey started in March 2015 in Abu Dhabi, and last year, the Solar Impulse 2 flew eight stages of its trip — including a record-breaking four-day, 21-hour leg from Japan to Hawaii. However, the craft was forced to wait out the winter on the Pacific island, spending 10 months in a hangar as repairs were carried out and the crew waited for optimum solar conditions.

the weight of a car, the wingspan of a Boeing 747

The Solar Impulse 2 is no heavier than a car, but has a wingspan of 72 meters — exceeding that of a Boeing 747. It's covered in 17,000 photovoltaic cells which power its motors and charge its batteries during the day, continuing to power the craft at night. The Solar Impulse can only carry a single passenger in its unheated, unpressurized cabin, and typically flies at speeds of around 30 mph (that's 18 times slower than a regular airplane). However, the plane could hypothetically fly perpetually, and the time it spends airborne is constrained primarily by the pilot's endurance.

Two pilots — Bertrand Piccard and Andre Borschberg — have managed alternate legs of the aircraft's round-the-world journey. The aim of the Solar Impulse 2, they say, is not to provide a template for future airplanes, but to show the potential of clean, solar energy.

"The Atlantic is the symbolic part of the flight," Piccard told The Guardian this morning, a few hours before landing in Spain. "It is symbolic because all the means of transportation have always tried to cross the Atlantic, the first steamboats, the first aeroplane, the first balloons, the first airships and, today, it is the first solar-powered aeroplane."

"it’s not the future, it’s now."

Piccard said that during his Atlantic crossing he saw whales breaching the water and an iceberg floating south away from the Arctic. "Every minute is a minute of suspense, a minute of challenge, and the fact I can stay [airborne] without fuel or pollution for four days and four nights is something so new," he said. "I have the impression I am in a science fiction story and it’s like I am already in the future. And then I look outside and I say, well it’s not the future, it’s now."

After Seville, the Solar Impulse 2 will fly on to Abu Dhabi, with this last section of its flight split into two or three legs. Greece and Egypt have been highlighted as possible pit-stops, but as with previous legs, this final journey will depend on the weather.