The solar airplane that flew across the United States is back, bigger, better and turned up to 11.

Bertrand Piccard and André Borschberg made history last year when they flew across the country in a giant airplane powered only by the sun. The two traded stints in the cockpit of a sophisticated aircraft with the wingspan of a Boeing 747, four small motors and a cruising speed of just 29 mph.

That airplane, known simply by its Swiss identification number HB-SIA, was an impressive bit of engineering, but merely a prototype for the more sophisticated machine the Solar Impulse team hopes to fly around the world next year.

A journey of that magnitude will require more of everything: More power, more energy and more room for the pilot. To that end, Solar Impulse 2 offers greater speed, reliability and comfort for Piccard and Borschberg, who will take turns squeezing themselves into the unpressurized, unheated cockpit for as long as five days at a stretch.

>Solar Impulse 2 shows what is possible, and is no less impressive than the Dreamliner

"We were facing the unknown,” Borschberg said during the plane’s unveiling this week. “There was no benchmark. We had a feeling that with the first one, we would not be close to what was possible.”

Solar Impulse 2 shows what is possible, and is no less impressive an engineering feat than, say, the Boeing 787 Dreamliner.

The design and engineering teams had three objectives. First and foremost, they needed to ensure utmost reliability. The last thing a pilot wants to experience is engine failure midway across the ocean—a point Borschberg conceded when he joked that he and Piccard “didn’t want to take too much time taking swimming lessons.”

Propulsion comes from four brushless motors generating 17.4 horsepower apiece, a significant upgrade from the previous plane’s 10-horsepower units. The props are bigger, too, and spin faster—up to 525 rpm. At cruising speed, the motors produce 15 horsepower to maximize efficiency.

The wings—which measure 72 meters from tip to tip–are covered with 17,248 monocrystalline silicon photovoltaic cells. A 164 kilowatt-hour lithium-ion battery pack weighing a whopping 2,077 pounds ensures power flows through the night.

The new motors, coupled with a more efficient electrical system and batteries with an energy density of 260 watt-hours per kilogram will allow Solar Impulse 2 to fly continuously for five days and nights at a maximum speed of 77 knots and a flight ceiling of 27,000 feet.

The unusual propulsion system and delicate airframe aside, Solar Impulse 2 shares its basic architecture and components with a typical airplane. The cockpit is slightly ahead of the wing. Two engines hang from each wing, and at the back you’ll find a horizontal and vertical tail surface.

Image: Solar Impulse

Although the plane has a wingspan greater than that of a 747, it weights just 5,000 pounds—about as much as a Mercedes-Benz S-Class sedan. The airframe is of carbon fiber, as are the wings, which feature 140 carbon-fiber ribs spaced 20 inches apart.

As big as the plane is, there’s precious little room for the guy flying it. He’s squeezed into an unpressurized, unheated cockpit with just enough room for food, oxygen and, well, let’s just say rudimentary facilities for when nature calls. The seat is about as big as you’d find in business class, and it serves double duty as a reclining birth and the toilet—though neither Piccard or Borschberg went into detail on how that works.

Like all good pilots, the two men are hoping for the best but preparing for the worst, so the seat also includes a parachute and a life raft.

Although Solar Impulse could, in theory, fly nonstop, the pilots cannot. They’ll make five day stints before landing to hand things over to the next guy. An advanced autopilot system they’re calling “virtual co-pilot” will allow them to sleep while aloft.

The flight is slated to begin next March somewhere in the Middle East, a departure point selected to avoid the monsoon season. They’ll fly east over the Arabian Sea and central Asia to the Pacific Ocean, then across the United States to the Atlantic. From there they’ll cross Europe or north Africa. Test flights begin next month in Switzerland.

The flight will take just 20 days, but the journey will take three months. The team plans to make frequent stops along the way to let people see the plane—and glimpse the future. No one involved in the project believes solar airplanes will one day carry us around the world, but that’s not the point.

The point, they say, is to get people thinking about clean energies, minimizing our impact on the environment and dreaming about a better future.

“Solar Impulse is one example of what we can do when we believe that we can achieve the impossible," Piccard says, "and this brings hope."