An airplane has flown the expanse of U.S. airspace powered by photons alone. The coast-to-coast journey was but a warm-up for an attempt at circumnavigating the globe.

The transcontinental flight that didn’t use a drop of fuel took 105 hours and 41 minutes to cover 5,650 kilometers—and the next one will be nearly 10 times longer. The Solar Impulse HB-SIA completed its trip across North America last week, enduring bad weather and landing early due to a tear in the wing. But that's nothing compared with the challenge of completing the transglobal electric airplane marathon scheduled for 2015. "The mission was harder than foreseen, especially because of difficult weather conditions," says co-pilot André Borschberg, CEO of the Solar Impulse team, which completed the cross-country flight with five layovers. "We had to adapt very quickly and change flight strategy in a very short time."

The fabric underneath the left wing tore during the flight between Washington, D.C., and New York City on July 6 for reasons unknown, and strong headwinds made the landing in Dallas–Forth Worth on May 23 particularly challenging. (It also prompted multiple 911 calls about a possible UFO in the area.) And the aircraft, even when identified, is a strange flying object, boasting the wingspan of a 747 but weighing less than a compact car. Yet, the solar craft—and its pilots—performed flawlessly, even when damaged with a 2.5-meter-long tear. "It demonstrates that the structure is strong and totally reliable because it held on despite the wind flow into the wing," Borschberg argues.

The HB-SIA crossed the country on sunshine using the electricity provided by 12,000 solar cells, soaring to more than 8,000 meters and then slowly coasting down at 15 meters per minute after sunset as low as 1,500 meters on its 90-kilowatt-hour lithium ion battery pack before resuming a climb when the sun rose anew in the morning. This superefficient aircraft travels slower than a moped, averaging just 50 kilometers per hour on most trips, with its four electrical engines capable of putting out the maximum 10-horsepower each, solely for takeoff and landing. The longest single flight—from Dallas–Forth Worth to Saint Louis—took 21 hours and 21 minutes to complete.

And the solar airplane doesn't exactly have the lift capacity to start ferrying passengers. "Today we don't see how we could put 200 passengers in a solar airplane," says Bertrand Piccard, the first man to fly a balloon around the world nonstop as well as chairman and co-pilot of this effort. "This plane is more about demonstrating the incredible duration that is possible, rather than speed to destination."

One key improvement to allow for longer-distance flight will be a form of computer monitoring, so that the human pilot will be able to rest during the at least five-day and five-night Pacific Ocean overflight while the autopilot keeps the airplane stable. An "electrical box," in the words of Borschberg, will also supervise the state of the airplane and will only call the pilot's attention to specific challenges, in part via a new system of human–machine interface conveyed by vibrations on the left or right side when the bank angle exceeds more than five degrees. "At the beginning we will not know this new airplane very well," he says. "The first flight through the night will be full of suspense."

The pilots have already spent at least three days and nights in a flight simulator, in training. Borschberg notes that the key adaptation was changing his mental focus from what he wanted to achieve to what he was actually experiencing: "I was almost a little bit disappointed it was already over," he recalls of the training attempt. And that's nothing compared with the serene state induced by the slowly passing scenery from the hush of the cockpit disturbed only by the whistle of the air passing over it. "It is never boring to fly in the most incredible airplane existing, an airplane that burns no fuel and can fly day and night," Piccard wrote during a Reddit Ask-Me-Anything session on May 31, perhaps because the airplane's pitch, roll and yaw can all change simultaneously making it extremely challenging to fly.

The Solar Impulse team will concentrate now on improving energy efficiency even further in the next version, dubbed HB-SIB, allowing for more reserve power as well as working to improve reliability to enable the around-the-world attempt. Currently, humid conditions can ground HB-SIA because the airplane's electrical circuits are not completely waterproof and it cannot handle strong winds.

Critical to the success of these 21st-century would-be Phileas Foggs will be toilets integrated into the pilot's seat of the new aircraft. For this flight, the pilots made it through on bottles and will power. As Borschberg says: "What can be interesting for commercial and military aviation is how we will manage the long-duration flight in terms of sleep, rest, alertness and fitness of the pilot."