Two years ago, Solar Impulse's Bertrand Piccard and Andre Borschberg became the first pilots to fly a solar-powered plane across America without burning a drop of fuel. But that was just a warmup for the months-long, 22,000-mile odyssey that's due to begin Monday.

"The real thing, of course, is around the world," Piccard, the venture's co-founder and chairman, told NBC News. "That’s the ultimate flight. You cannot do more than around the world."

The globe-girdling adventure will unfold as a series of flights that hopscotch across continents and oceans, starting and ending in Abu Dhabi in the United Arab Emirates. Borschberg, who is Solar Impulse's chief executive officer and will take the first turn in the cockpit, is looking forward to Monday's sunrise takeoff.

"I've been thinking about this, not every day, but almost," he said. "So now it's the time to go, now it's the time to start."

Borschberg's solo flight to Oman will open the climactic chapter in a 12-year-long, $150 million saga aimed at rewriting the history books for aviation — and for environmentally friendly technologies as well.

The Solar Impulse prototype that took on 2013's two-month-long journey across America was a technological wonder, but the upgraded Solar Impulse 2 is even more so. The plane's wider than a Boeing 747, with a wingspan of 236 feet (72 meters), but weighs only about as much as a car (5,000 pounds, or 2.3 metric tons).

The aircraft draws its energy from more than 17,000 solar cells festooned on its wings, fuselage and tail. The electricity is stored in lithium polymer batteries that weigh 1,400 pounds (633 kilograms) — which accounts for a little more than a quarter of the airplane's weight. Four 17-horsepower electric motors, each packing roughly the power of a small motorbike, push the plane through the air.

The system is efficient enough to keep the plane flying at altitudes as high as 27,000 feet (8,500 meters) all through the night, even though it's totally solar-powered.

Four-month journey

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The trade-off for all that efficiency is that Solar Impulse isn't exactly a speed demon. Its maximum velocity is about 87 miles an hour (140 kilometers per hour), and it typically cruises at an even more sedate speed. Borschberg expects to take 12 hours to complete the first leg of the round-the-world trip, from Abu Dhabi to Muscat in Oman. A commercial airliner takes just an hour to fly the same course.

To make the full, 22,000-mile (35,000-kilometer) circuit, the Solar Impulse team has laid out a four- to five-month schedule that includes stops in India, Myanmar, China, Hawaii and a string of cities on the U.S. mainland, plus a spot in southern Europe or north Africa, and then back to Abu Dhabi. The plane will be on the ground for days or weeks at a time — in part to accommodate maintenance requirements and public events, but mostly to avoid inclement weather such as south Asia's monsoons and America's tornadoes.

Because of the weight constraints, the cockpit can accommodate only one pilot at a time. Piccard and Borschberg will alternate. Borschberg is taking the first leg, and Piccard will pilot the final flight. Piccard will fly across the Pacific, and Borschberg will span the Atlantic.

During those ocean crossings, Solar Impulse 2's pilot will have to stay in the cockpit for up to five days and five nights without stopping.

Using sustainable technology to power the plane turned out to be the easy part. "We had to think about how to make the pilot sustainable," Borschberg said.

Putting the seat down ... or up

For each flight, the cockpit is stocked up with portable meals, water and energy drinks. The seat opens up to provide a toilet, or reclines to provide a couch for catnaps. Piccard and Borschberg have trained themselves in the arts of meditation, yoga and self-hypnosis to stay in the right state of alertness during long, lonely flights in a cramped cockpit.

An onboard computer system will serve as a "virtual co-pilot," with the ability to wake up the human if a problem arises. But what if it's a problem the human can't handle? "If something really goes wrong, you have to bail out," Piccard said. He and Borschberg will be equipped with a parachute and a well-stocked life raft that can see them through several days of being stranded. If it takes longer than that for a rescue, the pilot will have to depend on a follow-up air drop — and his own wits.

So what's the point of all this? In the short term, the point is not to offer a solar-powered alternative for today's commercial air traveler, although Piccard doesn't count that out as a long-term result.

"It’s a little bit like the Wright Brothers Flyer, 110 years ago," he said. "You know, the Wright brothers had no technology to fly with passengers. Nevertheless, after 20 years, the industry developed the system, and there could be passengers in airplanes. So we don’t know. Maybe it will happen like this with Solar Impulse."

For now, the aim is to demonstrate advanced technologies that can be applied to life on the ground. For example, lightweight insulating foam has been built into the plane's structure to keep the batteries (and the pilots) at a comfort temperature level even when the thermometer outside rises above 100 degrees Fahrenheit (40 degrees Celsius) or falls to 40 below zero (-40 C). That same type of foam is being used to build more efficient refrigerators, Piccard said.

Flight of the human spirit

But Solar Impulse isn't just about Swiss industrial efficiency: It's also about the human spirit, Piccard said. He noted that his grandfather was on the first balloon flight into the stratosphere in 1931, and his father was on the first dive to the bottom of the Pacific's Mariana Trench in 1960.

"They gave me the taste for exploration, the taste for pioneering, to try things that haven't been done before," Piccard said.

A psychiatrist by training, Piccard and co-pilot Brian Jones became the first to make a nonstop balloon flight around the world in 1999. "That was fantastic," Piccard recalled. "But you know, I burned almost 4 tons of liquid propane, and for the entire flight I was afraid to be short of gas before the end. This was when I made a promise that the next time I would try a flight around the world, it would be with no fuel at all."

The venture will promote Swiss technology, but also pure pioneering.

"With Solar Impulse, we want to show that we can still achieve big dreams, or at least take the risk of trying," Piccard said. "Maybe we'll fail. But the worst is not to fail. The worst is to be afraid of trying."

Takeoff is set for 6:30 a.m. Monday Abu Dhabi time, which is 10:30 p.m. ET Sunday. Solar Impulse plans to offer real-time coverage of the round-the-world odyssey via its SolarImpulse.com website. On Twitter, follow @SolarImpulse or watch for the round-the-world hashtag #RTW.