The very first solar-powered flight around the world is underway

Who says there are no world records left to set? On Monday, two Swiss pilots flew from Abu Dhabi to Oman in a solar-powered airplane — the initial leg of an epic effort to complete the very first solar flight around the world.

The ultralight plane, the Solar Impulse 2, features 17,000 solar cells on top of its 236-foot wings, as well as four lithium-polymer batteries that store up energy during the day to power the craft at night. It builds on a previous solar plane, the Solar Impulse 1, which successfully crossed the United States.

The two pilots, Bertrand Piccard and André Borschberg, will take turns flying this new craft as it goes from city to city across the entire globe over the next five months. The plane's top speed is 43 miles per hour, so this will be a slow journey:

(Solar Impulse)

On Monday, the plane successfully completed the first leg of its trip, flying from Abu Dhabi, UAE, to Muscat, Oman. That was the relatively easy part — a mere 248 miles and 10 hours. The real challenge will be on some of the longer legs, like the stretch from Nanjing, China, to Hawaii, a 5,000-mile journey that could mean 120 hours — or five straight days — in the air.

Much like the audacious intercontinental flights in the days before jet engines, the trip will be mentally and physically tasking. As the Guardian reports, the cockpit is the size of a small car, and the pilots will spend about 250 hours apiece crammed in there, one at a time. To rest, they plan to switch off and take 20-minute naps every two to four hours.

As for the technology: The Solar Impulse 2's wingspan is actually wider than a 747's, but the plane is much lighter — a mere 5,000 pounds (about one quarter of that is batteries). And there's a good reason for that. Solar cells don't provide nearly the same energy density as jet fuel, so the craft has to carry far less weight:

Emirati men stand near the Solar Impulse 2, the first solar-powered plane to be able to fly for several days and nights, on January 20, 2015, at an Abu Dhabi airport. (Karim Sahib/AFP)

You can read much more about the plane, and the flight, here.

Will solar-powered airplanes ever be practical?

They almost certainly won't be commonplace anytime soon, though that doesn't mean this stunt is totally frivolous. (The pilots, for their part, say they are planning to raise awareness about clean energy around the globe during their trip.)

For commercial purposes, the Solar Impulse 2 obviously isn't very useful as is. It can only carry two people and travels at top speeds of just 43 miles per hour. By contrast, a Boeing 747-400 can carry more than 400 people and travel at top speeds of 570 miles per hour.

So unless there's some truly radical advance in solar-cell technology, we're unlikely to see solar-powered aircraft that can carry many passengers or heavy cargo in the foreseeable future. At most, solar cells might help provide a bit of electricity to planes, but that's about it.

Aviation is one of the world's fastest-growing sources of greenhouse-gas emissions

That said, the effort to develop cleaner plane technology is very much a hot issue right now. Flying, after all, produces more carbon-dioxide emissions per passenger mile than other forms of mass transport. And while aviation accounts for just 3 percent of humanity’s carbon-dioxide emissions, that’s projected to triple by 2050 at current growth rates. Europe has already begun regulating greenhouse-gas emissions from aviation, and the United States could well follow suit.

For decades, the aviation industry has mostly focused on reducing emissions by boosting fuel efficiency — which has risen 70 percent, industry-wide, since the 1960s. Boeing has said that it introduces newly designed planes every few years that nudge up efficiency 3 to 4 percent, through sleeker, lighter designs and improved engines. But those improvements tend to be slow, and a really big cut in emissions will require something more radical.

In the medium future, the industry is putting its hopes in biofuels — made from things like algae — that could potentially cut emissions in half by mid-century. But even this is a ways off: the first jet biofuels were introduced in 2011, and they're still years away from large-scale commercial deployment.

Further down the road, solar-powered fuel cells seem far more likely to power the clean planes of the future than putting solar panels on the wings (and even fuel-cell technology has a long ways to go).

But even if the Solar Impulse-2 isn't the future of flight, there might be a few things that can be learned from the project. As Borschberg told The Guardian, the plane's engines had to be engineered to be extremely energy-efficient. (More specifically, the four motors on the plane only lose about 3 percent of their energy as waste heat, whereas conventional motors lose about 70 percent.) "These technologies that provide energy efficiency can be used in your home, in your car, in the appliances that you buy," he said.

So, no, we likely won't be riding solar planes anytime soon. But it's an impressive feat all the same — and possibly one that could have a few spillover benefits elsewhere.

Further reading: The BBC has more detail on the two pilots — as well as some video of the flights.

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