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Sci-Tech

Solar-powered plane embarks on longest leg of round-the-world flight

The Solar Impulse 2, which is entirely powered by solar energy, is attempting to fly from Nagoya, Japan, to Hawaii. The flight will take approximately five days.

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Yasmine, Andre Borschberg's wife, says goodbye as he takes flight for Hawaii. Solar Impulse

Somewhere over the Pacific Ocean, a pilot is flying a plane to Hawaii relying solely on a single energy source: the sun.

Solar Impulse, a company that aims to legitimize the possibility of zero-fuel airplanes in the future, has hit the air on the eighth leg of a round-the-world trip on its solar-powered airplane. Andre Borschberg, the pilot and co-founder of Solar Impulse, took off in the single-seater aircraft on Sunday with Hawaii in his sights. The non-stop flight will take approximately five days to complete.

While there are no plans to bring a solar-powered airplane to the passenger industry anytime soon, the Solar Impulse 2, which has successfully completed seven missions and 24 percent of its distance goal, presents a possible alternative to gas-guzzling airplanes. Environmentalists have long panned airplane manufacturers for their products' heavy use of fuel, and have called on agencies around the world to seek new ways to power aircraft. A Boeing 747, for instance, burns approximately 1 gallon of fuel per second, or five gallons of fuel per mile.

Still, some have said that the real assault on green technology isn't in airplanes, but rather in cars. A fully packed Boeing 747, which can carry 568 people on a single flight, can travel 91 miles per gallon. The average passenger car offers a miles-per-gallon rating in the mid-30s, according to the US Department of Transportation, in 2013, the last year it provided figures.

Such arguments, however, have done little to quell unrest in the environmental-protection space, where critics say greater reliance on renewable energy and not fossil fuels, is critical. In a statement on Monday, Solar Impulse Chairman Bertrand Piccard said his company's airplane is "living proof that clean technologies and renewable energies can achieve incredible feats; and that all these energy efficient technologies should now be used globally in order to have a cleaner world."

The flight from Japan to Hawaii is the longest in the series of flights planned to take the Solar Impulse 2 around the world. The danger factor is high: there are no immediate landing zones if the plane runs into trouble. A future crossing over the Atlantic Ocean will provide similarly harrowing circumstances for the pilot.

The Solar Impulse 2 aircraft took off for its first flight in the 20,000-mile circumnavigation in March, successfully flying from Abu Dhabi in the United Arab Emirates to Muscat, Oman. The company plans to make 12 flights to successfully circumnavigate the globe, with the next flight heading from Hawaii to Phoenix.

The Solar Impulse 2 is completely powered by the sun and made of carbon fiber. At 236 feet (72 meters), the aircraft's wingspan is broader than that of the Boeing 747, but slightly smaller that the world's largest passenger plane, the Airbus A380.

Despite its size, the Solar Impulse 2 weighs less than 5,070 pounds (2,300 kilograms), or about the same as a pickup truck, and carries just one passenger: the pilot.

In order to take flight, the Solar Impulse 2 is powered by 17,248 solar cells that charge four lithium polymer batteries. When the sun is out, the aircraft collects the solar energy and recharges the batteries. At night, it runs on the power built into the batteries. There is no reserve fuel, meaning any loss in solar power collection or issues with the batteries leaves the airplane incapable of continuing on. The Solar Impulse 2 can fly at speeds between 22 miles per hour and 87 miles per hour.

Relying on the sun means Solar Impulse 2 requires more than just an experienced pilot. The company also has a Mission Control Center that will be tasked with predicting weather and guiding the airplane through troubled areas to ensure it captures enough solar energy to survive nightfall.

In addition to piloting Solar Impulse 2, Borschberg must also worry about his body. In a statement on Monday, the company said that the five-day flight will require Borschberg, who is in a cockpit 3.8 cubic meters in size, to be "physically and mentally alert throughout the entire journey." That will require him to sleep for only 20 minutes at a time during the entire trek. He will also use yoga and meditation "to keep his body energy and mindset functioning well." While he's sleeping, the Mission Control Center will monitor the plane remotely and ensure the safety of both Borschberg and the aircraft.

Solar Impulse is keeping followers abreast of the flight's progress on Twitter, reporting this morning that after 16 hours, Solar Impulse 2 had hit the point of no return and must now press on to Hawaii without the possibility of turning around to Japan. The company has also created a widget on its website, showing real-time views of the plane's current altitude, battery levels and speed.

As of this writing, the Solar Impulse 2 is over the Pacific, moving steadily and on course.

Solar Impulse did not immediately respond to a request for comment.