Photograph courtesy Jean Revillard, Solar Impulse

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The Solar Impulse plane sets off from San Francisco in the early morning hours May 3. The solar-powered plane is set to land in Phoenix very early Saturday and later make stops in four other cities.

Photograph courtesy Jean Revillard, Solar Impulse

Solar Impulse’s U.S. Expedition Begins

Solar Plane Aims for Coast-to-Coast U.S. Flight Without Using Fuel

A solar-powered aircraft able to fly day and night began the first leg of its journey this morning on a mission to become the first aircraft to fly coast-to-coast across the United States without using a single drop of fuel.

The aircraft, named Solar Impulse, took off from Moffett Airfield near San Francisco, California, just after 9 a.m. EDT (6 a.m. PDT) and is expected to land in Phoenix around 3 a.m. EDT (1 a.m. PDT) Saturday. Solar Impulse is expected to reach a cruising altitude of 21,000 feet (6,400 meters). (Related blog: "Solar Plane Team Sets Sights on U.S. Flight.")

Shepherding the cross-country flight are Bertrand Piccard and André Borschberg, Swiss pilots with extensive backgrounds in aviation. Piccard was the first pilot to complete a nonstop balloon flight around the globe and Borschberg trained as a fighter pilot in the Swiss air force. The duo, who designed the aircraft and will take turns piloting the solar plane across the country, are just warming up for a round-the-world solar flight they aim to make in 2015.

Weighty Batteries

The Impulse, which is completely solar-powered, requires no fossil fuels and emits no pollution. Instead, the aircraft is covered in almost 12,000 silicon solar cells that drive four electric motors and can turn the plane's propellers day and night with special batteries that store power. Weighing in at approximately 880 pounds (400 kilograms), the batteries account for more than 25 percent of the plane's total mass, something that required major weight reductions in the rest of the body.

The entire journey will take more than a month and is made up of four legs. In mid-May, Solar Impulse will depart from Phoenix and land in Dallas, Texas. The end of May will see Solar Impulse head to St. Louis, Missouri. On the fourth and fifth legs, the solar aircraft will fly from St. Louis to Washington, D.C., and then to New York City in late June or July.

In addition to completing the landmark flight, the Solar Impulse's trip is intended to launch a "Clean Generation" Initiative, which aims to promote clean technologies around the world. The initiative intends to "encourage governments, businesses and decision-makers to push for the adoption of clean technologies and sustainable energy solutions," Solar Impulse said in a statement.

Capitalizing on air currents, Piccard and Borschberg will travel west to east at an average speed of 40 knots (46 miles per hour) in the ultralight aircraft, which weighs about as much as an average automobile (3,525 pounds or 1,600 kilograms).

Global Dreams

In May 2012, the Solar Impulse achieved the first solar-powered intercontinental flight by flying from Spain to Morocco in just over 19 hours. Piccard and Borschberg plan on flying a solar-powered plane around the world in 2015—they're building a second plane with a larger cockpit and a few other adjustments for the journey, which they hope to complete in just 20 days. (See pictures: "Solar Plane Completes First Intercontinental Flight.")

Solar Impulse also completed a successful 26-hour overnight flight in 2010.

Each leg of the coast-to-coast flight will be streamed live on, as well as on Twitter and Facebook.

Christina Nunez and Brian Handwerk contributed to this report.

This story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.