Swiss pilot Bertrand Piccard prepares for a test flight of the solar-powered Solar Impulse 2 experimental aircraft in Payerne November 13, 2014. The aircraft, weighing 2.4 tons with a wingspan of 72 meters, is fitted with more than 17,000 solar cells. An attempt to fly around the world in stages using only solar energy will be made in 2015. (Photo by Ruben Sprich/Reuters)

The solar-powered Solar Impulse 2 experimental aircraft, piloted by Swiss Bertrand Piccard, prepares to take off for a test flight in Payerne November 13, 2014. (Photo by Ruben Sprich/Reuters)

Swiss pilot Bertrand Piccard prepares for a test flight of the solar-powered Solar Impulse 2 experimental aircraft in Payerne November 13, 2014. (Photo by Ruben Sprich/Reuters)

Swiss pilot Bertrand Piccard prepares for a test flight of the solar-powered Solar Impulse 2 experimental aircraft in Payerne November 13, 2014. (Photo by Ruben Sprich/Reuters)

A man watches a test flight of the solar-powered Solar Impulse 2 experimental aircraft, piloted by Swiss Bertrand Piccard, in Payerne November 13, 2014. (Photo by Ruben Sprich/Reuters)

The solar-powered Solar Impulse 2 experimental aircraft, piloted by Swiss Bertrand Piccard, is pictured during a test flight in Payerne November 13, 2014. (Photo by Ruben Sprich/Reuters)

Swiss pilot Bertrand Piccard prepares for a test flight of the solar-powered Solar Impulse 2 experimental aircraft in Payerne November 13, 2014. (Photo by Ruben Sprich/Reuters)

Swiss pilot Bertrand Piccard prepares for a test flight of the solar-powered Solar Impulse 2 experimental aircraft in Payerne November 13, 2014. (Photo by Ruben Sprich/Reuters)

Swiss pilot Bertrand Piccard prepares for a test flight of the solar-powered Solar Impulse 2 experimental aircraft in Payerne November 13, 2014. (Photo by Ruben Sprich/Reuters)

Swiss pilot Bertrand Piccard prepares for a test flight of the solar-powered Solar Impulse 2 experimental aircraft in Payerne November 13, 2014. (Photo by Ruben Sprich/Reuters)

Swiss pilot Bertrand Piccard prepares for a test flight of the solar-powered Solar Impulse 2 experimental aircraft in Payerne November 13, 2014. (Photo by Ruben Sprich/Reuters)

The solar-powered Solar Impulse 2 experimental aircraft, piloted by Swiss Bertrand Piccard, is pictured during a test flight in Payerne November 13, 2014. (Photo by Ruben Sprich/Reuters)

Swiss pilot Bertrand Piccard prepares for a test flight of the solar-powered Solar Impulse 2 experimental aircraft in Payerne November 13, 2014. (Photo by Ruben Sprich/Reuters)