The Solar Impulse 2 (foreground), a solar powered plane, sits on the tarmac after it landed at Nagoya airport in Japan June 1, 2015. REUTERS/Thomas Peter

The Nagano mountain area is pictured by Swiss pilot Andre Borschberg in the cockpit of the Solar Impulse 2 plane during the 7th leg of the round the world trip in this June 1, 2015 handout photograph released by Solar Impulse. REUTERS/Andre Borschberg/Solar Impulse/Handout via Reuters

Swiss pilot Andre Borschberg takes a selfie in the Solar Impulse 2 plane during the 7th leg of the round the world trip in this June 1, 2015 handout photograph released by Solar Impulse. REUTERS/Andre Borschberg/Solar Impulse/Handout via Reuters

Solar Impulse 2 -a solar powered plane- takes off in Nanjing, China May 31, 2015. REUTERS/Solar Impulse/Handout via Reuters

ar powered plane- in Nanjing, China, May 31, 2015. REUTERS/Solar Impulse/Handout via Reuters

The Solar Impulse 2 plane is seen on the tarmac before taking off at Chongqing Jiangbei International Airport, to fly to Nanjing in Jiangsu province, April 21, 2015. REUTERS/Stringer

"Solar Impulse 2" descends to land in Muscat March 9, 2015. REUTERS/Jean Revillard/Handout via Reuters

Two youths take a picture of themselves as the Solar Impulse 2, a solar powered plane, ascends in the sky behind them after taking off at the airport in the western Indian city of Ahmedabad March 18, 2015. REUTERS/Amit Dave

Two youths take a picture of themselves as the Solar Impulse 2, a solar powered plane, ascends in the sky behind them after taking off at the airport in the western Indian city of Ahmedabad March 18, 2015. REUTERS/Amit Dave

The Solar Impulse 2, a solar powered plane, circles above Nagoya airport in Japan before a planned landing, June 1, 2015. REUTERS/Thomas Peter

A man uses a phone to take a picture as the Solar Impulse 2, the first round-the-world solar-powered plane, lands at Mandalay International Airport March 19, 2015. REUTERS/Soe Zeya Tun