Visitors take pictures in front of the world's largest solar-powered plane-Solar Impulse 2-in Nanjing, capital of Jiangsu province, on Friday. The Swiss-produced Solar Impulse, a fuel-free airplane, began its global journey from Abu Dhabi, capital of the United Arab Emirates, on March 9.It has already flown for more than 75 hours and about 6,000 kilometers on solar energy. Nanjing was the sixth leg of its tour. [Photo / Xinhua]