A model that shows how the relay satellite Queqiao provides a communication link with ground control during the exploration of China's Chang'e-4 probe on the moon's far side is displayed at an exhibition at the China Science and Technology Museum in Beijing on Sept. 16. The exhibition is part of nationwide activities to mark the 2019 Science Day that lasts from Sept. 14 to 20. [Photo by Yuan Fang/China.org.cn]