Because of a phenomenon called gravitational locking, the Moon always faces the Earth from the same side. This proved useful in the early lunar landing missions in the 20th century, as there was always a direct line of sight for uninterrupted radiocommunications between Earth ground stations and equipment on the Moon. However, gravitational locking makes exploring the hidden face of the moon - the far side - much more challenging, because signals cannot be sent directly across the Moon towards Earth.
Still, in January 2019, China's lunar probe Chang'e-4 marked the first time a spacecraft landed on the far side of the Moon. Both the lander and the lunar rover it carried have been gathering and sending back images and data from previously unexplored areas. But how does Chang'e-4 probe communicate with the Earth? The answer is Queqiao, a relay communications satellite, explains Dr. Lihua Zhang from DFH Satellite Co., Ltd., China.
As explained by Dr. Zhang in a review paper recently published in Space: Science and Technology, Queqiao is an unprecedented satellite designed specifically for one purpose: to act as a bridge between Chang'e-4 probe and the Earth. Queqiao was launched in 2018 and put into orbit around a point 'behind' the Moon.