On May 21, China’s Queqiao (Magpie Bridge) relay satellite of Chang’e 4 lunar probe was launched into space by a Long March 4C rocket at 5:28 a.m. Beijing Time. This satellite carries a laser corner cube retro-reflector (CCR), which has been developed by Prof. Jun Luo, academician of the Chinese Academy of Science (CAS) from Huazhong University of Science and Technology (HUST), and his team since 2015, to conduct a satellite laser ranging experiment. The CCR employed in the mission is developed by the Center for Gravitational Experiments (CGE) of HUST.

Dr. Qi Liu, and Dr. Yun He, Post-doctors of CGE, and other colleagues have been participating in the CCR manufacture. According to their introduction, the CCR, with an effective aperture of 170 mm, is fabricated by three critically polished Ultra-Low Expansion glass (ULE) panels using a special technology. To ensure the precision of laser ranging, the angle errors for the CCR need to be less than 2 µrad, so that the CCR can coordinate with the ground-based ranging facility with large telescope to realize the Earth-Satellite laser ranging.

It is reported that this experiment serves as a pioneering research for the “Tianqin” mission to detect the gravitational waves in space in the future. On January 27, 2018, the Applied Astronomy Group of Yunnan Observatories, CAS, realized the laser ranging between the Earth and the Moon. However, the relay satellite is about 80,000 km farther from the Earth than the Moon (the longest Earth-Moon distance is about 400,000 km). The project will attempt the pure reflection laser ranging test over the Earth-Moon distance for the first time in the world; and it is also the first step of the "0123 plan" in the Tianqin roadmap, that is, part of the "0" phase. The purpose of the project is to develop the lunar and deep-space satellite laser ranging technology to provide technical verification and to reserve for the precise orbit determination of the later Tianqin satellites. Moreover,it is likely that the CCR will be deployed on the lunar surface in the future to achieve the next generation Lunar Laser Ranging (LLR) with higher accuracy.