Why did the five girls write: Big Sister, Second Sister, Third Sister and Fourth Sister when they went to the Moon Palace?

Elder sister, second sister, third sister and fourth sister are Chang 'e detectors. The five girls of Chang 'e are the most complicated detectors in the Chang 'e family. The first is' appearance', that is, the appearance is different. According to Ruan Jianhua, deputy chief designer of Chang 'e-5 probe in the Fifth Academy of China Aerospace Science and Technology Corporation, compared with Chang 'e-1, Chang 'e-2, Chang 'e-3 and Chang 'e-4, the technology span of Chang 'e-5 probe is larger and the structure is more complicated. It is composed of four devices, including orbiter, lander, riser and returner. These four devices are like "candied haws", each of which is a separate individual, and can be combined together. For example, the lander and the riser are "hugged" together to form a "landing assembly", and the orbiter and the returner are combined to form a "track return assembly". The four devices are connected in series to form a complete detector.