Queqiao: The Bridge Between Earth And The Far Side Of The Moon

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Researchers explain the design of the relay communication satellite that enabled a glimpse at the hidden face of the Moon

The far side of the Moon always faces away from the Earth, making communications from lunar equipment there much more challenging. Fortunately, relay communication satellites can act as a bridge or stepping stone between transmission from the far side towards Earth ground stations. (Image credit: DFH Satellite Co., Ltd.; taken by the camera onboard Queqiao.)

Due to a phenomenon called gravitational locking, the Moon always faces the Earth from the same side. In the lunar landing missions of the 20th century, this phenomenon proved useful 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 since signals cannot be sent directly across the Moon towards Earth.

In an interesting development, China’s Chang’e-4 probe spacecraft present on this far side of the Moon has been able to communicate with ground stations, 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 that orbits a point behind the Moon and bridges Earth and Chang’e-4.

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 known as the Earth-Moon Libration point 2. Over here, a special case of gravitational balance allows Queqiao to maintain an orbit such that it has an almost constant direct line of sight with both the far side of the Moon and the Earth. Getting the satellite into this peculiar orbit required careful planning and maintenance management. The success of this operation set a precedent for future attempts at putting satellites in orbit around other Earth-Moon libration points.

From its stable place in space, Queqiao helped guide the soft-landing and surface operations of Chang’e-4 probe and has been our intermediary with it ever since. The satellite is equipped with two different kinds of antennas. The parabolic antenna has a large diameter of 4.2m, which has been designed to send and receive signals on the X band (7-8GHz) to and from the rover and lander on the surface of the Moon. Its large size is related to the expected noise levels and the low intensity of the transmissions that are sent by surface equipment.

On the other hand, the spiral antennas operate on the S-band (2–4 GHz) and communicate with Earth ground stations, forwarding commands to the lunar surface equipment and exchanging telemetry and tracking data. Most notably, all these different links can transmit and receive simultaneously, making Queqiao highly versatile.

“Scientists in both China and other countries have conducted analysis and research based on the retrieved data, and they have produced valuable scientific results. The longer the operational life of Queqiao, the more scientific outcomes will be achieved,” remarks Dr Zhang, spacecraft system engineer and project manager -Queqiao at DFH Satellite Company Ltd, China.

Dr Zhang also addressed the prospects for future lunar missions and how relay communication systems should evolve to support them. Many unexplored areas on the Moon, such as the largest crater at the South Pole, call for multiple relay satellites to maintain constant communication links, which poses an expensive and time-consuming challenge.

On the prospects of deploying relay satellites for more than a single mission, he says, “A sustainable communication and navigation infrastructure should be established to benefit all lunar missions rather than dealing with each mission independently. This infrastructure should adopt an open and extensible architecture and provide flexible, interoperable, cross-supportable and compatible communications services, which are critical to the success of future lunar explorations.”

Future endeavours on the far side of the Moon will likely be a test on how well we can cooperate to unveil the secrets of our natural satellite. Based on current predictions, Queqiao should be operable on mission orbit for at least five years.

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