China's Chang'e 6 mission is set to return samples from the far side of the Moon, but since the same side of the Moon always faces Earth, how do ground control communicate with the lunar lander?
One of the biggest problems with landing a mission on the lunar far side is that spacecraft can’t communicate directly with Earth from the surface, as the Moon itself blocks radio signals.
To resolve this, China uses relay satellites to convey signals from the surface back to Earth.
The Queqiao lunar constellation
The first of these, Queqiao, launched on 20 May 2018.
It is in a halo orbit around the Earth–Moon L2 Legrange point, which is directly behind the Moon with respect to Earth.
Queqiao helped support the Chang’e 4 mission, the first ever to land on the lunar far side when it touched down within the Von Kármán crater in January 2019.
The Queqiao-2 relay satellite, launched in March 2024, is a larger craft weighing 1,200kg (2,645lb).
Queqiao-2 was placed in frozen elliptical lunar orbit, meaning it can support missions across the whole of the Moon.
The next step
A future constellation of Queqiao satellites has also been proposed, which could provide not just communications but also navigation and remote-sensing services.
Details are still largely unknown, but it’s thought the first phase, Queqiao v1.0, could be in place by 2030.
It would consist of Queqiao-2 and the future Queqiao-3, as well as the Tiandu-1 and Tiandu-2 test satellites which would fly in formation to test lunar satnav and communication technologies.
Queqiao v2.0, comprising 16 satellites, would then be deployed between 2030 and 2040, and would enable improved remote-sensing capabilities, higher bandwidth (1–10 GB/s) and better positioning resolution.
The final generation, deployed in the 2040s, would have even higher performance and could also act as interplanetary relay stations.
Click here to read about the how the sample arrived back on Earth
This article appeared in the May 2024 issue of BBC Sky at Night Magazine
