The concept of deploying a relay satellite in the halo orbit was first put forward by U.S. space experts in the 1960s, but was realized by Chinese space engineers.

"We will let Queqiao work as long as possible. It could also provide communication for probes from other countries if they intend to explore the moon's far side within the lifetime of the satellite," said Ye Peijian, an academician of the Chinese Academy of Sciences and a senior space expert.

"And that will be a Chinese contribution made to the world," Ye said.

The relay satellite will also be used for scientific and technological experiments.

It has a low-frequency radio spectrometer, jointly developed by Dutch and Chinese scientists, to help astronomers "listen" to the deeper reaches of the cosmos.

It also carries a reflector developed by the Sun Yat-sen University, in south China's Guangdong Province, to conduct the world's longest laser-ranging test between the satellite and an observatory on the ground.

Researchers hope to use the cameras on the satellite to capture asteroids hitting the far side of the moon, said Sun Ji.

"It's extremely difficult, but we hope to try," Sun said.

To control the cost of the Chang'e-4 mission, the relay satellite was designed to be relatively small, weighing about 400 kg.

Chinese experts designed several antennas for it, including one shaped like an umbrella with a diameter of almost 5 meters.

"We learned from textile technologists and watchmakers in the development of the metal mesh and ribs on the antenna," Zhang said.

"It must endure temperature changes of more than 300 degrees centigrade. We conducted countless experiments for that."

His team had just 30 months to develop the satellite, putting them under tremendous pressure.

To promote public interest in space exploration, the China National Space Administration invited people to write down their wishes for lunar and space exploration, and the relay satellite carries the names of tens of thousands of participants and their messages.