Relay satellite plays key role in exploring moon

China's Chang'e 4 probe has started sending information from the far side of the moon thanks to a relay satellite that provides a communication link with ground control.

The relay satellite, named Queqiao, or Magpie Bridge, after a Chinese legend, was launched on May 21 and operates nearly 500,000 kilometers from Earth.

The maximum distance between the satellite and the Chang'e 4 probe on the far side of the moon is 79,000 km. The satellite processes data from the probe and transmits it to Earth, said Sun Ji, one of the satellite's designers from the China Academy of Space Technology.

The satellite can stay in orbit for a long time due to its relatively low fuel consumption, as the earth's and moon's gravity balance its orbital motion, said Zhang Lihua, chief designer of the satellite.

While in orbit, it can "see" both the earth and the far side of the moon, Zhang said.

"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."

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 Sun Yat-sen University in 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 see asteroids hitting the far side of the moon, designer Sun said.

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

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

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 C. 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. The relay satellite carries the names of tens of thousands of participants and their messages.