An exciting landing process of Chang'e-4 lunar probe (月球探测器 ) was seen through the monitor at Beijing Aerospace Control Center in Beijing, Jan. 3, 2019. It touched down on the far side of the moon at 10:26 a.m. Beijing Time, becoming the first spacecraft soft-landing on the moon's unknown side never visible from Earth.
China's Chang'e-4 probe has started the exploration on the far side of the moon thanks to the relay satellite that provides a communication link with ground control. The relay satellite, named Queqiao, meaning Magpie Bridge, after a Chinese legend (传说) , was launched on May 21, 2018, and became the first communication satellite operating in the halo orbit, nearly 500,000 km from the earth. It can also provide communication for probes from other countries. The relay satellite will also be used for scientific and technological experiments. 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 sends it to Earth.
The satellite can stay in its orbit for a long time due to its relatively low fuel consumption, as the earth's and moon's gravity balances its orbital moving. While in orbit, it can "see" both the earth and the far side of the moon.
The concept of using 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.
Researchers hope to use the cameras on the satellite to record asteroids (小行星 ) hitting the far side of the moon. To control the cost of the Chang'e-4 probe, the relay satellite was designed to be relatively small, weighing about 400 kg.
Researchers had just 30 months to develop the satellite, putting them under high 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.
