After orbiting the moon for more than 20 days, the Chang'e-4 probe, launched from the Xichang Satellite Launch Center in southwest China on Dec. 8, 2018, has seen countless craters, mountains and valleys on the moon.
Finally, its destination on the far side, the South Pole-Aitken (SPA) Basin, the largest, deepest and oldest crater in the solar system, was illuminated by the rising sun.
The developers of Chang'e-4 decided on Thursday it was the time to come down on this barren world.
At 10:15 a.m., a variable thrust engine was ignited with the assistance of the relay satellite Queqiao (Magpie Bridge), operating in the halo orbit around the second Lagrangian (L2) point of the Earth-moon system, about 65,000 km from the moon, where it can see both Earth and the moon's far side.
Chang'e-4's relative velocity to the moon was lowered from 1.7 km per second to close to zero, and the probe was adjusted to face the moon and descend vertically towards the Von Karman Crater in the SPA Basin.
When it descended to an altitude of about 2 km, its cameras captured the shadows of the hills and valleys on the lunar surface. Its computer identified and assessed large obstacles such as rocks and craters, so the probe could avoid them.
At 100 meters up, the probe hovered to identify smaller obstacles and measured the slopes on the surface. Its computer calculated again and selected the safest site.
At 2 meters above the surface, the engine stopped, and then the golden lander with a silver rover on top touched down on the desolate gray surface with four legs, throwing up some dust.
The probe performed the entire landing process, lasting about 12 minutes with no intervention from ground control, and the relay satellite transmitted the first close-up photos of the moon's far side back to a control center in Beijing.
The China National Space Administration later announced that the probe landed at the preselected landing area at 177.6 degrees east longitude and 45.5 degrees south latitude on the far side of the moon.
Named after Chinese moon goddess "Chang'e," China's lunar exploration program, which began in 2004, includes orbiting and landing on the moon, and bringing samples back to Earth.
After Chang'e-3 completed China's first soft landing on the moon in 2013, Chinese space experts aimed high, hoping Chang'e-4 could carry out unprecedented and more challenging tasks.
The moon is tidally locked to earth, rotating at the same rate that it orbits Earth, so one side of the moon is seen from Earth, leaving the far side a mystery, until now.
About 60 years ago, the Soviet Union's Luna 3 probe sent back the first images of the moon's far side. And about 50 years ago, three astronauts on the United States Apollo 8 mission became the first people to see it with their own eyes.
Lunar orbiters have shown the moon's two sides are very different: the near side is relatively flat, while the far side is thickly dotted with impact craters of different sizes.
Scientists believe that the lunar crust on the far side is much thicker than the near side. However, the reason is still a mystery. Only on site exploration might reveal the secrets.
The moon and Earth shared a similar "childhood." But traces of the remote past on Earth have been erased by geological activities.
Exploring the Von Karman Crater in the SPA Basin is meaningful in another sense. The crater was named after a Hungarian-American mathematician, aerospace engineer and physicist in the 20th century, who was also the teacher of Qian Xuesen and Guo Yonghuai, the founders of China's space industry.
Nearly 50 years have passed since people first stood on the moon. Can we return? How will radiation on the moon affect astronauts? How much water is there?
Scientists from China, Germany and Sweden hope to find the answers through Chang'e-4, and make preparations for people to return to the moon.
The Chang'e-4 mission, including the probe, the relay satellite Queqiao and a micro satellite orbiting the moon, is equipped with four payloads developed through international cooperation, providing more opportunities to the world's scientists and combining human expertise in space exploration.
For astronomers, the far side of the moon is a place of ideal tranquility, as the body of the moon shields against radio interference from Earth. From there, they can study the origins and evolution of stars and galaxies, peering into the dawn of the universe.
Chang'e-4 carries low-frequency radio astronomical instruments developed by Chinese and Dutch scientists.
The probe also took six live species - cotton, rapeseed, potato, arabidopsis, fruit fly and yeast - to the lifeless environment to form a mini biosphere, which is expected to produce the first flower on the moon.
Chinese space engineers also plan to get data by constantly measuring temperatures on the surface of the moon.
China's Yutu-2, the first rover on the far side of the moon, has found materials from deep inside the moon that could help unravel the mystery of the lunar mantle composition and the formation and evolution of the moon and the earth.
Using data obtained by the visible and near infrared spectrometer installed on Yutu-2, a research team led by Li Chunlai, of the National Astronomical Observatories of Chinese Academy of Sciences (NAOC), found that the lunar soil in the landing area of the Chang'e-4 probe contains olivine and pyroxene which came from the lunar mantle deep inside the moon.
After Chang'e-4 successfully landed on the Von Karman Crater in the South Pole-Aitken Basin on Jan. 3 this year, the Yutu-2 rover obtained good quality spectral data at two sites.
Analysis showed the lunar soil in the landing area contains a large amount of olivine, low-calcium pyroxene and a small amount of high-calcium pyroxene, which are very likely from the lunar mantle.
The first important scientific discovery of the Chang'e-4 probe since it made the first-ever soft landing on the far side of the moon was published online in the latest issue of the academic journal Nature. (Xinhua)