China's Chang'e 5 spacecraft is nearing the end of its mission after a three-week roundtrip to the moon and back to Earth.
The spacecraft's re-enter capsule is due to land, carrying 2 kilograms (4.4 pounds) of ground samples from the moon, in Inner Mongolia during the night of December 16 to 17.
It is expected to land between 01:32 a.m. and 02:07 a.m. local time (6:32 p.m. and 7:07 p.m. CET) in Siziwang "banner," a county or district, in the autonomous, north China region.
Siziwang is a known Chinese space landing site. It is where the country's crewed Shenzhou missions landed in the early 2000s, and it was selected especially for the Chang'e 5 return capsule.
But Inner Mongolia is vast and finding the small capsule in the harsh, snowy winter weather will be difficult during the night.
The return capsule is one-seventh the size of the one used for China's crewed spaceship and it will land in an area is 16 times larger.
Ground and retrieval teams have practiced the search, as shown in video from China's Xinhua news agency, using powerful search lights. They have conducted 30 land surveys. Helicopter teams and ground vehicles are at the ready.
Like a skipping stone
To make things harder for the retrieval team, the return capsule will land using a "skip reentry method."
A skip reentry method describes the way that a spacecraft reenters the Earth's atmosphere.
Like a stone that you throw at an angle at water — a pond or the ocean — to let it skip across the surface, spacecraft can also skip on reentry.
It is, however, a very difficult maneuver.
If, for instance, a spacecraft fails to slow down enough as it hits and skips on the Earth's atmosphere, it may bounce off the atmosphere and get sent back out into space.
If, on the other hand, it hits the atmosphere at the wrong angle — if it "splashes" on the surface — the spacecraft could fail completely and burn up.
But we've all tried skipping stones on water, and some go farther than you'd expect. And that may also go for spacecraft, potentially. Which poses another challenge for the retrieval team as the return capsule may land — somewhere — over a very large area.
Historic mission, either way
It is the third time that China has landed a spacecraft on the moon, starting with Chang'e 3 and then Chang'e 4.
Chang'e 4 was the first to ever land on the far-side of the moon — the side we cannot see from Earth.
And if successful, Chang'e 5 will be the first so-called "sample return" mission to bring back rock, dust and other minerals from the moon since Luna 24. That was Soviet Russia's last sample return mission in 1976.
Before that, the US Apollo missions also brought back kilos of lunar samples, but the program was disbanded in 1972.
Now, the global space community is working on a growing number of sample return missions to the moon and beyond.
Japan's Hyabusa 2 just returned with samples from an asteroid called Ryugu. Japan is also planning a sample return mission to the moons of Mars in 2024.
Then there is a European and American sample return mission to the Red Planet itself. All this, within this decade.
Chang'e 5's lander stayed back on the surface of the moon to carry out surveys and measurements. It was equipped with a panoramic camera to map the topography of the landing site, an infrared spectrometer to determine the physical composition of stones and dirt around the landing site, and a soil measurement instrument to detect and analyze the subsurface structure where the lander drilled for samples.
The China National Space Agency (CNSA) says Chang'e 5 will help foster the country's knowledge, technology and talent pool for future manned missions to the moon and other deep space expeditions.
Pei Zhaoyu, deputy director of CNSA's Lunar Exploration and Space Program Center, says scientists around the world will are welcome to participate in the research related to the new lunar samples.