China mission primed for Moon landing
Chang'e-5 and 6 are sample return missions, delivering lunar rock and soil to laboratories on Earth.
By Paul Rincon
China is preparing to make the first attempt at landing robotic spacecraft on the Moon's far side.
A static lander and rover are expected to be deployed to the surface in the next day, state media reports.
The vehicles are carrying a suite of instruments designed to characterise the region's geology, as well as a biological experiment.
In recent days, the Chang'e-4 spacecraft had lowered its orbit in preparation for landing.
At the weekend, Chinese state media said the probe had entered an elliptical path around the Moon, bringing the vehicles to within 15km (9 miles) of the lunar surface at its closest point.
Authorities have not specified the exact time of the attempt to touch down in the Von Kármán crater. But a report in the state-run China Daily newspaper suggests Chang'e-4 could begin descending on its thrusters sometime from 2-3 January.
Targeting the far side turns this mission into a riskier and more complex venture than its predecessor, Chang'e-3 - which touched down in the Moon's Mare Imbrium region in 2013. But China's latest moon shot will pave the way for the country to deliver samples of lunar rock and dust to Earth.
Andrew Coates, professor of physics at UCL's Mullard Space Science Laboratory in Surrey, told BBC News: "This daring mission will land nearly 50 years on from the historic Apollo landings and will be followed in late 2019 by a Chinese sample return mission."
Because of a phenomenon called "tidal locking", we see only one "face" of the Moon from Earth. This is because the Moon takes just as long to rotate on its own axis as it takes to complete one orbit of Earth.
The lunar far side is often referred to as the "dark side", though "dark" in this case means "unseen" rather than "lacking light". In fact, both the near and far sides of the Moon experience daytime and night-time.
But the far side has a thicker, older crust that is pocked with more craters. There are also very few of the "mare" - dark basaltic "seas" created by lava flows - that are evident on the near side.
The Von Kármán crater is located within a much larger feature - the South Pole-Aitken (SPA) Basin - thought to have been formed by a giant impact early in the Moon's history.
"This huge structure is over 2,500km in diameter and 13km deep, one of the largest impact craters in the Solar System and the largest, deepest and oldest basin on the Moon," Prof Coates told me.
And therein lies the scientific interest. The event responsible for carving out the SPA basin is thought to have been so powerful, it punched through the outer layer of the Moon, known as the crust, and down into the zone called the mantle. Researchers will want to train the instruments on any mantle rocks exposed by the calamity.
The science team also hopes to study parts of the sheet of melted rock that would have filled the newly formed South Pole-Aitken Basin, allowing them to identify variations in its composition.
A third objective is to study the far side regolith, the broken up rocks and dust that make up the surface.
"The in-situ composition information in particular will be hugely valuable in understanding the formation of the Moon," Andrew Coates commented.