Chang'e-4: China space mission lands on Moon's far side

China says it has successfully landed a robotic spacecraft on the far side of the Moon, the first ever such attempt and landing.

At 10:26 Beijing time (02:26 GMT), the unmanned Chang'e-4 probe touched down in the South Pole-Aitken Basin, state media said.

It is carrying instruments to characterise the region's geology, as well as a biological experiment.

State media called the landing "a major milestone in space exploration".

While past missions have been to the Earth-facing side, this is the first time a craft has landed on the unexplored far side.

The probe has sent some first pictures from the surface. With no direct communications link possible, all pictures and data are first sent to a separate satellite and then relayed from there to earth.

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.

Risky mission

The BBC's China correspondent John Sudworth says that more was at stake than just science: the propaganda value of a leap forward in its space race ambitions was underscored by the careful media management - with very little news of the landing attempt before the official announcement that it had been a success.

China has been a late starter when it comes to space exploration. Only in 2003, it sent its first astronaut into orbit, making it the third country to do so, after the Soviet Union and the US.

#BREAKING: China’s Chang’e-4 probe successfully made the first-ever soft landing on the far side of the Moon on the South Pole-Aitken basin Thursday morning, a major milestone in space exploration. #ChangE4 pic.twitter.com/mt2YTWqlxs

— Global Times (@globaltimesnews) January 3, 2019

Targeting the far side has turned this mission into a riskier and more complex venture than its predecessor, Chang'e-3 - which in 2013 touched down on the near side of the Moon, in the Mare Imbrium region.

China's latest moon shot will pave the way for the country to deliver samples of lunar rock and dust to Earth. 

Ahead of the landing, Andrew Coates, professor of physics at UCL's Mullard Space Science Laboratory in Surrey, told the BBC: "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."

The 'dark side of the Moon'

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.

The near side (L) and far side (R) of the Moon have some key differences

 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 Chang'e-4 probe is aiming to explore a place called Von Kármán crater, located within the much larger 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 said.

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," Mr Coates explained.

Seeds and eggs

The lander and rover were originally built as back-ups for 2013's Chang'e-3 mission. However, they have received important modifications for the ambitious touchdown on the far side.

Chang'e-4's static lander is carrying two cameras; a German-built radiation experiment called LND; and a spectrometer that will perform low-frequency radio astronomy observations.

Scientists believe the far side could be an excellent place to perform radio astronomy, because it is shielded from the radio noise of Earth. The spectrometer work will aim to test this idea.

A mock-up of the Chang'e-4 lander and rover, on display in Dongguan, China

The lander carries a 3kg (6.6lb) container with potato and arabidopsis plant seeds - as well as silkworm eggs - to perform biological studies. The "lunar mini biosphere" experiment was designed by 28 Chinese universities.

The rover also carries a panoramic camera; a radar to probe beneath the lunar surface; an imaging spectrometer to identify minerals; and an experiment to examine the interaction of the solar wind (a stream of energised particles from the Sun) with the lunar surface.

In an article for the US-based Planetary Society in September, Dr Long Xiao from the China University of Geosciences (Wuhan), said: "Chang'e-4 will be humanity's first landed exploration of the lunar far side. The challenge faced by a far side mission is communications. With no view of Earth, there is no way to establish a direct radio link."

Thus, the landers must communicate with Earth using a relay satellite named Queqiao, launched by China in May this year.

Queqiao orbits 65,000km (40,000mi) beyond the Moon, around a Lagrange point - a kind of gravitational parking spot in space where it will remain visible to ground stations in China and other countries such as Argentina.

The lander and rover was launched from Xichang Satellite Launch Center in China on 7 December; the vehicles arrived in lunar orbit on 12 December.

The mission is part of a larger Chinese programme of lunar exploration. The first and second Chang'e missions were designed to gather data from orbit, while the third and fourth were built for surface operations. 

 

Chang'e-5 and 6 are sample return missions, delivering lunar rock and soil to laboratories on Earth.

BBC