Exploring the far side

Off the beaten track The first images from the Chang’e-4 lander on the far side of the Moon show the South Pole-Aitken Basin and the rover making its first journey. *(Chinese Lunar Exploration Program; CLEP/Imaginechina/Shutterstock)*

The far side of the Moon remained an untouched territory until a Chinese lander, called Chang’e-4, touched down in the region earlier this year. Ling Xin examines the consequences for our understanding of Earth’s nearest neighbour

When the Soviet spacecraft Luna 3 sent back the first images of the far side of the Moon in October 1959, scientists were shocked to see a world very different from what they expected. The photos, although patchy and blurry, showed few of the large, flat, dark expanses that dominate the near side of the Moon – the side that is tidally locked to face Earth. Instead, the previously hidden far side proved to be densely peppered with mountains and impact craters. However, six decades after Luna 3’s pioneering journey, every Moon landing since then – 27 by the end of 2018 – has touched down on the near side. The far side had been studied via orbiting spacecraft but remained an unexplored and enigmatic territory for both manned and unmanned missions to the surface.

That was, however, until earlier this year, when a Chinese probe, Chang’e-4, touched down on 3 January 2019 in the Von Kármán crater in the South Pole-Aitken Basin – one of the Moon’s most scientifically rich regions. About 2500 km in diameter, the basin is the largest and most ancient impact crater on the Moon and is thought to have formed from a collision that penetrated through the Moon’s crust. The basin may even have exposed parts of the lunar mantle itself – allowing scientists to peer directly into the body’s interior.

Sharing its name with a lunar goddess in Chinese mythology, the Chang’e missions to the Moon began in 2007 with the launch of the Chang’e-1 lunar orbiter. It was followed three years later by the similarly designed Chang’e-2 probe. Having thus demonstrated it had the technical prowess to reach and orbit the Moon, China’s next target was to land on it. That was achieved when Chang’e-3 successfully touched down on the near side in December 2013. While the mission provided the first in situ measurements of the lunar surface in four decades, the accompanying small rover travelled barely 100 m before it shut down as a result of a short circuit, meaning that it failed to execute commands.

Chang’e-4 was originally designed as a back-up for Chang’e-3 but after that mission largely succeeded, it was re-purposed with the ambitious intention to become the first mission to land on the Moon’s far side. Launched on 7 December 2018, Chang’e-4 consists of a lander containing four instruments – two cameras, a low-frequency radio receiver and a neutron detector – as well as a small 135 kg rover that carries a camera, radar, spectrometer and a particle analyser. To help mission controllers communicate with the lander, which being on the far side is out of view from Earth, the relay satellite Queqiao was launched in May 2018 (see box below).

By 11 May 2019 the Chang’e-4 lander and rover had already completed the fifth lunar day of their adventure in the Von Kármán crater (one lunar day being 29 Earth days long). In that time the mission produced around 7 GB of data, with scientists now beginning to analyse and publish the first batch of results. As the rover finished its work that night, it had already accomplished its design lifespan of three months and driven almost 200 m on the Moon’s surface – sparking widespread relief among officials following the problems with the Chang’e-3 rover.

Chang’e-4, touched down on 3 January 2019 in the Von Kármán crater in the South Pole-Aitken Basin – one of the Moon’s most scientifically rich regions

Taking on the mantle

A long-standing puzzle about the Moon is the composition of its mantle – a layer inside a planetary body bounded below by a core and above by a crust. Studies suggest that the Moon’s crust is dominated by a mineral called plagioclase. We know that the mantle beneath is rich in iron and magnesium but otherwise has a somewhat unknown composition. Gaining a better understanding of the far-side mantle would provide an insight into the formation of planetary interiors and magma oceans as well as answer questions such as why the lunar crust is thicker on the far side than on the near side.

Before Chang’e-4, researchers had managed to obtain only indirect measurements of the Moon’s far side using satellites orbiting our nearest neighbour. The conventional thinking is that craters such as Von Kármán would have been flooded by basaltic lava flows soon after they had formed and so would not easily yield information about the mantle. Chang’e-4’s location in the South Pole-Aitken Basin now gives us the first opportunity to test this notion by carrying out in situ geochemical measurements. As the Chang’e-4 rover roams around, its visible and near-infrared imaging spectrometer is examining the mineralogy of rocks while the lunar-penetrating radar is looking down to about 100 m beneath the surface, probing the depth of the regolith – the loose material that covers the solid rock. It is also searching for subsurface structures.

In May this year, researchers reported the first results from Chang’e-4 (Nature 569 378). Having analysed the spectra of material collected during the first lunar day, they have already identified signals that seem different from those seen on the near side. On the near side, the lunar surface is mostly made up of plagioclase and low-calcium rock-forming silicate minerals, known as pyroxene. But Chang’e-4 found that on the far side it is made up of olivine – a magnesium-iron silicate – as well as pyroxene. Scientists think that both these materials originated from the upper mantle of the Moon – particularly olivine – and conclude that they were excavated from below the basin’s floor during the creation of the nearby large crater Finsen and then transported to where Chang’e-4 is currently located.

Patrick Pinet, a lunar geologist from the Institute of Research in Astrophysics and Planetology in Toulouse, France, notes that the findings, although initial, are “very exciting”. “If it can be established that we are indeed observing lunar mantle materials, it will be very important for setting up a sample-return mission in this region of the Moon,” he says. “The mission is already an amazing success and I predict that it is going to be considered as quite a remarkable landmark in lunar exploration in the coming years.”

However, Ian Crawford, a planetary scientist from University College London, says that more samples need to be analysed from the landing site to confirm the results, particularly the levels of olivine in the mantle. “The argument that deep crustal and/or mantle materials may be mixed into the Chang’e-4 landing site from the nearby Finsen crater is plausible,” he adds. “And the dominance of orthopyroxene in the spectra is convincing and interesting in itself.”

If further measurements by Chang’e-4 in the crater can confirm the existence of mantle materials, it would be the perfect reason for a future sample-return mission to the region. That is because more precise measurements of materials are currently possible through carbon-dating techniques here on Earth. Indeed, China is already planning its next lunar adventure that will involve two sample-return missions. First up is Chang’e-5, which is set to launch in December 2019. If it successfully lands on the Moon’s near side, it would mark the first lunar-sample return in over 40 years and would aim to collect 2 kg of material, reaching down to 2 m below the surface. Meanwhile, Chang’e-6 will target the South Pole-Aitken Basin when it launches in 2023 or 2024, though the details of what and how much material it will collect will partly depend on the success of Chang’e-5.

The story of Queqiao – the lunar gateway

Fly me to the Moon Scientists and engineers prepare the Queqiao lunar satellite for launch. (China Academy of Space Technology)

When engineer Lihua Zhang at the DFH Satellite Company in Beijing was asked in early 2015 if he wanted to design a communications satellite for China’s upcoming Chang’e-4 mission to the far side of the Moon, his first reaction was one of excitement. But it quickly dawned on him that building such a craft would be an enormous challenge. Zhang had successfully developed several previous satellites, but all of them orbited the Earth.

As the far side of the Moon is not visible from Earth, a satellite is crucial when relaying data to a lunar lander and rover. However, before Chang’e-4, no spacecraft had attempted to land on the lunar far side, so the satellite needed to be designed from scratch. Young and adventurous, Zhang embraced the challenge and his team beat strong competition to win the contract in June 2015.

The biggest question facing Zhang was where to place the satellite. After studying the literature, Zhang and his team noticed one particularly attractive possibility: the Earth–Moon Lagrange Point 2, or L₂. This virtual, gravitationally balanced point in space behind the Moon offered the perfect option as the satellite could see both the Earth – via a “halo” orbit around L₂ – and the far side of the Moon, thereby making it easier to transmit data between the two.

This option had originally been proposed by the aeronautical engineer Robert Farquhar as part of his PhD thesis at Stanford University in the late 1960s. Farquhar became a pioneer of orbital trajectories during his 23-year career at NASA, but much to his disappointment, the space agency considered landing a mission on the far side of the Moon too risky.

Zhang was intrigued by Farquhar’s proposal, but implementing it would be another story. His team needed to figure out the most time-saving, energy-efficient way to arrive and remain in that orbit. This involved taking into account the influences of the Sun, because once in orbit, the satellite would have to regularly go through cycles of complete darkness and extreme temperatures as low as –230 °C.

The other major challenge was designing a communication antenna to receive and relay signals between the lander and rover and about a dozen ground stations on Earth. As the satellite could be as far as 79 000 km away from the possible landing site, the antenna needed to be as large as possible. But it also had to be light and easy to deploy. After months of work, Zhang’s team came up with a 4.2 m-diameter umbrella-shaped antenna – the largest of its kind ever used in deep-space exploration.

After 30 months of development, the relay satellite took off from Xichang Satellite Launch Center on 21 May 2018. By 14 June it had entered the planned orbit and was given a new nickname by Chinese “netizens”: Queqiao. This name, taken from Chinese mythology, signifies a bridge formed by magpie birds to reunite lovers in heaven. “Two things went beyond our expectations,” says Zhang. “The antenna was our big concern, but we developed a novel pointing-control method and in-orbit calibration showed the pointing accuracy to be well above the design threshold. The other is that by precision control we needed fewer orbital corrections than originally thought.” Thanks to this, Queqiao was able to save some fuel, meaning that it may now operate for up to a decade – well beyond its design lifespan of three years.

Farquhar visited China in 2015 where he learnt that his L₂ design would finally be implemented by the Chang’e-4 mission and was apparently very happy to hear the news. Sadly, Farquhar died in October that same year at the age of 83. He never saw how this orbital design would become a reality and, in doing so, open a new chapter in lunar exploration and science.

To the dark ages

But Chang’e-4 is not just about studying the far side of the Moon itself. That’s because this region is also a dream location for making low-frequency radio observations. There is no atmosphere on the Moon and behind it there is little radio interference from Earth. Indeed, for radio astronomer Linjie Chen from the National Astronomical Observatories, Chinese Academy of Sciences, the last few months since Chang’e-4 touched down have been busy. Chen earned his PhD degree from the Netherlands when very-low-frequency radio astronomy in China was in its infancy. But his training and contacts have been pivotal while working on the Netherlands–China Low-frequency Explorer (NCLE), which is set to be deployed on the Queqiao relay satellite by the end of July.

One key target is detecting the radio emissions from hydrogen from a period in the early universe known as the cosmological “dark ages”. This is the era before the first stars were born and so to understand it better, scientists need to study the hydrogen during the early universe that emitted radiation with a wavelength of 21 cm. While this emission is still present, the expansion of the universe means it now has a wavelength of around several metres, causing it to be easily reflected by the Earth’s ionosphere.

To attempt to detect this radiation, the NCLE consists of three 5 m antennas to map the radio sky at 1–80 MHz. It will also carry out joint observations with the low-frequency spectrometer on board the Chang’e-4 lander to form a prototype Earth–Moon–space very-long-baseline experiment. This will aim to study solar bursts, space-weather events and the near-Moon low-frequency radio environment.

Marc Klein Wolt from Radboud University Nijmegen in the Netherlands, who is the NCLE’s principal investigator, is cautious about what the NCLE will uncover given its experimental nature. “While we are trying to measure a very faint signal from the very early universe, the satellite itself will be giving off noise signals that may be difficult to remove,” he says. “But if we succeed, we pave the way for a large-scale radio facility on or near the Moon that will allow us to trace the hydrogen in the dark ages.”

Probing the dark ages The Netherlands–China Low-frequency Explorer instrument aboard the Queqiao lunar satellite will perform radio astronomy jointly with Chang’e-4. *(NCLE team, Radboud University, ASTRON, Innovative Solution In Space)*

All together now

One feature of Chang’e-4 that sets it apart from previous Chang’e missions is the involvement of countries from outside China. Besides the Dutch involvement in the NCLE, there are two further European-led payloads riding along with Chang’e-4. One is the German-led Lunar Lander Neutrons and Dosimetry experiment, which will measure the radiation dose on the lunar far side to help prepare for possible future human exploration of the Moon. The other is the Swedish-led Advanced Small Analyzer for Neut-rals on the Chang’e-4 rover, which explores how solar wind might be involved in the production of water on the Moon.

“Chang’e-4 is the first time that space scientists in China and Europe have worked together in such a broad and deep level,” says Chi Wang, director general of the National Space Science Center, Chinese Academy of Sciences, who is deputy chief engineer of the Chang’e-4 mission. Indeed, discussions between Europe and China began around two years ago and a joint lunar research team has recently been established to co-ordinate future work. In April the China National Space Administration (CSNA) released a call for payloads for Chang’e-6. Both its orbiter and lander will reserve 10 kg for payloads selected from proposals made by researchers at Chinese universities, private companies and foreign research institutions. “The call is open to all countries without exceptions,” adds Wang.

That call, however, might be challenging for US scientists to answer. Although NASA’s Lunar Reconnaissance Orbiter worked briefly with the CNSA in January and released photos of the Chang’e-4 landing site, NASA scientists, who are federally funded, have been prohibited from co-operating with China on space activities. In Chang’e-4’s case, NASA had to request Congress’s permission in advance. According to space-policy expert John Logsdon from George Washington University in the US, this is likely a one-time move rather than a request for a change of policy.

US researchers interested in the far side of the Moon may then have to go it alone. Clive Neal from the University of Notre Dame in the US says lunar geologists have already submitted three separate proposals to NASA for a sample-return mission from the basin – dubbed MoonRise – to understand how old it is. But each time, NASA has considered it too risky to give it the go-ahead. Neal adds that the successful landing of Chang’e-4 shows that such a mission is technically feasible, and the Chinese lander’s scientific findings now give strong support for a future NASA mission to the far side.

For now, though, the world has only one craft on the far side of the Moon and scientists will be anxiously waiting for more results to emerge from the Chang’e-4 mission, especially its newly commissioned radio-astronomy payloads and additional analyses of the lunar mantle. Indeed, planetary geologist Jim Head from Brown University in the US even compares Chang’e-4’s mission to the 16th-century Chinese novel Journey to the West – a legendary pilgrimage of a Tang-dynasty monk who travelled to central Asia and India to obtain sacred Buddhist texts through much personal suffering. “The messages from Chang’e-4’s journey are fundamentally meaningful,” he says. “They inspire us to pursue samples from that area for further study and interpretation.”