How did China decide where to land its upcoming Moon missions?

I became interested in landing site selection in 1987. As a child of Apollo, a teenager watching live TV coverage of people on the Moon, I didn't pay much attention to the selection of landing sites, but in 1987 I was unexpectedly introduced to the process. It was my first LPSC—the Lunar and Planetary Science Conference, a meeting of planetary scientists which in those days was held at the Johnson Space Center (JSC), south of Houston. This, the 18th LPSC, was held in the JSC gym building, the Gilruth Center, and on the last day it shut down at noon. Having a free afternoon I wandered over to the Lunar and Planetary Institute (LPI) to see what was happening there. Today LPI is housed in a building several kilometers away near the University of Houston at Clear Lake, but in those days it occupied a beautiful old mansion just outside the JSC fence. I poked my head in the building, but it seemed to be empty as the staff worked on the conference closeout, so I walked through to the LPI library in another building. There I met the librarian, Fran Waranius, who showed me some of their holdings including the voluminous minutes of the Apollo Site Selection Board and related groups who chose the Apollo sites. It hadn't occurred to me before that there was a long story behind landing site selection.

Fast forward 30 years. I have studied lunar and Mars exploration history and published books about missions to the Moon and Mars, and from the earliest stage of planning I decided to tell the story of site selection for every mission. Often I had to go back to the original source materials like those volumes of minutes, or to the people who did the work, including Ewen Whitaker for the Ranger impact missions and Don Wilhelms for Apollo. Now I am updating my decade-old Moon book and trying to keep up with site selection work going on today around the world, for planned missions from Europe, Japan, India and Russia, for Google Lunar X Prize teams, and for two upcoming Chinese landers. Chang'E 5 will conduct a sample return mission late in 2017, and Chang'E 4 will carry a rover to the far side of the Moon, the first mission ever to do so, about a year later. The apparent reversal of numbering is a result of the original sequence of missions planned by China (two orbiters, two landers with rovers, two sample returns), which were given numbers according to their positions in that sequence rather than the launch date. As it turns out, there is now a possibility that the numerical order will be followed because a test launch of the large new rocket which Chang'E 5 will ride on failed on July 2nd this year, and it may not be ready to launch the spacecraft in 2017. 

Landing sites for NASA missions such as Curiosity or the next rover, Mars 2020, are often chosen through a series of open workshops and meetings, gradually reducing a long list of candidates to a shortlist before choosing a specific site. I have been told by Chinese researchers that their landing site planning is not yet developed to that level. Basic guidelines are established within the lunar exploration program, and though a few scientists usually offer suggestions for sites the final decision is made internally. A broad landing region will be selected first, and a specific point within it chosen closer to the time of flight. So, how were the Chang'e 5 and 4 landing sites chosen?

For Chang'E 5, a sample return mission, the basic requirement was for a site on the Earth-facing hemisphere at a latitude around 45º north or south to match thermal constraints on the hardware. The mid-latitudes are a bit cooler than the equatorial region in the middle of the lunar day. The sample should also be scientifically useful, a kind of material not sampled by the Apollo or Luna spacecraft more than 40 years ago. Opinion from the start seems to have favoured northern Oceanus Procellarum. As early as 2013 Associate Professor ZongCheng Ling of Shandong University had suggested the Aristarchus area as a sample return site because of its diverse volcanic rocks.

Fei Li, a Professor at Wuhan University, and his colleagues submitted an article about orbit tracking for this mission to the journal Advances in Space Research late in 2015. No site had been selected in the area of interest, so the study used a fictitious site about 75 km east of the Soviet Union's Luna 13 lander as an example. By the time that paper was published in early 2016 the area around the Rümker Plateau (or Mons Rümker, a volcanic upland more than 20 degrees further north) had become the favored region. Jiannan Zhao, a graduate student at the Planetary Science Institute, China University of Geosciences (also in Wuhan) and his colleagues presented arguments for two landing sites on the Rümker Plateau itself at the Lunar and Planetary Science Conference in March 2016 (figure below). A version of that study published in the Journal of Geophysical Research later that year had two different sites, but both pairs of sites had something in common. One site in each pair was on a volcanic dome in the southern part of the plateau, the other was in a more rugged region on the north edge of the plateau, possibly covered by ejecta from a large impact older than the surrounding lava flows.

By 2017 the favored site had shifted off the plateau to younger volcanic rocks in the plains east of the plateau, and arguments for this location were presented at LPSC in March 2017 by Zongcheng Ling, who had suggested Aristarchus earlier. The young basalts here should be substantially younger than any collected by previous missions and will fulfill a major goal of lunar science, helping to determine how long volcanism persisted on the Moon. 

But those young basalts cover a large area. The colourful image below is a composite of Lunar Reconnaissance Orbiter images and multispectral data from NASA's Clementine mission. The bright colors are obviously far from realistic, but they indicate variations in composition derived primarily from infrared wavelengths. The blue material is the young basalt unit. At a meeting in Vienna in April 2017, Xingguo Zeng, a researcher in the China Lunar Exploration Program at the National Astronomical Observatories in Beijing, and colleagues identified a rectangular landing area and presented a list of seven suggested sites in that area. These were still only suggestions, and the final decision will be made closer to the launch date by the mission management.

Chang'E 4 is a lander and rover mission directed to the far side of the Moon. A relay satellite will be launched first, to circle around the L2 Lagrangian point above the lunar farside. There are plenty of interesting targets on the far side, but most attention has been directed at the gigantic South Pole-Aitken basin (SPA). A basin is a giant crater, often characterized by multiple concentric rings of mountains rather than a single crater rim, and though some have individual names (e.g. Hertzsprung, Schrödinger) many others do not. Those basins tend to be named after things which roughly delineate the extent of the basin, such as Mendel-Rydberg basin south of the well-known Orientale basin. Its extent is roughly defined by the two craters Mendel and Rydberg. Similarly the South Pole-Aitken basin spans the region between the Moon's South Pole and the large crater Aitken, which is near 20º south in the central far side highlands. Don't be misled by statements like 'Aitken basin at the south pole' which are all too common in press reports. Chang'E 4 will NOT land at the South Pole, or in Aitken.

A sample return mission called Moonrise has been suggested for NASA's New Frontiers program, so a lot of work on its preferred landing site has been done in the last few years, especially by Brad Jolliff at Washington University in St. Louis and his colleagues. Their goal is material which will establish the age of the SPA, especially rock from the vast pool of molten material (the melt sheet) which formed the floor of the basin just after it formed in a giant impact. I think it was somewhat taken for granted that Chang'E 4's target would be the same. But Chang'E 4 has other goals including radio astronomy from the radio-sheltered far side, and it is not bound by the same site selection constraints. 

In 2016 Qiong Wang and Jizhong Liu of the Lunar Exploration and Space Engineering Center, part of the China National Space Administration (CNSA), identified five impact basins which might be suitable targets (Orientale, Australe, Moscoviense, Apollo and Ingenii), but most of these are at latitudes too low for a Chang'E mission. Thermal constraints require that the site should be near 45º north or south, as for Chang'E 5 and Chang'E 3. At LPSC in 2017, Yangxiaoyi Lu of the Beijing Planetarium and Vladislav Shevchenko from the Sternberg State Astronomical Institute in Moscow advocated for a site near a collapse pit or 'skylight' giving access to an underground cavity in Mare Ingenii. This is at 36º south. Apollo basin is near 45º south and has been rumoured to be a likely site for the mission. Since the southern parts of Apollo cut into the SPA melt sheet, sampling its ejecta might resolve the age of SPA.

But new information suggests that Apollo is not the target. China has been actively recruiting foreign partners in the mission, including Saudi Arabia and Germany, and a presentation on the mission at the German Aerospace Centre (DLR) included a slide which was illustrated on the www.chinaspaceflight.com website on 6 July 2017. It showed nine midlatitude regions considered for the mission (white boxes in the figure above). Two of them lie south and west of the Apollo basin, but three others are further west in SPA. One of those contains the crater Von Kármán, named after the Caltech professor instrumental in creating JPL, and a site in Von Kármán was described as the likely landing site.

Von Kármán's floor is very smooth and would make a safe site, but it is covered with lava flows rather than SPA impact melt, so it does not satisfy some researchers. The lava flows are covered in places by ejecta from the crater Finsen just outside Von Kármán, ejecta which may contain SPA fragments, so very small amounts of SPA material may be collected here. However, a site outside Von Kármán would sample much more SPA rock, and Paul Spudis of LPI has identified a suitable site east of Von Kármán in his blog. 

The date of Chang'E 5 might be delayed, possibly into 2019, because of its launch vehicle failure. The schedule is not clear yet, but possibly we will see Chang'E 4 in 2018, Chang'E 5 in 2019, and a second sample return mission in 2020. This is Chang'E 6, the final mission in the series, and it was recently suggested that it will land on an illuminated ridge near the south pole. Mission simulations used the rim of Shackleton crater, very close to the pole, as target. Earlier, people had assumed that it might go to the Chang'E 4 area in SPA. There is still some possibility for confusion here because there are also suggestions of two future polar landers to follow the Chang’E program, and the Shackleton simulations may be related to those missions. Whatever happens, these Chinese missions will be exciting additions to the story of lunar exploration. The landing site work still has a way to go, and it will be interesting to see if any further changes take place.