The future Chang'e 4 lunar farside landing mission is rapidly taking shape. As I described earlier this year, a relay orbiter will launch in 2018 for the Moon-Earth L2 point and a lander and rover (upgraded from Chang'e 3) will follow six months later. Now the mission's team is coming to a consensus on the landing location, as well as on the mission's instrument package. But, to the surprise of geologists in the international lunar science community, the package does not seem to include an instrument dedicated to studying the elemental chemistry of those never-before-sampled farside rocks.
News on Chang'e 4 has been coming out from a variety of sources, including mainstream media articles like this one (in Chinese), as well a paper by Wang Qiong and Liu Jizhong, recently accepted in Acta Astronautica: "A Chang’e-4 mission concept and vision of future Chinese lunar exploration activities." The launch dates quoted in the Wang and Liu article have shifted forward in time a little bit since my last article, with the relay orbiter launch expected "in the end of 2018" and the lander/rover "in the first half of 2019." Unlike missions to more distant worlds, Moon launch dates are fairly flexible and can usually be shifted a month at a time to ensure mission readiness and success at launch. I suspect Chang'e 4's schedule is sensitive to any changes to the Chang'e 5 sample return mission schedule. Chang'e 5 should launch and return in 2018, before the Chang'e 4 mission.
Both the media reports and Wang and Liu state that the Apollo basin is the most likely target for Chang'e 4. This is a large, double-ring impact basin on the lunar farside. They cite a paper about Chandrayaan-1 data indicating that Apollo may harbor materials excavated from the lower crust. But the Wang and Liu paper also mentions other mare basalt sites inside and outside south pole-Aitken, including Moscoviense, Orientale, Ingenii, and Australe. The mare basalts present in all of these locations give them comparatively flat floors -- flat, that is, for the farside. But the lunar farside is much more topographically variable than the nearside, and Wang and Liu that the lander's guidance systems will need improvements to ensure that they can land safely in the more rugged terrain.
So far, the named scientific payload elements include:
- Relay satellite:
- There will be a low-frequency radio spectrometer provided by the Netherlands, which also seems to have an element on the lander.
- According to Wang and Liu, the payload may include an impact flash camera and a sodium emission monitoring camera, either "developed from scratch or acquired through international cooperation."
- Two of the instruments will be the same as for Chang'e 3: descent camera and topography camera. On Chang'e 3, the topography camera failed in the first lunar night; I wonder if they will be able to modify it to last longer, or if it will be the same instrument.
- New instruments include "a Lunar Dust Analyser (LDA) for lunar dust physical characteristics measurements, an Electric Field Analyser (EFA) to measure magnitude of electric field at different elevations, a Plasma and Magnetic Field Observation Package (PMFOP), a Lunar Seismometer (LS) for lunar internal structure and impact investigations, [and] a VLF Radio Interferometer (VRI) for radio astronomical observation." The seismometer is mentioned only in the Wang and Liu paper, not in the media reports.
- There will also be a neutron dosimeter to take measurements relevant to future human exploration, to be contributed by Germany for the lander. According to the Kiel University press release, the dosimeter is a thermal neutron spectrometer and therefore could measure water content of the regolith beneath the lander.
- The rover will carry three of the four Yutu instruments, including panoramic camera, ground-penetrating radar, and an infrared spectrometer. It will, however, not carry an alpha-particle X-ray spectrometer. While I was at the Lunar and Planetary Science Conference in March, I learned that the APXS is being removed because the Chang'e 4 rover will not have a robotic arm to place it.
- New instruments include "an Active Source Hammer (ASH) for active source seismic experiments, and a second VLF Radio Receiver (VRR)." Again, the seismology-related instrument is mentioned in the Wang and Liu paper but not in the media reports.
- There will also be an energetic neutral atom analyzer, to be contributed by Sweden for the rover. It has heritage from a similar instrument flown in lunar orbit on Chandrayaan-1.
- Finally, as reported here before, two instruments will be selected from a public competition. According to Xinhua news, the public competition has yielded 257 submissions. The results of the competition will be announced in September.
Lunar geologists worldwide have been pushing for a landing on the lunar farside -- and, even better, sample return -- for a long time. Much of the farside is covered by a feature called the South Pole-Aitken Basin. It is an enormous impact basin, stretching between the south pole and Aitken crater, that is the oldest discernible feature on the Moon and among the largest impact basins in the whole solar system. Analysis of data from orbital missions suggests that its rocks are unique and have likely not been sampled by any previous lunar landing. The rocks may have a unique composition because they have sampled lunar mantle, or because they represent the composition of what would have been an enormous sheet of impact melt, or some other reason.
Although the Chang'e 4 mission has a stated goal to "study regional geochemistry", the removal of the APXS from the Chang'e 3 Yutu instrument package -- and the lack of any replacement instrument that can get at the elemental composition of the surface -- will make it hard to do geochemistry. There is still a visible and near-infrared spectrometer, but it's not easy to get to mineralogy from spectrometry without some help from elemental chemistry. I was in a room with lunar geologists receiving a briefing on Chang'e 4 at the Lunar and Planetary Science Conference in March, and I think it's not too strong a statement to say the scientists present were dismayed by the thought of a lander being sent to the south pole-Aitken basin without such an instrument.
Instead, Chang'e 4's robust science package has more of a physics focus. It may have a seismology station with a mobile roving hammer, and ground-penetrating radar to look at the subsurface, and several instruments focused on the farside space environment, taking advantage of the bulk of the Moon to shield its sensitive radio instruments from radio frequency interference from Earth. Hopefully Chang'e 4 can set a precedent for farside lunar landings, and we'll get our dedicated geology mission someday, from China or NASA or somebody else. While I'm daydreaming: maybe successes of Chang'e 4 and Chang'e 5 will pave the way for a future farside sample return mission -- a future Chang'e lander? or something like MoonRise? We can do lunar geochemistry much more effectively with returned samples than with the limited capabilities of a lander.
On that note, one other interesting items from the Wang and Liu article: The relay satellite will use "standard CCSDS protocols...to ensure international compatibility," and they suggest that other nations could use it for relay in future missions. It wouldn't be the United States, because federal funding can't be used for cooperation with China. But it could be available for anyone else.