Chang'e-4 far side Moon-landing Mission of China
China plans to send the Chang'e-4 lunar probe to land in the south pole region of the far side of the moon in 2018, according to CNSA (China National Space Administration). This lunar exploration mission will incorporate an orbiter, a robotic lander and rover. Chang'e-4 will be China's second lunar lander and rover. The Chang'e-4 lander was a backup to the Chang'e-3 mission, so it will have the same basic structure, but a different scientific payload, holding 11 instruments. 1) 2)
With its special environment and complex geological history, the far side of the moon is a hot spot for scientific and space exploration. However, landing and roving there requires the relay satellite to transmit signals. Hence, the project plans to send a relay satellite for Chang'e-4 to the halo orbit of the Earth-Moon Lagrange Point L2 in late May or early June 2018, and then launch the Chang'e-4 lunar lander and rover to the Aitken Basin of the south pole region about half a year later, said Tongjie Liu, deputy director of the CNSA's Lunar Exploration and Space Program Center. 3) 4)
The lander of Chang'e-4 will be equipped with descent and terrain cameras, and the rover will be equipped with a panoramic camera. Like China's first lunar rover Yutu, or Jade Rabbit, carried by Chang'e-3, the rover of Chang'e-4 will carry subsurface penetrating radar to detect the near surface structure of the moon, and an infrared spectrometer to analyze the chemical composition of lunar samples.
But unlike Chang'e-3, the new lander will be equipped with an important scientific payload especially designed for the far side of the moon: a low-frequency radio spectrometer.
"Since the far side of the moon is shielded from electromagnetic interference from the Earth, it's an ideal place to research the space environment and solar bursts, and the probe can 'listen' to the deeper reaches of the cosmos," Liu said.
The Chang'e-4 probe will also carry three scientific payloads, respectively, developed by the Netherlands, Sweden and Germany, according to Liu.
The low-frequency radio spectrometer, developed in the Netherlands, will be installed on the Chang'e-4 relay satellite. The Dutch and Chinese low-frequency radio instruments will conduct unique scientific studies such as measuring auroral radio emissions from the large planets in the solar system, determining the radio background spectrum at the Earth-Moon L2 points, creating a new low-frequency map of the radio sky, and detecting bright pulsars and other radio transient phenomena.
"The Chinese and Dutch low-frequency radio spectrometers on the lander and relay satellite of Chang'e-4 might help us detect the 21 cm hydrogen line radiation and study how the earliest stars were ignited and how our cosmos emerged from darkness after the Big Bang," said Xuelei Chen, an astronomer with the National Astronomical Observatories of the Chinese Academy of Sciences.
The rover will also carry an advanced small analyzer for neutrals, developed in Sweden, to study the interaction between solar winds and the moon surface.
And a neutron dosimeter, developed in Germany, will be installed on the lander to measure radiation at the landing site. Scientists say it is essential to investigate the radiation environment on the lunar surface, in preparation for human missions to the moon.
The Chang'e-4 mission will also be the first time that a satellite is sent to an unexplored region on the far side of the Moon. This region is none other than the South Pole-Aitken Basin, a vast impact region in the southern hemisphere. Measuring roughly 2,500 km in diameter and 13 km deep, it is the single-largest impact basin on the Moon and one of the largest in the Solar System. 5)
This basin is also source of great interest to scientists, and not just because of its size. In recent years, it has been discovered that the region also contains vast amounts of water ice. These are thought to be the results of impacts by meteors and asteroids which left water ice that survived because of how the region is permanently shadowed. Without direct sunlight, water ice in these craters has not been subject to sublimation and chemical dissociation.
Since the 1960s, several missions have explored this region from orbit, including the Apollo 15, 16 and 17 missions, the LRO (Lunar Reconnaissance Orbiter) and India's Chandrayaan-1 orbiter. This last mission (which was launched in 2008) also involved sending the Moon Impact Probe to the surface to trigger the release of material, which was then analyzed by the orbiter.
The significance of soft landing exploration on the Lunar farside (Ref. 9).
At Lunar farside, numerous highland terrains are distributed all over, including the highest peak, which is up to 10.9 km. In the highland area, craters and mountains are widely spread. The well-known South-Pole Aitken Basin (SPA) is located in the southern part of the farside, with the central area at latitude 40°–60°S, longitude around 180° and the diameter ranging from 2000–600 km. This is the largest-scaled and eldest impacted basin in the solar system and of high scientific interest. 6)
The farside of the Moon, for its special location, is of unique peculiarity that the nearside cannot match. On one hand, the farside shields all kinds of radio waves emitted from the Earth, thus becomes the best place for cosmic radio spectrum detection. On the other hand, the original information of the Moon is hidden in the largest, deepest and eldest SPA. It is crucial for the study of the history, evolution, composition and components of the deep-layer of both the Moon and the Earth system. Besides, how SPA is formed remains controversial and deserves further research. Soft landing on the SPA as well as rovering exploration of it are of great scientific significance mainly in the following two aspects.
Planetary formation and evolution:
1) The study of SPA may benefit the discovery of material composition of Lunar crust and mantle. So it opens an important window to the study of the deep-layer material composition of the Moon.
2) SPA is a basin (its altitude is 13 km lower than its surrounding highlands) and of thin crust. Whether in the passive or active modes that bring out the Lunar mare basalt, there should have emerged large amount of basalt in SPA. However, currently obtained data cannot effectively prove that the basin has abundant basalt. On the other hand, absence of basalt may indicate something happened in the process of Lunar thermal evolution and differentiation in early times.
3) Comparing the craters in SPA with the Lunar mare we can see that the degradation situation of SPA is not obvious. Also no crater with Lunar rays has been discovered. Therefore the formation, evolution, topography and chemical characteristics of craters in the SPA are apparently different from those of other terrains.
Figure 1: Elevation diagram of the South-Pole Aitken Basin (scale in km). This LOLA (Lunar Orbiter Laser Altimeter) image on NASA's LRO (Lunar Reconnaissance Orbiter) mission centers on the SPA (South Pole-Aitken) basin, the largest impact basin on the Moon (diameter = 2600 km), and one of the largest impact basins in the Solar System. The distance from its depths to the tops of the highest surrounding peaks is over 15 km, almost twice the height of Mount Everest on Earth. SPA is interesting for a number of reasons. To begin with, large impact events can remove surficial materials from local areas and bring material from beneath the impact craters to, or closer to, the surface. The larger the crater, the deeper the material that can be exposed. As SPA is the deepest impact basin on the Moon, more than 8 km deep, the deepest lunar crustal materials should be exposed here. In fact, the Moon's lower crust may be revealed in areas within SPA: something not found anywhere else on the Moon (image credit: NASA/GSFC)
Ideal observation site for low-frequency radio (Ref. 9)
The astronomical observation of radio waves is one of the most effective methods to study and understand the universe. At present, most portion of the spectrum has been detected, such as ultraviolet wave (in 1890s), radio wave (wavelength less than meters, in 1930s), X-ray (in 1940s), infrared and millimeter wave (in 1950s), Gamma-ray (in 1960s). But no myriametric wave (<30 MHz) has been detected yet. The detection of myriametric wave is of much importance for all-sky imaging obtained by continuous sky scanning of discrete radio source, cosmic dark times study (21 cm radiation in dark times), solar physics, space weather, extreme-high-energy cosmic ray and neutrino study. 7)
Interfered by ionosphere and Earth radio waves, it is impossible to detect myriametric wave on the Earth. In earlier times, wave detection satellites are RAE-A/B (NASA). RAE-A was launched in 1968 and operated in near-Earth orbit. Its scientific objective was to detect the intensity of cosmic ray (0.2–20 MHz). But it was interfered by radio waves in Earth orbit. RAE-B was launched in 1973 and was injected into the lunar orbit, whose scientific objective was to detect the long-wavelength radio waves (working frequency 25 kHz–13.1 MHz). It demonstrated that the lunar farside is ideal for myriametric wave detection. 8)
At present, low-frequency radio detection was mainly achieved via spacecraft operating in circumlunar orbit by foreign countries but none of them has done this on the Lunar surface.
The exploration of Change'4 will further promote people's understanding of the farside of the Moon. With comprehensive analysis and study on the nearside exploration data, more general understanding about the Moon will be obtained and the reliability of a theoretical system will be increased.
To land on the farside and carry on in-situ exploration has become one of the great targets for lunar exploration of all nations. Nevertheless, no country had success in landing on that side. One of the major reasons is that landing on the farside of the moon needs to meet more technical challenges. Compared with the nearside, the technical difficulties of landing on the farside involve the following three aspects.
Safe landing under complex landform situation (Ref. 2):
To ensure the landing safety, guidance, navigation and control method should be optimized to adapt to the complex terrain. On the other hand, with careful orbit design and control, the distribution of landing sites is reduced for landing safety.
1) Influence of the topographic relief. In the descent process, the length of path of the lander is almost 450 km on the Lunar surface. During this process, the distance and velocity should be measured for navigation of the descent. At the farside of the Moon, the topographic relief is more apparent than that at the nearside, with the topographic elevation difference increased from 3 km to 7 km. The difference brings out the jump of navigation information and extreme difficulties for control strategy. Therefore, the sectional control target and navigation information utilization time as well as navigation algorithm should be optimized to decrease the influence of the great topographic relief.
2) Decrease of the backscattering coefficient. Research results show that the site on which Change'3 has landed at the nearside of the Moon is opposite to the SPA(South-Pole Aiken) basin at the farside of the Moon, and the contents of FeO and TiO2 are 15%–25% and 0%–15%, while the average value of them on the nearside of the Moon are 15%–20% more than that on the farside. The low contents of FeO and TiO2 decrease backscattering coefficient of microwaves and directly influence the echo characteristics of range and velocity microwave sensor (RVS). Therefore, the signal emission energy and signal-noise-ratio should be increased to obtain useful measuring data.
3) Reducing the distribution of landing sites. Due to the complex terrain of the farside, it is difficult to find out a large flat area. For different launch windows, the landing sites may be widely distributed without precise orbit control and descent process control. Different from Change'3, the circumlunar orbits with various inclination angles for different launch windows are designed for Change'4 specific requirements. Meanwhile, the orbit is carefully regulated in the circumlunar phase. With in-orbit calibration of the 7500 N thrust-variance engine, the powered descent process path is precisely controlled and the landing sites are reduced to a flat and small area.
Power supply with RTG (Radioisotope Thermoelectric Generator) and thermoelectric technology application:
For data collection of the temperature of the Lunar soil in moon night, the temperature sensor has to have a power supply. Currently, the most realistic method is to utilize the RTG, which is based on Seebeck principle to transform the heat energy into the electric energy. When generating electric power, the RTG can also supply great heat energy to regulate temperature. Generally Pu-238 is used as the RTG source.
Compared with the RTG used in Change'3, the thermoelectric module should be increased in the RTG of Change'4, which may mean various technical difficulties, including high-performance thermoelectric material manufacture, high-performance thermoelectric components design and assembly, high-efficiency heat transformation, etc.
1) High-performance thermoelectric material manufacture technology. Thermoelectric material is the core material in the RTG. The electric and mechanical performance and the thermal stability directly determine the reliability and stability of the RTG. Currently, PbTe and CoSb3 are used, while PbTe is well developed but with lesser performance. For further design, the chemical dose ratio adjustment, the doping impurity selection and manufacture process should be optimized to make the material meet the engineering requirement.
2) High-performance thermoelectric components design and assembly technology. The thermoelectric components are comprised of π type pairs of the P-type and N-type materials and directly determine key parameters such as the output voltage, the inner resistor and the output power of the RTG. The design and assembly of the energy transfer components determine the reliability, including the components joint process design, the matching design for the thermal expansion coefficient of the electrode material and substrate material and the shelter layer and transition layer optimization. For future improvements, the different temperatures in both nighttime and daytime on the Moon should be considered. Calculation, simulation, test and verification should be done to satisfy engineering requirements.
3) High-efficiency thermoelectric transformation technology. The thermoelectric transformation efficiency and decay rate of the RTG directly determines the lifetime and the performance of the RTG and is a vital segment for the success of the mission in moon night. The key technology is thermal flow design of the RTG, which aims to maximize the heat transformation from thermoelectric components to heat collector. Meanwhile, the RTG shell should have the layer of high emission ratio and low-absorption ratio to ensure the safe of RTG under the high temperature at Lunar daytime.
For decades, this basin has been a source of fascination for scientists; and in recent years, multiple missions have confirmed the existence of water ice in the region. Determining the extent of the water ice is one of the main focuses of the rover mission component. However, the lander will also to be equipped with an aluminum case filled with insects and plants that will test the effects of lunar gravity on terrestrial organisms.
These studies will play a key role in China's long-term plans to mount crewed missions to the Moon, and the possible construction of a lunar outpost. In recent years, China has indicated that it may be working with ESA (European Space Agency) to create this outpost, which ESA has described as an "international Moon village" that will be the spiritual successor to the ISS.
Figure 2: The Chinese lunar rover, part of the upcoming Chang'e-4 mission to the far side of the Moon Image credit: CASC/China Ministry of Defense)
The Chang'e-4 mission will consist of a relay orbiter, a lander and a rover, the primary purpose of which will be to explore the geology of the South Pole-Aitken Basin. Chang'e-4 is the backup to the Chang'e-3 mission which put a lander and rover on Mare Imbrium in late 2013. Following that success, the lunar craft have been repurposed for a pioneering landing on the moon's far side s to communicate via the Chang'e-4 Relay (Queqiao) satellite. 9)
Figure 3: Illustration of China's Chang'e-4 rover in the South Pole-Aitken Basin on the far side of the moon (image credit: CNSA, CASC, Ref. 10)
The launch mass of Chang'e-4 spacecraft is ~ 3780 kg; the lander has a mass of ~1200 kg, while the rover has a mass of 140 kg.
Development status of Chang'e-4 mission:
Figure 4: Payload fairing for the Chang'e-4 lunar far side mission (image credit: CASC)
• On 16 August 2018, Chinese scientists unveiled their Chang'e 4 lander and rover mission, saying it will be launched in December 2018 on a mission to land on the far side of the moon. 10)
- The announcement was made at a news conference in Beijing, held by SASTIND (State Administration of Science, Technology and Industry for National Defense), which oversees China's space activities. The configuration of the spacecraft and a contest to name the mission rover were also unveiled.
- The Chang'e-4 spacecraft will target a landing region within the South Pole-Aitken Basin, a vast impact crater of immense scientific interest, with potential landing areas previously identified in and around the Von Kármán crater. The final landing site is understood to have been selected but has not been revealed.
Figure 5: Artist's rendering of the Chang'e-4 lunar far side lander, released in August 2018 (image credit: CNSA, CASC)
Legend to Figure 5: Visible on the newly released image of the lander are the antennas for the LFS (Low Frequency Spectrometer), which will take advantage of the uniquely quiet electromagnetic environment offered by the far side of the moon.
- Images displayed displayed at the press conference showed the rover was a rectangular box with two foldable solar panels and six wheels. It is 1.5 meters long, 1 meter wide and 1.1 meters high.
- Wu Weiren, chief engineer of China's lunar exploration program, said the Chang'e 4 consists of two parts - a lander and a rover, and both carry multiple scientific instruments. The probe's design is based on its predecessor, the Chang'e-3 (Yutu), but with some modifications.
Launch: The Chang'e-4 lander and rover mission was launched on 7 December 2018 (18:24 UTC, local time 02:24 on 8 December) from the XSLC (Xichang Satellite Launch Center) in the southwest of the country atop a Long March 3B/E launch vehicle (Ref. 10). 11) 12)
Figure 6: Launch of the Long March 3B rocket carrying Chang'e-4 on 7 December 2018 at the Xichang Satellite Launch Center in southwest China (image credit: CASC)
CASC (China Aerospace Science and Technology Corporation), the main contractor for China's space program, officially announced success of the launch following trans-lunar injection, just under an hour after launch. The spacecraft now enters a five-day voyage to the moon before lunar orbit injection.
No official date has been released for the landing attempt, but CASC announced shortly after launch that the landing will take place in the first days of January 2019, following sunrise over the main candidate landing within the Von Kármán crater in late December.
As the far side of the moon never faces the Earth, communications with the spacecraft will be facilitated by the ‘Queqiao' relay satellite launched in May and inserted into a halo orbit around the second Earth-moon Lagrange point in June 2018. - From this vantage point between 65,000-85,000 kilometers beyond the moon the Queqiao satellite will have constant line-of-sight with both the Chang'e-4 spacecraft and Chinese ground stations in China, at Kashi and Jiamusi, Namibia and Argentina.
Possible landing sites of Chang'e-4
The South Pole-Aitken Basin (SPA) is large—roughly 2,500 km in diameter. A number of teams, including some scientists from outside China, are looking at possible landing areas around Von Kármán crater and the Apollo basin. Phil Stooke, who produced a fascinating post on how China decides where to land its upcoming Moon missions, has listed and mapped the areas noted in papers on potential landing sites in the SPA, listed S1 through S5 (Ref. 22).
Figure 7: A map of the lunar far side showing known candidate landing areas for Chang'e-4. The red strip represents the most likely region as of August 2018 (image credit: Phil Stooke)
The candidate landing region is 45°S-46°S and 176.4°E-178.8°E, which is in the southern floor of the Von Kármán crater, within the SPA basin (Figure 8a). Von Kármán is a pre-Nectarian crater of 180 km in diameter. Mare basalt flows filled the crater floor subsequently at ~3.35 Ga, but a portion of possible central peak remains near the center of the crater (Figure 8 b). NRC (National Research Council) has previously identified goals for future sample return mission in Von Kármán crater, including the possibility to study the existence and extent of differentiation of the SPA melt sheet and possible exposed upper mantle materials. 13)
The Chang'e-4 project analyzed the topography of the candidate landing region using the LRO (Lunar Reconnaissance Orbiter) LOLA (Lunar Orbiter Laser Altimeter) and Kaguya (Selene mission of JAXA) TC (Terrain Camera) merged DEM (Digital Elevation Model), the spatial sampling is 512 ppd (59 m/pixel). Spectral analyses were performed using the Kaguya Multiband Imager (MI) and Chandrayaan-1 M3 (Moon Mineralogy Mapper) data. Geomorphologic characteristics were studied with LRO WAC (Wide Angle Camera), NAC (Narrow Angle Camera), and TC images.
Figure 8: a) The red star indicates the candidate landing region within the Von Kármán crater in SPA basin (Background = LOLA topography). b) Context of candidate landing region in the southern portion of Von Kármán crater floor (LROC WAC mosaic). c) TC morning mosaic of the landing region shown in b; a large number of secondary craters can be observed (image credit: Chang'e-4 landing site team)
Figure 9: Alternate view of the landing site scenario (image credit: Chang'e-4 landing site team)
Power descent of Chang'e-4
The following two aspects are carried out during the power descent phase: first, the oblique forward motion trajectory is changed to the vertical down trajectory after the main deceleration phase. Furthermore, the pointing of ranging is consistent with the location of the landing point. Second, in order to ensure the correctness of navigation results in altitude, the range sensor is introduced to modify the navigation filtering algorithm. The power descent of Chang'E-4 is shown in Figure 10. 14)
Figure 10: Planned power descent sequence of Chang'e-4 (image credit: Beijing Aerospace Control Center)
Sensor complement of the Chang'e-4 lander (LFS, LCAM, TCAM, LND)
Bearing in mind the features of the farside of the Moon, scientific payloads usually include a low-frequency radio spectrometer (LFS), an infrared spectrometer, a panoramic camera, a lunar radar, etc. The LFS is newly developed for the farside exploration and is implemented in the lander and the rover for the comparison and analysis of data collected (Ref. 9).
LFS (Low Frequency Spectrometer)
Low-frequency radio frequency spectrum analyzer for detection of low-frequency radio frequency characteristics of the sun and the moon's low-frequency radio environment to perform low-frequency radio astronomy observations.
The LFS is used for detecting the low-frequency electric field of the solar storm and to study the Lunar plasma. By detecting the low-frequency electric field from the Sun, the planetary space and the galactic space, the information of electric magnitude, phase, time variance, frequency spectrum, polarization and DoA (Direction of Arrivals) are collected for analysis. With features of variation of the spatial low-frequency electric field, the Lunar plasma environment above the landing site will be analyzed. The LFS is configured with a three-component decomposition active antenna to receive electromagnetic signals from the Sun and from space. Each of the three antenna units receives one of the three orthogonal components of the electromagnetic signals. According to radio transmission theory, information such as the electromagnetic intensity, the frequency spectrum, the time variance, the polarization features and the direction of radiation source are obtained by analyzing and processing the exploration data.
LCAM (Landing Camera)
LCAM is used for optical imaging of the landing area during descent to investigate surface morphology and geological structure.
TCAM (Terrain Camera)
The objective of TCAM is for optical imaging of the landing area to investigate surface morphology and geological structure.
LND (Lunar Lander Neutrons and Dosimetry Experiment)
The LND effort is led by the Christian-Albrechts-University in Kiel (CAU), Germany, with contributions from the Institute for Aerospace Medicine of the German Space Center, the National Space Science Center (NSSC), the National Astronomical Observatories (NAOC) from the Chinese Academy of Sciences (CAS), and the China Academy of Space Technology (CAST). LND is supported by DLR (German Space Agency) through the Federal Ministry of Economics and Technology.
The LND instrument will be accommodated on the Chang'e-4 Lander and has two major science objectives: 15)
4) dosimetry for human exploration of the Moon and
5) contribute to heliospheric science as an additional measuring point.
To achieve the first objective, LND is designed to measure time series of dose rate and of linear energy transfer (LET) spectra in the complex radiation field of the lunar surface. For the second objective, LND is capable to measure particle fluxes and their temporal variations and thus will contribute to the understanding of particle propagation and transport in the heliosphere. Its stack of 10 silicon solid-state detectors (SSDs) allows to measure protons from 10-30 MeV, electrons from 60-500 keV, alpha particles from 10-20 MeV/n and heavy ions from 15-40 MeV/n. In addition, LND can measure fast neutrons in the energy range from 1-20 MeV and, using two Gd-sandwich detectors, measure fluxes of thermal neutrons, which are sensitive to subsurface water and important for understanding lunar surface mixing processes.
Instrument concept: The zenith-pointing LND is mounted inside the payload compartment of the Chang'E 4 lander and consists of a thermally decoupled sensor head and an electronics box (Figure 11). The sensor head consists of a stack of 10 SSDs (Figure 4) as well as two Printed Circuit Boards (PCBs). One PCB is used to pre-amplify the detector signals, the other PCB contains shaper circuits and the Analog to Digital Converters (ADCs). These digitized signals are sent from the sensor head to the electronics box which accumulates the data into histograms, count rates, PHA words, etc., packetizes them and sends them to the Instrument Control Unit (ICU). The latter serves as the electrical and data interface with the lander.
Figure 11: Left: The sensor head of LND; Right: The electronics box of LND (image credit: LND Team)
Measuring fast neutrons: Fast neutrons are generated by the interaction of the galactic cosmic rays with the lunar regolith and are an important source of the radiation dose reaching the interior of an astronaut's body. LND uses three segmented Si SSDs which are as closely packed as possible to detect fast neutrons. The innermost segment of the C detector in Figure 12, C1, measures neutral radiation in anti-coincidence with all outer segments (B1&B2, D1&D2, C2). LND's response to neutral radiation (n, γ) is shown in Figs. 7 and 8.
Figure 12: Measurement of fast neutrons (image credit: LND Team)
Sensor Head Design: LND is largely based on developments which were made for IRAS (Ionizing Radiation Sensor) in an early phase of ExoMars. As shown in Fig. 4, LND is basically a telescope consisting of ten segmented SSDs (A-J). Three detectors (B, C, and D) are packed as close together as possible to measure neutrons in the energy range 1-20 MeV (see section 3). The lower six detectors (E-J) are mounted in two different sandwich configurations. In one each sandwich clamps a very thin (~20 µm) Gd foil (shown in red) to detect thermal neutrons. To discriminate thermal neutrons which are emitted from the lunar soil (and are thus sensitive to the subsurface proton (water) content), the GH sandwich is shielded from above by a thicker Gd foil which is encased in two thick Al sheets. The GH sandwich then measures thermal neutrons from below and the EF sandwich thermal neutrons from above. The lowermost sandwich is a copy of the IRAS BCsandwich and serves as the final detector in the stack. The J detector serves as an anticoincidence to discriminate stopping particles from penetrating ones.
Figure 13: The particle telescope of LND (image credit: LND Team)
Measuring thermal neutrons using Gd converters: Natural gadolinium (Gd) has a very large cross section for thermal neutrons (48'890 b). After neutron capture, the Gd nuclei have a large probability to decay via internal conversion emitting electrons with energies of 29, 72, 78, and 132 keV (Figure 14). LND uses a 20 µm thin natural Gd foil as a neutron converter. The electrons which escape from the foil can then be measured in the adjacent Si SSDs in anticoincidence with the surrounding detectors.
LND will have to use a fairly thick (20 µm) foil. While our initial sputtering tests were successful, the sputtered Gd on the flight detectors showed bubbles (Figure 15).
Figure 14: Spectrum of natural Gd conversion electrons (Kandlakunta, 2014), image credit: LND Team
Figure 15: We encountered difficulties in sputtering Gd onto Si (image credit: LND Team)
Geant4 Simulations: We used Geant4 to simulate the performance of LND. Figure 16 shows the instrument response functions (IRFs) for the inner C segment (C1) for gamma and neutron particles. It is dominated by neutrons for energy deposits above 1 MeV.
Figure 16: Instrument response functions for gammas, and neutrons for the C inner detector segment (image credit: LND Team)
As discussed in section 3, LND measures fast neutrons by using an anti-coincidence of C1 with (B1&B2, D1&D2, and C2). Thus, if only the C1 segment was triggered, this was due to a neutron or gamma, because both of them only interact weakly with the Si-nuclei in the detector compared with charged particles, e.g., protons and electrons. The expected spectrum of neutral radiation energy deposits in C1 shown in Figure 17 was obtained using a GEANT4-model of the lunar surface neutron and gamma spectra folded with the LND IRF. It is dominated by gammas at low energies. Above about 700 keV, neutrons start to dominate, e.g., at energies above 1 MeV, we expect only about 0.01 gamma, but ~0.03 neutron counts per second.
Figure 17: Expected spectrum energy deposits in C1 only (image credit: LND Team)
Data Products: The raw data of LND is processed by LND on board and telemetered to Earth via an Earth-Moon L2 relay satellite. LND data products and their cadences are given in the table to the left. LND provides measurements of interest for dosimetry, lunar regolith science, as well as heliophysics. After receipt on ground, instrument response functions will be applied to the data, and visual inspections will be performed at CAU (Christian-Albrechts-University in Kiel, Germany) and at NSSC (National Space Science Center) China. LND data will be made available to the scientific community via the usual channels.
Table 1: Parameters measured
Sensor complement of the Chang'e-4 rover (PCAM, LPR, VNIS, ASAN)
PCAM (Panoramic Camera)
The objective of PCAM is to obtain three-dimensional images of the landing and patrolling lunar surface for investigation of surface morphology and geological structure.
LPR (Lunar Penetrating Radar)
LPR is of Chang'e-3 heritage. The objective of LPR are the mapping of lunar regolith and detection of subsurface geologic structures.
VNIS (Visible and Near-Infrared Imaging Spectrometer)
VNIS is of Chang'e-3 heritage. VNIS is capable of simultaneously in situ acquiring full reflectance spectra for objects on the lunar surface and performing calibrations. VNIS uses non-collinear acousto-optic tunable filters and consists of a VIS/NIR imaging spectrometer (0.45–0.95 µm), a shortwave IR spectrometer (0.9–2.4 µm), and a calibration unit with dust-proofing functionality.
ASAN (Advanced Small Analyzer for Neutrals)
ASAN was developed by the Swedish Institute of Space Physics (IRF) in Kiruna.
• Moon radar for surveying the lunar sub-surface structure to investigate surface morphology and geological structure
• Infrared imaging spectrometer for patrol area lunar surface infrared spectroscopy and imaging exploration to survey lunar surface material composition and available resources
• Lunar neutron and radiation dose detector (LND, Germany) neutral atomic detector (ASAN, Sweden) for observations of energy-neutral atoms and positive ions in patrol area to investigate the particle radiation environment in the patrol area.
Figure 18: The ASAN instrument on Chang'e-4 (image credit: IRF)
On April 7, 2018, the Swedish Institute of Space Physics (IRF) successfully delivered the flight model of the ASAN (Advanced Small Analyzer for Neutrals) instrument to the National Space Science Center of the Chinese Academy of Sciences in Beijing, China. The ASAN instrument will be launched at the end of 2018 onboard the Chinese Chang'e-4 mission to the Moon. 16)
Chang'e-4 relay satellite / Queqiao of China
The Chang'e-4 relay satellite, named Queqiao ('magpie bridge'), is a precursor to an unprecedented attempt to soft-land the Chang'e-4 satellite on the lunar far side in late 2018, when a lander and rover will be send to the Moon. Since the lunar far side does not face the Earth as the moon's orbital period matches its rotational period, a relay satellite is required to facilitate communications between the Chang'e-4 lander on the far side of the moon and ground stations on Earth.
The nickname Queqiao was announced by CNSA (China National Space Administration) on 25 April 2018, China's Space Day. In a Chinese folktale, magpies form a bridge with their wings on the seventh night of the seventh month of the lunar calendar to enable Zhi Nu, the seventh daughter of the Goddess of Heaven, to cross and meet her beloved husband, separated from her by the Milky Way. 17) 18)
This is the main role of the 425 kg spacecraft, developed by the China Academy of Space Technology (CAST), which is being sent into position around six months before the landing mission in order to test and verify is functions.
The 425 kg relay satellite is based on the three-axis stabilized CAST-100 minisatellite bus featuring an 130 N hydrazine propulsion system. It carries a deployable 4.2 m dish antenna for the relay equipment. It provides four 256 kbit/s X-band links between itself and the lander/rover and one 2 Mbit/s S-band link towards Earth.
Figure 19: Illustration of the deployed Chang'-4 relay satellite (image credit: CAST)
Launch: The Chang'4 relay satellite (Queqiao) was launched on 20 May 2018 (21:28 UTC) on a Long March-4C vehicle from the XSLC (Xichang Satellite Launch Center) in China. 19)
Figure 20: Photo of the Queqiao relay satellite, launched ahead of Chang'e-4 lunar mission (image credit: CNSA)
Orbit: Halo orbit of the Earth-Moon L2 (Lagrangian Point 2), around 65,000 km on the far side of the moon, so as to be visible to both ground stations on the Earth and the lander and rover on the lunar far side at all times.
Figure 21: Flight profile of the relay satellite (image credit: CAST)
Two Chinese microsatellites, DSLWP -A and -B (Discovering the Sky at Longest Wavelengths Pathfinder), also referred to as Longjiang-1 and Longjiang-2, were launched with the Chang'e 4 relay mission to conduct astronomical observations from deep space (Selenocentric, elliptical orbit).The two microsatellites were developed by the Harbin Institute of Technology. Each microsatellite has a mass of 47 kg. 20)
They will be inserted into 200 km x 9000 km lunar orbits. The satellites are three-axis stabilized and carry a radio-astronomy payload featuring two linear polarization antennas mounted along and normal to the flight direction, which uses the moon as a shield to avoid radio emanations from earth. Additionally, the microsatellites carry a KACST (King Abdulaziz City for Science and Technology) developed micro-optical camera and an amateur radio communications system.
Figure 22: Photo of the Longjiang-1 and -2 microsatellites at the launch site, which were launched to the Moon with the Queqiao/Chang'e-4 lunar relay satellite on May 20, 2018 (image credit: Harbin Institute of Technology)
Figure 23: The map of the EML (Earth-Moon Liberation) points (image credit: CAST)
The relay satellite orbiting around the Earth-Moon L2 point is about 60000–80000 km away (halo orbit) from the lander and the rover working on lunar surface. Under the constraints of the launching mass and size, the relay communication link should be optimized in multiple aspects such as the relay transmission modes and the high gain relay antenna development to achieve high-bit-rate communication.
Relay communications: The link between Change'4 relay satellite and lander/rover is designed to work on X-band. The forward link uses unified carrier-wave TT&C regime and the backward uses BPSK. The forward relay signal emitter should be able to scan with complex frequency carrier waves, similar to ground stations. The relay satellite should be able to transmit data to Earth and relay with the lander and the rover simultaneously. To avoid interference, the relay link adopts X-band channel, and TT&C to Earth chooses S-band unified carrier wave regime and the data transmission utilizes S-band and BPSK carrier wave regime. Channel encoding is not used for telecommand transmission and RS+concatenated convolution channel encoding is adopted for telemetry and data transmission. Meanwhile for the purpose of synchronous encoding with the ground, both telemetry and data pseudo-random coded.
Relay communication link design: The link profile among the relay satellite, the lander and the rover and ground stations is as shown in Figure 24. The forward data is emitted from the relay satellite, the lander and the rover receive data via omnidirectional antenna and the signals are modulated in PCM/PSK/PM. The backward data is transmitted via an omnidirectional antenna, a medium gain antenna or a directional antenna and is received by the relay satellite. The backward link data of the rover is transmitted by the omnidirectional antenna or the directional antenna and is received by the relay satellite. The lander backward link adopts omnidirectional antenna, medium gain antenna and directional antenna corresponding to low, medium and high bit rates. The modulation mode of the backward link is BPSK.
During the powered descent process, except for setting up X-band omnidirectional forward/backward relay communication links, the lander and the relay satellite also communicate via a medium gain antenna to return landing camera data back to Earth.
While working on the Lunar surface, the lander and the rover respectively receives forward telecommand signals via an omnidirectional antenna from the relay satellite. The relay satellite can send data at two frequency points at the same time to realize simultaneous control of the lander and the rover. Following the command, the lander and the rover send backward data (including telemetry and scientific exploration data) via an omnidirectional antenna or a directional antenna.
The relay satellite can receive backward data from both the lander and the rover simultaneously.
Table 2: Relay communication phases
Figure 24: Relay communication link profile (image credit:CNSA, CAST)
Figure 25: Artist's rendering of the Chang'e-4 relay satellite, launched in May 2018, and lander and rover to set down on the lunar far side in late 2018 (image credit: CAS)
Figure 26: An illustration of China's Queqiao relay satellite near the moon (image credit:CNSA/CAS)
Status of the Chang'e-4 relay satellite / Queqiao mission
• September 4, 2018: One of the two microsatellites, launched along with a required communications relay satellite in May, has quietly been allowing radio operators to download images from the spacecraft taken along its elliptical lunar orbit. 21)
- Longjiang-2, aka DSLWP-B, was developed by students at the Harbin Institute of Technology (HIT) in the Heilongjiang Province, northeast China. Despite having a mass of just 47 kg, the tiny satellite managed to use its own propulsion to slow down and enter lunar orbit while the relay satellite continued past the Moon to its special destination.
- During its time in orbit Longjiang-2 has used a student-developed camera to take images of the Moon, Mars, the Sun and other objects. UHF tests have seen data transmitted by Longjiang-2 and received and decoded by radio operators on Earth.
- Unfortunately its partner microsatellite, Longjaing-1/DSLWP-A, was lost shortly after trans-lunar injection, and likely remains in distant Earth orbit after a lunar flyby.
• August 16, 2018: Longjiang-2 (aka DSLWP-B), one of two microsatellites launched along with the Queqiao relay satellite in May, is still functioning in an elliptical orbit around the Moon, with amateur radio enthusiasts using the 47 kg spacecraft for experiments. 22)
- Longjiang-2 carries a camera developed by KACST of Saudi Arabia, accounting for the final of four internationally-provided payloads for Chang'e-4, which sent back these cool images of the Earth and Moon.
- Another onboard imager developed by students at the Harbin Institute of Technology (HIT) in northeast China has allowed radio operators to download images. Here is one such image, of Mare Nubium on the near side of the Moon.
Figure 27: The Moon's Mare Nubium imaged by the student camera aboard Longjiang-2/DSLWP-B (image credit: HIT)
• June 18, 2018: Two microsatellites, Longjiang-1 and Longjiang-2, were sent into space on 20 May (21:28 UTC) together with the Chang'e-4 lunar probe's relay satellite from southwest China's Xichang Satellite Launch Center. 23)
- Longjiang-2 successfully reached its destination near the Moon on May 25, and entered a lunar orbit with a perilune at 350 km and an apolune at 13,700 km. However, Longjiang-1 suffered an anomaly and failed to enter lunar orbit, according to CNSA (China National Space Administration).
- With a mass of 47 kg, Longjiang-2 has become the world's first lunar orbiter developed by a university. The Longjiang-2 microsatellite carries an optical camera developed by KACST (King Abdulaziz City for Science and Technology) of Riyadh,Saudi Arabia, as well as a low-frequency radio detector developed by the National Space Science Center of CAS (Chinese Academy of Sciences).
- The camera, which began to work on 28 May , has conducted observations of the Moon and acquired a series of clear lunar images and data, according to Xinhua News.
Figure 28: The released photo shows part of the Mare Imbrium on the moon. On 14 June 2018, China and Saudi Arabia jointly unveiled three lunar images acquired through cooperation on the relay satellite mission for the Chang'e-4 lunar probe (image credit: CNSA, CLEP, KACST)
• June 15, 2018: China has provided an update on its Chang'e-4 relay satellite, launched in May in preparation for a later landing on the far side of the Moon, while also revealing the status of the Longjiang microsatellites intended for lunar orbit. A series of stunning images from a Saudi Arabia-developed camera aboard one of the lunar microsatellites, namely Longjiang-2, were also released. 24)
Figure 29: A demonstration of the Lissajous/halo orbit to be used by the Queqiao Chang'e-4 relay satellite mission (image credit: CASC)
Figure 30: The Earth and Moon imaged on June 8 by the KACST-developed camera on China's Longjiang-2 microsatellite. The image shows Saudi Arabia on the distant Earth, as well as the northern hemisphere of the lunar far side, near the Petropavlovskiy crater (image credit: CNSA/CLEP/KACST)
• June 14, 2018: The relay satellite for the Chinese Chang'e-4 lunar probe, named Queqiao and launched on 20 May 2018, entered the Halo orbit around the second Lagrangian (L2) point of the Earth-Moon system (EML-2), about 65,000 km. from the Moon, at 11:06 a.m. on Thursday, June 14, after a journey of more than 20 days. 25)
- "The satellite is the world's first communication satellite operating in that orbit, and will lay the foundation for the Chang'e-4, which is expected to become the world's first soft-landing, roving probe on the far side of the Moon," said Hongtai Zhang, President of the China Academy of Space Technology (CAST). 26)
- The concept of the Halo orbit around the EML-2 (Earth-Moon L2 ) point was first put forward by international space experts in 1950s. While in orbit, the relay satellite can see both the Earth and the far side of the Moon. The satellite can stay in the Halo orbit for a long time due to its relatively low use of fuel, since the Earth's and Moon's gravity balances the orbital motion of the satellite.
- "From Earth, the orbit looks like a halo of the Moon, which is where it got its name," said Lihua Zhang, project manager of the relay satellite. He said the Halo orbit was a three-dimensional ,irregular curve. It is extremely difficult and complex to maintain the satellite in orbit. "If there is a tiny disturbance, such as gravitational disturbance from other planets or the Sun, the satellite will leave orbit. The orbit period is about 14 days. According to our current plan, we will conduct orbit maintenance every seven days. It's a new type of orbit, we don't have any experience. We ran a number of simulations to make sure the design is feasible and reliable," Zhang added.
- In order to establish a communication link between Earth and the planned Chang'e-4 lunar probe, space engineers must keep the satellite stable and control its altitude, angle and speed with high precision. Next, the team will test the communication function of the relay satellite.
- With a mass of 400 kg, and with a design life of three years, the satellite carries several antennas. One, shaped like an umbrella with a diameter of 4.2 meters, is the largest communication antenna ever used in deep space exploration, this according to Lan Chen, deputy chief engineer of the Xi'an Branch of CAST.
- Tidal forces of the Earth have slowed the Moon's rotation to the point where the same side always faces the Earth, a phenomenon called tidal locking. The other face, most of which is never visible from Earth, is the far side or dark side of the Moon, not because it's dark, but because most of it remains unknown. With its special environment and complex geological history, the far side is a hot spot for scientific and space exploration. The Aitken Basin of the lunar south pole region on the far side has been chosen as the landing site for Chang'e-4. The region is believed to have great research potential.
• June 1, 2018: Queqiao made its lunar swing-by on May 25, performing a braking burn at 13:32 UTC to send the communications satellite towards EML-2, some 60-80,000 km beyond the Moon. 27)
Sensor complement of the Chang'e-4 relay satellite: (NCLE)
On board is a Dutch radio antenna, the NCLE (Netherlands Chinese Low-Frequency Explorer). The radio antenna is the first Dutch-made scientific instrument to be sent on a Chinese space mission, and it will open up a new chapter in radio astronomy. 28)
The NCLE instrument was developed and built by engineers from ASTRON, the Netherlands Institute for Radio Astronomy in Dwingeloo, the Radboud Radio Lab of Radboud University in Nijmegen, and the Delft-based company ISIS — in collaboration with a team from the Chinese NAOC (National Astronomical Observatory of the Chinese Academy of Sciences). With the instrument, astronomers want to measure radio waves originating from the period directly after the Big Bang, when the first stars and galaxies were formed.
Why is it so important for the measuring instruments to be placed behind the Moon? Professor of Astrophysics from Radboud University and ASTRON Heino Falcke: "Radio astronomers study the universe using radio waves, light coming from stars and planets, for example, which are not visible with the naked eye. We can receive almost all celestial radio wave frequencies here on Earth. We cannot detect radio waves below 30 MHz, however, as these are blocked by our atmosphere. It is these frequencies in particular that contain information about the early universe, which is why we want to measure them."
Special about the radio antenna is that it will receive low frequency radio waves with a large frequency range. The instrument passed an important risk assessment review by the Chinese space agency at the end of April.
"In the past this was not possible and therefore a receiver with a narrow frequency band was used, in order to avoid electromagnetic interference of the satellite itself," explains project leader Albert-Jan Boonstra of ASTRON. "We have now succeeded in avoiding the electromagnetic interference and making a broadband receiver. That is, of course, good news for subsequent missions and can, for example, be used for future nano-satellites."
In 2016, the NSO (Netherlands Space Office) and its Chinese counterpart CNSA signed an agreement to cooperate in this project, which was an elaboration of the Memorandum of Understanding the two space agencies signed the year before during a trade mission in presence of the Chinese President Xi Jinping and the Dutch King Willem Alexander.
A digital controller unit, designed and supplied by the Somerset West-based SAC (Space Advisory Company), is part of the NCLE (Netherlands-China Low-Frequency Explorer) as a science payload on the Chang'e-4 relay satellite which lifted atop a Long March 4C Rocket from Xichang Satellite Launch Center at 21:28 UTC on Sunday, 21 May 2018. 29)
While the Chang'e-4 satellite's main mission is to relay messages between Earth and the Moon, the NCLE instrument will ride along and conduct experiments into deeper space.
Duncan Stanton, CEO of SAC said that they are ecstatic to be part of such an unique mission and especially proud of their engineering team who proved themselves to be world-class by meeting the ambitious timeline and performance requirements of the project. They may just have embarked on proudly flying the South African flag the furthest ever.
Stanton continued that the controller unit supplied by them forms a critical part of the digital receiver system for the NCLE instrument. The instrument was built by the Radboud Radio Lab from the Radboud University, the Netherlands Institute for Radio Astronomy (ASTRON), and ISIS (Innovative Solutions In Space) in Delft. The instrument has a primary science objective to detect low frequency 21 cm hydrogen line emissions from the ‘dark ages' period of the universe before stars began to shine.
SAC is a member of the Somerset West-based SCS Aerospace Group (SCSAG), Africa's largest privately-owned group of satellite design and manufacturing companies with more than 25 years of experience in this domain.
Figure 31: The digital controller unit designed and supplied by SAC (Space Advisory Company) integrated into Radboud Radio Labs NCLE Digital Receiver Instrument bound for an orbit beyond the moon (image credit: SAC, RadBoud Radio Labs)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).