China launched its fourth and most ambitious lunar exploration mission on December 7, 2018. Liftoff of the Chang’e-4 probe with latest lunar rover took place at 18:24 UTC from the Xichang Satellite Launch Center using a Long March-3B/G3Z launch vehicle thundering from the LC2 Launch Complex. If successful, this will be the first mission to land on the far side of the Moon.

The launch took place on the first opportunity of the launch window available from 18:15 UTC to 18:34 UTC.

The Chang’e-4 mission is part of the second phase of China’s lunar program, which includes orbiting, landing and returning to Earth. It follows the success of the Chang’e-1, Chang’e-2 and Chang’e-3 missions in 2007, 2010 and 2013.

The launch mass of Chang’e-4 spacecraft is around 3,780 kg, while the lander has a mass of around 1,200 kg and the rover has a mass of 140 kg.

The mission is composed of two distinct elements: the lander and the rover. The lander is equipped with a radioisotope thermoelectric generator (RTG) to power the lunar operations during the three-month mission. The energy will be used to power the scientific payload of seven instruments and cameras.

The lunar rover will explore the lunar surface after departing the lander and is equipped with a solar panel to power the vehicle during the lunar day on a three month mission.

With 1.5 m high, the rover has a payload capacity of 20 kg, the rover will be capable of real time video transmission and will also be able to dig and perform simple analysis of soil samples.

The lander will be equipped with an important scientific payload, a low-frequency radio spectrometer, specially designed for the far side of the moon.

The rover carries the panoramic camera (PCAM) to obtain three-dimensional images of the landing and patrolling lunar surface for investigation of surface morphology and geological structure, the Lunar Penetrating Radar (LPR) for surveying the lunar sub-surface structure to investigate surface morphology and geological structure, the Visible and Near-Infrared Imaging Spectrometer (VNIS) for patrol area lunar surface infrared spectroscopy and imaging exploration to survey lunar surface material composition and available resources.

The lander carries the Landing Camera (LCAM), the Terrain Camera (TCAM), and the Low Frequency Spectrometer (LFS).

There are also three international joint collaboration payloads installed on the Chang’e−4 explorer, which are the Lunar Lander Neutrons and Dosimetry (LND) installed on the lander and developed in Germany, the Advanced Small Analyzer for Neutrals (ASAN) installed on the rover for observations of energy-neutral atoms and positive ions in patrol area to investigate the particle radiation environment in the patrol area, and the Netherlands-China Low-Frequency Explorer (NCLE) installed on the relay satellite Queqiao.

The LFS is newly developed for Chang’e−4 lander, and another five kinds of payloads are the inherited instruments from Chang’e−3.

The LFS will be used for the 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.

The LCAM landing camera will be used for optical imaging of the landing area during descent to investigate surface morphology and geological structure, while the objective of the TCAM terrain camera will be for optical imaging of the landing area to investigate surface morphology and geological structure.

The LND was developed 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 to dosimetry for human exploration of the Moon and to contribute to heliospheric science as an additional measuring point.

LND is designed to measure the 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.

The PCAM panoramic camera will obtain three-dimensional images of the landing and patrolling lunar surface for investigation of surface morphology and geological structure.

Via the Chang’e-3 heritage, the objective of lunar penetrating radar LPR is to the map the lunar regolith and to detect the subsurface geologic structures.

The Visible and Near-Infrared Imaging Spectrometer is also from Chang’e-3 heritage. The 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.

The Advanced Small Analyzer for Neutrals – ASAN was developed by the Swedish Institute of Space Physics (IRF) in Kiruna, and will be used for 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, and to make lunar neutron and radiation dose detection and neutral atomic detection for observations of energy-neutral atoms and positive ions in patrol area to investigate the particle radiation environment in the patrol area.

Lunar navigation will be made using special sensors to prevent it from colliding with objects such as small rocks or big boulders.

The autonomous moon rover, which will detach from the lander, will be controlled when necessary by scientists on Earth. The rover uses six wheels that are individually powered. The wheels use a suspension system very similar to the one used on the NASA MER rovers and also on Curiosity.

After entering lunar orbit, Chang’e-4 will go through six stages of deceleration to descend from 15 km above to the lunar surface using its only variable thrust engine. During the descent, the attitude of the probe will be controlled using 28 small engines.

In lunar orbit, Chang’e-4 will prepare itself for the descent. After primary deceleration the probe will quickly adjust its attitude, approaching the lunar surface. In this phase the instruments will analyze the planned descent area, hovering over it. If needed, Chang’e-4 will maneuver in a hazard avoidance maneuver and entering in the final descent phase in a constant low-velocity descent. The main engine is automatically shut down at an altitude of 4 meters, free falling on the surface.

The probe will land on the Von Kármán crater at the South Pole-Aitken.

The soft-landing processes of the U.S. and former Soviet Union’s unmanned spacecraft had no capacity to hover or avoid obstacles. Chang’e-4, on the other hand, can accurately survey landforms at the landing site and identify the safest spots on which to land. In order to land quickly, the probe is equipped with high-precision, fast-response sensors to analyze its motion and surroundings. The variable thrust engine (completely designed and made by Chinese scientists) can generate up to 7,500 newtons of thrust.

The South Pole-Aitken basin is the largest and oldest recognized impact basin on the Moon with a diameter of roughly 2,500 km. The moon’s circumference is just under 11,000 km, meaning the basin stretches across nearly a quarter of the Moon, extending from the crater Aitken in the north and all the way down to the South Pole. Topographic data has shown the enormous effect the South Pole-Aitken impact had on the moon, with the basin being than 8 km deep.

Stratigraphic relationships show that the South Pole-Aitken is the oldest impact basin on the Moon. Lunar samples suggest that most of the major basins on the moon formed around 3.9 billion years ago in a period called the late heavy bombardment. By this time most of the large debris within the solar system should have already accreted to form the planets, so such a large number of big impacts occurring at nearly the same time may have been due to unusual gravitational dynamics in the early Solar System.

Was the impact that caused the South Pole-Aitken basin also a part of some cataclysmic event that occurred 3.9 billion years ago? If so, that impact is strong evidence for an extreme event that would have affected all of the terrestrial planets, including Earth at a time when life was just beginning. If the basin is much older, that may suggest that instead of a spike in the impact rate at 3.9 billion years, the number of impacts simply trailed off from a peak earlier on.

With a diameter of around 186 km, the Von Kármán crater, lying in the north western South Pole-Aitken basin, was formed in the pre-Nectarian. The Von Kármán crater floor was subsequently flooded with one or several generations of mare basalts during the Imbrian period. Numerous subsequent impact craters in the surrounding region delivered ejecta to the floor, together forming a rich sample of the South Pole-Aitken basin and far side geologic history. The topography of the landing region is generally flat at a baseline of around 60 meters.

The Chang’e-4 mission, landing on the far side of the Moon, will be important for the study of planetary formation and evolution and will be an ideal observation site for low-frequency radio.

The study of South Pole-Aitken basin may benefit the discovery of the 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.

The South Pole-Aitken is a basin with an altitude 13 km lower than its surrounding highlands and is composed 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 South Pole-Aitken basin. However, currently obtained data cannot effectively prove that the basin has abundant basalt. On the other hand, an absence of basalt may indicate something happened in the process of Lunar thermal evolution and differentiation in early times.

Also, comparing the craters in the South Pole-Aitken basin with the lunar mare we can see that the degradation situation in that basin is not obvious. No crater with lunar rays has been discovered at the South Pole-Aitken basin, therefore the formation, evolution, topography and chemical characteristics of craters there are apparently different from those of other terrains.

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, radio wave (wavelength less than meters), X-ray (), infrared and millimeter wave and Gamma-ray. 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.

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 far side is ideal for myriametric wave detection.

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 far side of the Moon. With a 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.

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