Following a December 7, 2018 launch on a Long March-3B/G3Z rocket, China has now landed its fourth and most ambitious lunar exploration mission on Thursday, January 3, 2019. This is the first mission to land on the far side of the Moon.
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 was 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 was 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 was 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 accommodated on the Chang’e-4 Lander 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 went 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 was controlled using 28 small engines.
In lunar orbit, Chang’e-4 prepared itself for the descent. After primary deceleration the probe quickly adjusted its attitude, approaching the lunar surface. In this phase the instruments analyzed the planned descent area, hovering over it. If needed, Chang’e-4 could maneuver in a hazard avoidance maneuver and enteri 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 was set to land on the Von Kármán crater at the South Pole-Aitken, pending confirmation of the actual landing zone.
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, could 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.
China’s Long March to the Moon started in 1998 when the Commission for Science, Technology and Industry for National Defense (COSTIND) began planning the lunar mission, the tackling major scientific and technological problems. The lunar orbiter project was formally established in January 2004 and the next month the program is named “Project Chang’e” after a mythical Chinese goddess who flew to the Moon.
The first mission, Chang’e-1 is successfully launched on October 24, 2007, entering in lunar orbit on November 7.
On November 26, Chang’e-1 transmits to Earth the voice of the probe and a Chinese song “Ode to the Motherland”. China’s first picture of the lunar surface is published by Xinhua News Agency. On January 31, 2008, COSTIND publishes the first picture of the lunar polar region taken by Chang’e-1. The first lunar hologram with a resolution of 7 meters is published on November 12, based on data collected by Chang’e-1. In the meantime, the Chang’e-2 mission is approved in October 2008 by the Chinese State Council.
The mission of Chang’e-1 ends when the probe impacts the moon under control on March 1, 2009.
Chang’e-2 is launched successfully aboard a Long March-3C launch vehicle on October 1, 2010. One of the objectives of the mission was to verify the key technologies ahead of the soft-landing. Arriving October 9 on a circular orbit 100 km over the lunar surface after a 112 hour flight, on October 26 the spacecraft transitioned to a closer elliptical orbit after finishing in-orbit tests and took a series of 1.5-meter resolution pictures of the moon’s Sinus Iridium landmark, the chosen landing site of Chang’e-3. After taking pictures of the area, the probe maneuvers again to its original orbit on October 29. The pictures of Sinus Iridium are published on November 8 by the State Administration for Science, Technology and Industry for National Defence (SASTIND).
The six engineering objectives and the four scientific missions of Chang’e-2 are completed on April 1, 2011, and until the end of May the probe surveys the south and north poles of the moon, taking high-resolution pictures of the chosen landing site for Chang’e-3. The extended mission in lunar orbit ends on June 8 and then the probe departs to the second Lagrange Point (L2) orbit, arriving there on August 22. At this point, the gravity of the sun and Earth act in a way that balances the motion of the probe. The main objective of this part of the mission was to test the Chinese tracking and control network.
Chang’e-2 departs from the L2 point on April 15th 2012, going now for an extended mission that took her to a close 3.2 km encounter with the asteroid Toutatis on December 13, taking pictures with a 10 meter resolution.
Launched on December 1st, 2013, Chang’e-3 was the third robotic lunar mission within the China Lunar Exploration Program. The objective was to soft-land on the moon’s surface and deploy an unmanned lunar rover (Yutu) to explore the areas surrounding the landing site. The mission was headed by SASTIND (State Administration of Science, Technology and Industry for National Defence) and the primary contractor for the probe was CAST (China Academy of Space Technology) of the China Aerospace Science & Technology Corporation (CASC). CAST, in turn, contracted the Shanghai Aerospace System Engineering Institute to design and develop the spacecraft.
Chang’e-3 was part of the second phase of China’s lunar program. Yutu was China’s first lunar rover, and the first spacecraft in 37 years to make a soft landing on the moon, since the Soviet Luna-24 mission in 1976. It is named after Chang’e, the goddess of the moon in Chinese mythology. The landing site of Chang’e-3/Yutu was northern Mare Imbrium, south of Montes Recti.
Chang’e-3 and the lunar rover Yutu (Jade Rabbit) landed on the lunar surface at 1:11 pm UTC on December 14, 2013. Following deceleration, the vehicle quickly adjusted its attitude, approaching the lunar surface. During this phase, the instruments analyzed the planned descent area. The main engine automatically shut down at an altitude of four meters, allowing the rover to free fall on the surface.
The landing sequence was executed perfectly, resulting in the vehicle selecting its preferred landing spot almost immediately, even landing without delay, technically 30 minutes ahead of schedule.
After the soft landing, Chang’e-3 charged and initialized the Yutu rover that soon began to communicate with mission control. After communications were established, Yutu unlocked the docking mechanism and then drove to the ladder transfer mechanism. The transfer mechanism then descended to the surface of the moon and moved away from Chang’e-3.
Some nine hours after the separation, the Chang’e-3 and Yutu began to capture some photographs of each other using the onboard cameras. 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 Chang’e-3 lander also carried four instruments: the MastCam, the Descent Camera, the Lunar-based Ultraviolet Telescope (LUT) and the Extreme Ultraviolet Imager (EUV).
Yutu was equipped with a solar panel to power the vehicle during the lunar day on a three month mission. During this time, Yutu was to explore a three square kilometer area, traveling a maximum distance of 10 km from the landing point. However, failed just over 100 meters into this trip. The rover was capable of real-time video transmission, while it was able to dig and perform simple analysis of soil samples. For the real-time video transmissions Yutu used the PanCam. These cameras provided stereo images in high-resolution.
In total, the Yutu rover carried four instruments: the PanCam; the Ground Penetration Radar (GPR); the VIS/NIR Imaging Spectrometer (VNIS); and the Alpha Particle X-Ray Spectrometer (APXS).