Chang'e-2 (Lunar-2 Mission of China) / CE-2
Chang'e-2 was the second Chinese robotic probe to orbit the moon, a follow-up mission to the Chang'e 1 lunar probe, which was launched in 2007. Chang'e- 2 is part of the first phase of the Chinese Lunar Exploration Program (CLEP), undertaken by CNSA (China National Space Administration). The main goal of Chang'e-2 is to collect high-resolution imagery of possible lunar landing sites of the future Chang’e-3 satellite mission and to test some crucial techniques for the follow-up lunar and deep space explorations of China. The Chang’E-3 is planned to land on the Sinus Iridum landing area in 2013.
Note: Chang'e-2 is also spelled as Chang'E-2 as well as CE-2 (Chang'E-2).
The specific objectives of the Chang'e-2 mission are to demonstrate the following key technologies (Ref. 1):
1) The direct injection of the spacecraft into the lunar-transfer orbit without first settling into an Earth orbit
2) Test of the orbital brake technology and capture by the moon at a location 100 km away (100 km lunar orbit insertion)
3) Test of the technology on a 100 km x 15 km lunar orbit maneuver and orbit measure
4) Capture of high-resolution imagery of the landing area in the Sinus Iridum (Bay of Rainbows, 43°N 31°W), a possible landing site of Chang'e-3.
5) Test of the spacecraft by X-band TT&C measurement system
6) Demonstrate new technology such as: LDPC (Low-density Parity-check Code), high-speed (12 Mbit/s) Lunar-Earth data transmission system, the brand-new landing camera, micro CMOS-camera.
The scientific objectives of Chang’e-2 mission are as follows: improving the performance of payloads on the basis of Chang'e-1, improving the accuracy and precision of the lunar scientific data, deepen the following four types of scientific exploration themes:
• To obtain three-dimensional images of the lunar surface with a spatial resolution < 10 m.
• To explore the composition of lunar surface material
• To observe the Earth-Moon and near-moon space environment.
Figure 1: Illustration of the deployed Chang'e-2 spacecraft (image credit: CAST)
The Chang'e-2 lunar orbiter is based on the DFH-3 geostationary communication spacecraft bus series of CAST (China Academy of Space Technology). The spacecraft is 3-axis stabilized with a launch mass of about 2480 kg, including 1300 kg of fuel and 140 kg of payload mass. The spacecraft was originally built as a back-up satellite for the Chang’e-1 probe. 1) 2)
Figure 2: System structure of Chang'e-2 (image credit: CAST)
Figure 3: Spacecraft integration of Chang'e-2 (image credit: CAST)
RF communications: Compared with the current S-band TT&C system, the newly established X–band transmission system is more suitable for deep space exploration, which features higher data transfer rate, less signal attenuation, and higher data rates (12 Mbit/s). Data transmission between China’s new X-band ground TT&C station and the miniature X-band deep space transponder was demonstrated in the mission.
Figure 4: Photo of the X-band transponder system (image credit: CAST)
Chang’e-2 is a more capable probe than is predecessor which underwent numerous technical upgrades for its mission. The probe sports new lunar capture maneuvers, orbit control, an improved high-resolution stereo camera and a new CCD camera – with an increased higher resolution camera compared to Chang’e-1.
Table 1: Comparison of Chang'e-1 and Chang'e-2 probes
Table 2: Spacecraft parameters of Chang'e-2
Launch: The Chang'e-2 spacecraft was launched on October 1, 2010 on a CZ-3C (Chang Zheng-3C) vehicle from the LC2 launch complex of XSLC (Xichang Satellite Launch Center) in southwest China.The Probe's launch rocket had two more boosters to accomplish the more direct route to the Moon. 3)
Orbit: The Chang'e-2 was launched directly into an Earth-Moon transfer orbit with a perigee 200 km and an apogee of about 380,000 km. After < 5 days, when the spacecraft was captured by lunar gravity, Chang’e 2 fired its thrusters; the retrofire lasted for 32 minutes and settled the spacecraft into a 12 hour lunar orbit. After three further retrofires, the spacecraft settled into its final orbiting pattern, orbiting at 100 km above the moon’s surface once every 118 minutes.
Flight Navigation System: The CE-2 navigation project of the Beijing Aerospace Control Center employed the short baseline interferometry concept for orbit determination. The signal processing method is the kernel of the interferometry technology. The CE-2 lunar satellite downlink signal s establish a short baseline interferometry experiment system. The recorded satellite signals are effectively analyzed. Clear interferometry fringes appear by analyzing the CE-2 lunar satellite signal, and high precision delay and delay rate measurements are obtained. 4) 5)
Compared with a VLBI (Very Long Baseline Interferometry) system,the short baseline DOR (Differential One-way Ranging) interferometry technique has its own advantages:
1) The time synchronization problem between the stations is effectively solved by using the same source of time and frequency.
2) The transmission media errors included troposphere, ionosphere and solar plasma. They are approximately the same between the two tracking stations; thus, the transmission media errors offset each other for the two tracking stations.
In this experiment, the distance between two antennas is about 300 m, that is to say, the baseline length B is about 300 m. Meanwhile, the same high stability frequency source, cesium atomic clock 5585B, is utilized in the two tracking stations, this guarantees strict time synchronization of the two tracking stations.
Figure 5: Schematic view of the short baseline DOR interferometry technique (image credit: Beijing Aerospace Control Center)
Figure 6: Flight profile of Chang’e-2 satellite (image credit: CAST, Ref. 2)
• On July 14, 2013, the Chang'e-2 Probe had reached a distance of 50 million km from Earth, marking a new height in the nation's deep space exploration. The various extended missions of Chang'e-2 have the goal to test China's spacecraft tracking and control network, including two newly built measuring and control stations in the northwest Xinjiang Uygur Autonomous Region and northeast Heilongjiang province. The Probe is still in good condition and on a trajectory leading still further out into the deep space direction. 6)
Scientific data and achievements (Ref. 2):
Chang’e-2 is equipped with 5 scientific payloads, including imaging, detecting of the Gamma-ray spectrum, solar wind ion detection, high energy practices, and so on. Chang’e-2 obtained about 6 TB of raw data, including high resolution overall moon images, local images of the Moon’s Sinus Iridium region and Earth-Moon environmental data. - According to the data distribution policy of the Chinese Lunar Exploration Program, the scientific data have been distributed to Chinese universities and academy institutes, including the ones in Hongkong and Macao, which will booster China’s moon and space scientific data analysis. Several scientific achievements resulted from the Chang'e-2 mission, including an overall moon image with 7 m resolution (Figure 7) and a distribution map of multiple elements on the moon's surface (Figures 8 and 9). By analysis of the data, scientists will obtain a better understanding of the Moon and of space science in general.
Figure 7: Whole Moon image obtained by Chang’e-2 (image credit: CAST)
Figure 8: Thorium distribution map on the lunar surface (image credit: CAST)
Figure 9: Potassium distribution map on the lunar surface (image credit: CAST)
Table 3: Summary of Chang'2-2 technological achievements (Ref. 2)
• On December 13, 2012, the Chang'e-2 space probe had completed a close flyby of Asteroid Toutatis acquiring close-up images of the asteroid and using its instruments to acquire scientific data. The asteroid has a size of ~ 5 km in length. 7)
At a distance of 240 to 93 km, Chang’e-2 took a set of images with a maximum resolution of 10 m per pixel. The image resolution is not better than the radar maps, but with color images of the asteroid, scientists can better determine albedo properties and use the photos to better understand data acquired with radars.
Only four of the world's space efforts have managed close encounters with asteroids: NASA with NEAR/ Shoemaker and Dawn, the ESA with Rosetta, JAXA with Hayabusa — and now China with Change-2.
Figure 10: Sequence of Toutatis images during flyby (image credit: Weibo/Xinhua)
• In the fall of 2012, the Chang'e-2 spacecraft is an operational mission at the Sun-Earth Lagrangian Point L2, operating in a Lissajous orbit. 8)
• Deep space mission: Chang’e-2 Probe departed from L2 on April 15, 2012, and is now heading for a deep space mission. The objective is to obtain important technical data to be used for the implementation of China's future deep-space exploration, regarding such aspects as trajectory design, deep space tracking, and telecommunication (Ref. 1).
• On Feb. 6, 2012, China's space agency released an amazingly detailed map of the moon' surface, made up of many high-resolution images observed by the Stereo Camera of the Chang'e-2 spacecraft during a seven month period between October 2010 and May 2011. 9) 10)
Figure 11: Full coverage high-resolution (7 m) images of the moon observed by Change-2 (image credit: SASTIND)
• On August 25, 2011, the Probe reached Sun-Earth Lagrange Point L2 after a 77-day cruise, becoming the first object ever to reach the L2 point directly from lunar orbit, and travelling further than any previous Chinese space probe. It is expected to remain there until the end of 2012, according to the State Administration of Science, Technology and Industry for National Defence (SASTIND). 11)
- In September 2011, Chang'e-2 transmitted the first data observed while orbiting the L2 in a Lissajous trajectory. The data sent back was observed by GRS (Gamma-Ray Spectrometer), HSPD (High-energy Solar Particle Detector), and SWID (Solar Wind Ion Detector), part of the orbiter's payload while it traveled from the moon's orbit to its current position. 12) 13)
- This was the first time for China’s spacecraft to fly beyond the moon. It was also an initiative in the international area to explore multiple targets starting from the Earth to the moon, and then to the L2 point and further to the asteroids (Ref. 2).
Figure 12: A diagram showing the Sun–Earth L2 point, which lies well beyond the Moon’s orbit around the Earth (image credit: NASA) 14)
• End of lunar mission: On June 8, 2011, Chang'e-2 completed its extended mission, and left its lunar orbit for the Sun-Earth Lagrangian point L2 (a stable gravity point on the side of the Earth opposite the sun), to test the Chinese tracking and control network. Before flying away, the orbiter had finished two additional tasks as of May 23, 2011: 15)
- One was to take photos of the northern and southern poles of the moon.
- The other was to descend again to the perilune orbit, about 15 km away from the surface, to catch high-resolution images of the Sinus Iridum, or Bay of Rainbows, the proposed landing ground for future moon missions.
• By May 23, 2011, 2011, CE-2 has obtained the entire coverage of the lunar surface, and is now continuing to work in orbit for an extended mission phase.
• By April 2011, the Chang'e-2 mission had reached the end of its half-year design life and achieved all of its scheduled engineering and scientific goals. Hence, the project decided to take full advantage of the remaining propellant and the operational payload and further expand lunar and deep-space exploration. 16)
- During the 150-day nominal lunar mission, Chang’e-2 experienced twice attitude transition for improvement the sun-tracking of the probe’s cell panel, three times orbit maintenance and one eclipse control. In accordance with the scientific plan, Chang’e-2 finished each scientific exploration mission, including the whole lunar surface high-precision imaging (Ref. 2).
- At the end of its 6 month mission, Chang'e-2 had 520 kg of propellant left. At the same time, the probe was in good condition. According to the ideas of “improving capability more quickly and promoting technological development”, in order to verify the further interplanetary exploration technology and accumulate deep space exploration engineering experiences, Chang’e-2 continued to carry out extended tasks. This was a great opportunity to explore further goals. Hence, a three-phase and multi-target mission was designed. The Sun-Earth L2 point and near-Earth asteroids were selected as the route and direction of the extended missions. The basic purpose was to go further into interplanetary space (Ref. 2).
• On November 2, 2010, the VLBI system along with the USB (Unified S-band ) range and Doppler system, completed the tracking task of the real-time phase of the CE-2 satellite. 17)
- Also on Nov. 2, 2010, Chang’e-2 started the long-term mission operations of the 150-day circumlunar flight (Ref. 2).
• On Oct. 26, 2010, the Chang'e-2 probe was maneuvered into an elliptical lunar orbit of 100 km x 15 km (apoapsis x periapsis) which was maintained throughout the mission. This orbit enabled the cameras to obtain higher-resolution imagery of the lunar surface.
• On Oct. 9, 2010, the Chang'e-2 spacecraft reached its preliminary circular lunar orbit of 100 km x 100 km.
• Lunar orbit insertion of the Chang'e-2 spacecraft from Oct. 6-9, 2010. The probe implemented a total of three times braking at perilune and one time orbital plane maneuver.
• On October 2, 2010, the first midway correction control was implemented. The 490 N engine fired for 70 s. The speed increment was 16 m/s. The control result was accurate and met the requirement of inserting the probe into lunar mission orbit, so the scheduled twice midway orbit corrections were cancelled.
Sensor complement: (MRM, Stereo Camera, LAM, GRS, XRS, HPD, SWID)
The Chang’e-2 project shares similar goals with Chang’e-1: Obtaining three-dimensional images of the lunar surface, analyzing and mapping the chemical elements on the lunar surface, and probing the features of the lunar soil and the space environment near the moon will all be undertaken.
The sensor complement has a mass of ≤140 kg and a power consumption of ≤200 W.
Table 4: Instrument summary on the two spacecraft Chang'e-1 (CE-1) and Chang'e-2 (CE-2) 18)
Most instruments are described in the file of Chang'e-1.
The CCD stereo pushbroom camera is one of the key payloads carried by the Chang’e-2 satellite, whose mission is to image the lunar surface with high spatial resolution and provide scientific guiding data for choosing a safety-landing site for Chang’e-3. The improved stereo camera design features the TDI (Time Delay Integration) method to increase the detection sensitivity of the instrument. 19)
The CE-2 CCD stereo camera adopts a stereo imaging solution with the single lens and two angles of view in the same track and a pushbroom imaging mode with a high sensitivity TDI capability. Compared with the three-line array mode of the CE-1 stereo camera, CE-2 adopts a two-line pushbroom scheme to reduce the data volume of the imagery. The two-line array CCD stereo camera acquires forward and backward high-resolution imagery of the lunar surface. 20)
Figure 13: Imaging configuration of the CE-2 stereo camera (image credit: Institute of Remote Sensing Applications, CAS)
Table 5: Technical parameters of CE-2 CCD camera
Chang’E-2 stereo camera has obtained data of both 7 m (coverage 100%) and 1.5 m (coverage 71%) resolution for the landing area of Chang'e-3. 21)
Figure 14: Photo of the CCD stereo camera (image credit: Xinhua)
The Chang'e-2 spacecraft features one landing camera and three surveillance cameras. These cameras feature a smart and low mass, low power, and a highly-integrated camera design. Technologies like auto-exposure, high-speed compression of color imagery and static gray image, and sub-sampling methods are introduced.
The four cameras have taken satisfying imagery of the moon's physiognomy, deployment of satellite solar battery wing, the working state of the 490 N engine, and the deployment of the directional antenna.
Figure 15: Photo of the Micro-CMOS camera (image credit: CAST)
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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.