Mars program of the People's Republic of China

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The Mars program of the People's Republic of China ( Chinese  中國 火星 探測 工程  /  中国 火星 探测 工程 , Pinyin Zhōngguó Huŏxīng Tàncè Gōngchéng , English China Mars Exploration Project, CMEP for short ) is a program for exploring the planet Mars with space probes , coordinated by the National Space Agency of China. Components of the program so far are a Mars orbiter ( Yinghuo-1 ), a Mars rover ( Tianwen-1 ) and a return mission to bring soil samples from Mars to Earth.

history

Russia had been working since 1996 on the development of a probe that would land on the Martian moon Phobos and from there bring soil samples back to Earth. However, this was initially just a theoretical project. It was not until 2004 that the Russian government provided sufficient funds to start building the first components. In June 2005, Russia and the People's Republic of China first discussed cooperation, which was officially confirmed in mid-August 2006 by Ye Peijian , chief designer of the lunar probes at the Chinese Academy of Space Technology since 2004 . On March 26, 2007, during a three-day visit to Russia, President Hu Jintao signed a contract between the China National Space Administration and Roscosmos , according to which China could send a small satellite with what was then known as the "Phobos Explorer" for financial participation.

This secured the financing for the project called “ Fobos-Grunt ” from 2008 (from Russian Грунт, “soil”). The launch of the two probes was originally planned for October 2009, but had to be postponed to 2011 at the last minute (there is only a launch window to Mars every 26 months). The launch actually took place on November 8, 2011, but the probes could not reach the transfer orbit to Mars and burned up in the Earth's atmosphere on January 15, 2012. In contrast to the lunar program , which specifically deals with the search for and mining of mineral resources on the earth's satellite, the Chinese Mars program has primarily scientific goals. In August 2012, the Chinese government was not ready to provide larger funds for an exploration of Mars in view of the higher degree of difficulty and less usefulness compared to the moon and instead still relied on international cooperation despite the failure.

Already in August 2010, before the failure of the Fobos-Grunt mission, 8 members of the Chinese Academy of Sciences had the national authority for science, technology and industry in their national defense , because of the English name State Administration for Science, Technology and Industry for National Defense mostly abbreviated to SASTIND, proposed under the impression of the successful Chang'e-1 mission to work out a comprehensive plan for deep space exploration beyond the moon. SASTIND immediately formed a group of experts to work out such a plan and examine the possibilities of its implementation. But then nothing happened for a long time. Only in 2013 - in March of that year, with the election of Li Keqiang and Xi Jinping as prime minister and president, respectively - the State Council of the People's Republic of China commissioned Dong Zhibao (董 治 宝, * 1966) from the then Lanzhou Desert Institute of the Chinese Academy of Sciences (中国科学院 兰州 沙漠 研究所) to lead a group of around a dozen researchers who were supposed to find a place on the highlands of Tibet where they could simulate the environmental conditions on Mars, set up a research base and store the technologies for a Mars rover and other equipment Could test extreme conditions.

The planning for the first purely Chinese mission to Mars was fundamentally changed three times in terms of the individual steps and mission goals. On June 23, 2014, Ouyang Ziyuan , then scientific advisor to the lunar exploration project leadership group at the National Space Agency, announced in a lecture on the successful landing of the Chang'e-3 lunar probe at the 22nd Conference of the International Planetarium Society in Beijing that China I have now launched a new, own Mars program. At that time, Ouyang Ziyuan announced a specific schedule for the mission with orbiter, lander and rover in 2020 and for a return mission in 2030, which was then still known as “Yinghuo-2”. A few months later, the China Aerospace Science and Technology Corporation exhibited a model of the Lander and Rover (with the solar panels arranged differently than the current version ) at the International Air and Space Exhibition in Zhuhai (November 11-16, 2014) organized by the State Council ), with explanations of the mission, namely that an orbiter should put the lander-rover group into orbit around Mars and then drop it off at the right time. On the one hand, the orbiter should make scientific observations and at the same time act as a relay satellite for the rover. The launch of the probe should take place with the heavy-lift rocket Changzheng 5, which is currently under development, from the Wenchang cosmodrome , which was completed in September 2014 .

This was still a matter of preliminary planning. It was not until January 11, 2016 that Prime Minister Li Keqiang released the corresponding funds from the Fund for National Scientific and Technical Large-Scale Projects , which officially launched the Mars program. On the afternoon of April 22, 2016, Xu Dazhe , the then director of the National Space Agency, announced the matter to the public at a press conference in the State Council building. Ionosphere researcher Wan Weixing , then head of the Laboratory for Earth Magnetism and Astrophysics at the Institute for Geology and Geophysics of the Chinese Academy of Sciences , which was restructured in 2017 and - still under the direction of Wan Weixing - became the chief scientist of the Mars program - became the chief scientist in the “Specialized Laboratory for Geophysics and Planetary Astrophysics ”was renamed. Communications engineer Zhang Rongqiao , who had previously worked at the Center for Lunar Exploration and Space Projects as Deputy Technical Director for the Lunar Program of the People's Republic of China , became the technical director of the Mars program . The astrochemist Li Chunlai , deputy director of the National Astronomical Observatories of the Chinese Academy of Sciences , and the engineer Zhang Tingxin (张廷 新) of the Chinese Academy of Space Technology became the deputy technical directors of the Mars program .

In the Chinese Academy of Space Technology, the same team that the lunar probe had Chang'e-3 with its Jadehase had developed -Rover, worked on studies for a Mars landing for years - the model of 2014 was formed from the former state of affairs. A generation change has now taken place there: in April 2016, Sun Zezhou , the previous deputy of Ye Peijian, was appointed chief designer for the Mars project (and the lunar probes). At the press conference on April 22nd, it was Ye Peijian who revealed that the launch of the probe, which was originally planned for 2018, had to be postponed to 2020 due to unresolved technical issues (size of the solar modules, landing procedures).

Program structure

Similar to the lunar program , the Mars program also consists of several steps:

  • Orbit (绕)
  • Landing (著)
  • Patrol (巡)
  • Return (回)

Yinghuo-1

The orbiter Yinghuo-1 , developed and built by the Shanghai Academy of Space Technology , a subsidiary of the China Aerospace Science and Technology Corporation , from 2006 onwards, was delivered to Russia in March 2009 to be used together with Fobos-Grunt during the launch window in September / October of that year to start Mars, where it should arrive in August 2010. As with the Chang'e-1 lunar probe , the scientific payloads were developed by the Center for Space Science and Applied Research, now the National Center for Space Science , in this case by a group led by Zhao Hua (赵华, * 1961); The current director of the center, Wang Chi , had overall responsibility for the payload systems .

Since the Russian Academy of Sciences wanted to carry out additional tests for safety reasons, Roskosmos made the decision on September 21, 2009, literally at the last minute, to postpone the start of the two probes to 2011. Despite all the tests, shortly after the actual launch on November 8, 2011, the on-board computer failed, triggered by cosmic radiation , the probes could not reach the transfer orbit to Mars and burned up two months later in the earth's atmosphere.

Tianwen-1

After the failure with Yinghuo-1, the next step was to land. Concepts for a Mars landing had been worked on in China for a long time, but originally a solution had been favored in which an orbiter dropped numerous miniature planes. It was known that about half of all Martian missions started worldwide had failed; if contact with one Minilander broke off or the other hit too hard, a number of their “colleagues” could still have worked on Mars.

Model of the rover at an exhibition in 2018

When the Martian program was seriously resumed after Xi Jinping's election in March 2013, however, a slightly modified version of the Chang'e 3 principle was decided: a simple lander with no additional payloads, a largely autonomous rover and another relatively intelligent one Orbiter, which was supposed to act as a relay station for transmitting the data collected by the rover to Earth and at the same time to take measurements itself. This time the probe was built by the Chinese Academy of Space Technology , which had a lot of experience with this concept from the moon lander; The National Center for Space Science was responsible for the development of the total of 13 payloads of the rover and orbiter, which then distributed the individual sub-projects to other suppliers. For example, the rover's ground penetrating radar was given to a group headed by Zhou Bin (周斌) and Shen Shaoxiang (沈 绍祥) from the Laboratory for Electromagnetic Investigation Techniques of the Institute of Electronics of the Academy of Sciences, who are already using similar devices for the Chang'e-3 and had built Chang'e-5 (the Institute of Electronics has its own workshop).

On April 24, 2020, the 50th anniversary of the launch of the first Chinese satellite, Dong Fang Hong I , it was announced that China's interplanetary missions would all be named “Tianwen” (天 问 or “Questions of Heaven”) after the eponymous , Qu Yuan's poem from the " Elegies of Chu " consisting of 183 puzzles . The Mars probe was the next pending mission at this time and was therefore called "Tianwen-1" (天 问 一号, Pinyin Tiānwèn Yīhào ). On July 23, 2020, the probe launched for Mars.

Return (2029+)

Those responsible only want to decide on the question of whether there will actually be a return mission to Mars after an assessment of the course of the Chang'e 5 return mission to the moon (end of 2020) and the Tianwen 1 mission. Since the Changzheng 9 heavy-duty rocket, which is required for the mission and is currently being developed, has to be tested in practice, the earliest launch window for another Chinese mission to Mars would be in April 2029, i.e. before the start of the typhoon season. With every further shift of 26 months, the weather at the Wenchang Cosmodrome gets worse. H. the risk of missing the start window for the fuel-efficient Hohmann transfer increases.

The mission is supposed to be similar to Chang'e-5, only that five components are used here:

  • Orbiter
  • Countries
  • rover
  • Transport capsule
  • Return capsule

After eight months of flight and going into orbit around Mars, the lander / rover / transport capsule group separates from the orbiter / return capsule group. The lander brings the rover and the transport capsule to the surface of Mars, where the rover examines the geomorphology at the landing site and collects rock and soil samples and stows them in the transport capsule. Then the transport capsule takes off with its own engine - whether a liquid rocket engine or a solid rocket engine will be used is currently still being discussed - and initially goes into a lower Mars orbit. After several orbit corrections, the transport capsule connects to the orbiter / return capsule group and stows the soil samples in the return capsule. The transport capsule then separates from the orbiter / return capsule group, the orbiter ignites its engine and begins its return flight to Earth. Close to the earth, the orbiter performs a turning and braking maneuver, then the return capsule separates, enters the atmosphere over the South Atlantic and begins the approach to the Dörbed landing site in Inner Mongolia via Somalia, Pakistan and Tibet . After examining the samples in the laboratory, they are compared with the results of the analyzes carried out by the rover on Mars and merged.

As a technically easier alternative, Meng Linzhi and his colleagues at the Chinese Academy for Space Technology considered in 2016 to do without a rover and to collect samples from the lander, similar to the Chang'e-5 mission. Since the samples collected in this way would only have a limited scientific value, the technology used would not represent any significant progress and could not serve as a preliminary stage for manned deep-space missions, this approach was initially postponed. One of the key questions in mission planning is the weight of the components, especially the transport capsule and the fuel required to move them out of the gravitational field of Mars and into orbit. On the one hand, you want to bring back as much material as possible and at the same time simulate the weight of a spacecraft suitable for humans, on the other hand you are bound to the capacity of the Changzheng-9 launcher, which can carry a maximum of 44 t payload into an Earth-Mars transfer orbit. In July 2020, thought was given to bringing the individual components to Mars, each divided into a Changzheng 5 and Changzheng 3B rocket .

Telemetry, tracking and control

conditions

When the Chinese deep space network was expanded from 2010 with antennas in Kashgar , Giyamusi and Zapala for the landing phase of the lunar program , the systems had already been designed so that flights to Mars, i.e. up to 400 million kilometers, could be monitored and controlled could. The basic problem here is that, on the one hand, the signal strength decreases with the square of the distance for the same transmission power , while at the same time the missions become more and more demanding, which makes an increased data transmission rate necessary. With the failed orbit around Mars with Yinghuo-1, a transmission rate of 2 megabits / second would have been possible, which corresponds to broadband internet access . For telemetry in a highly dynamic process such as the entry of a probe into the Martian atmosphere, for a rover that is supposed to deliver images in HDTV quality, or the operation of a multispectral camera , transmission rates of up to 250 Mbit / s are required.

In addition, the increasingly complex missions place extreme demands on navigation. The position of the spacecraft must be known precisely at all times in order, for example , to be able to carry out a rendezvous maneuver in Mars orbit on the return mission 2030 . For comparison: after the American Deep Space Network had introduced the Delta-DOR method for determining the location of space probes in the late 1980s , the accuracy there was around 20 nrad . In 2002 this was improved to 5 nrad, i.e. 0.001 arc seconds.

Group antennas (downlink)

The path that China is taking to meet these challenges are group antennas, both local groups consisting of several small, closely spaced antennas and nationwide groups from the tracking stations operated by the Xi'an satellite control center . Compared to building larger and larger antennas, this approach has several advantages:

  • Small antennas can be aimed faster and more precisely at a given target than a heavy and sluggish 60 m dish.
  • The individual antennas of a group can be serviced successively, while with a large individual antenna the station fails completely during maintenance.
  • The control systems of small antennas are less complicated, that is, the development costs are lower, as is the acquisition costs for mass production of the same antennas.
  • Antenna groups can be expanded flexibly; the group can continue to work while additional units are being built.

In contrast to the trajectory tracking of space probes, where interferometry is used, the so-called "Sumple algorithm" is used when receiving telemetry and payload data with such group antennas, in which several "fuzzy" versions received by the individual antennas in a group of a data set can be superimposed in order to achieve a unified set with a significantly better signal-to-noise ratio through their sum (the “sum” in “sum”) . This method was first proposed in 1994 by David Herbert Rogstad (* 1940) of the Jet Propulsion Laboratory . In 2013, a group headed by Lu Manhong (卢 满 宏, * 1968) from the Polytechnic University of Northwest China in Xi'an , which was funded by the National Program for the Development of High Technology, also known as " Program 863 " after the founding date, published an improved version with the data sets over the entire frequency spectrum can be superimposed in real time, including the calculation of time delays, which is particularly important for nationwide groups with antennas 3000 km apart in Qingdao and Kashgar .

David Rogstad's Sumple algorithm was tested by Chinese engineers from October to December 2010, when the Chang'e-2 probe was on its way to the moon. The four 12 m parabolic antennas of the University of Space Technology in the north of Beijing were used for this. At a distance of 140,000 km and 400,000 km from the earth, Chang'e-2 sent test images on the S-band that she had taken of the earth. If only one of the 12 m antennas received the image, a round globe with cloud formations could be seen on it, but in the wrong colors and very washed out. Then four antennas received the sent photo. With the Sumple algorithm superimposed, the signal mutilation could be compensated to more than 90%. The result was not quite as good as the image received at the same time by the 50 m antenna of the Miyun observatory (the difference could hardly be seen with the naked eye), but at least corresponded to the quality that can be achieved with a parabolic antenna with a 24 m diameter .

In December 2013, Lu Manhong's improved version of the algorithm was tested with the Chang'e-3 lunar probe , specifically with a nationwide group who, in addition to the four 12-m antennas near Beijing, also tested the 18-m tracking antennas from Qingdao and Kashgar and the 15 m antenna of the newly built Sanya ground station on Hainan were involved. In a photo of her rover Yutu taken by Chang'e-3 on the surface of the moon and radioed to earth , if only a 12 m antenna received the image, nothing could be seen except noise. On the image received from Qingdao, the rover was at least vaguely recognizable, with the - older - 18 m antenna in Kashgar still noisy, but much better. However, after the technicians superimposed four images from Beijing with those from Qingdao and Kashgar, 92% of the noise losses were compensated, the effective signal strength had increased by 3.7 dB compared to a single 18 m antenna , and not just the rover , but also his tire tracks were razor-sharp.

After a group consisting of the 35 m and 66 m antennas from the Kashgar and Giyamusi deep-space stations had already been able to coincide the telemetry signals received on a trial basis from the ESA probe Venus Express with 92% and the effective signal strength in Compared to a single 35 m antenna by 4.25 dB, the decision was made to add three identical antennas to the existing 35 m antenna south of Kashgar for the 2020 mission , so that the deep-space station there will have the same reception performance like the 66 m antenna in Giyamusi.

Group antennas (uplink)

While it is relatively easy to improve the performance of a ground station by adding additional antennas when receiving the data transmitted from the probe to earth, the so-called " downlink ", this is difficult in theory when sending control commands to the probe, the " uplink " Although the transmission power of an antenna group increases with the square of the number of antennas, in practice phase shifts between the control signals sent by the individual antennas, caused by delay times in the systems for frequency change, the amplifiers, etc., also lead to different atmospheric effects at widely spaced apart systems Locations as well as disturbances in the interplanetary medium lead to a drop in the effective transmission power, which becomes more noticeable the more antennas are involved in a group. For example, the loss of real transmission power caused by the phase shift is almost twice as much with four antennas as compared to a group that only consists of two antennas. This can be partially compensated for with complex procedures, also developed with funding from the 863 program. In an experiment with three 3 m antennas set up next to each other and transmitting test signals on the C-band to a geostationary communication satellite, it was possible to achieve 80% of the transmission power theoretically expected by such a group.

Nevertheless, there is ultimately no way around the development of stronger broadcasters. Currently (2019) in the underground stations Kashgar, Giyamusi and Zapala Klystron transmitters from domestic production with a power of 10 kW are installed, which after amplification a transmission power of 18 kW in the S-band and 15 kW in the X-band emitted by the antenna deliver. For several years, work has been carried out on the development of a 50 kW transmitter for the X band. At the beginning of 2018, the prototype of such a klystron with a bandwidth of more than 95 MHz was completed and tested. However, by the Chinese TT&C engineers, this example is only seen as a first step in gaining experience in building such transmitters. For comparison: the European deep space antennas work with a transmission power of 20 kW, the American Deep Space Network with 80 kW in the X-band and 400 kW in the S-band.

Orbit modeling

In the early summer of 2018, the engineers at the Beijing Space Control Center were able to maneuver the Elsternbrücke relay satellite into a halo orbit around the Lagrange point L 2 behind the moon in real time and, so to speak, manually using the orbit data determined by the Chinese deep space network . On a Mars mission, this is not possible due to the signal propagation time of almost 10 minutes between Earth and Mars. The engineers therefore use computer models here, where they can determine the position of the probe and its solar modules on the basis of the last determined orbit data (position, direction, speed) and factors such as the gravitational pull of the earth, Mars and its moons or the radiation pressure of the sun on the probe and its solar modules Predict spacecraft and initiate orbit maneuvers on this basis .

This is particularly important for determining the point at which the probe's engine must be ignited in order to swivel into Mars orbit. The NASA currently succeed with an accuracy of 400 meters. In China this is still a long way off. In 2018, the Beijing Research Institute for Orbit Tracking and Communication Technology of the Strategic Combat Support Force was able to determine the periapsis , i.e. the point of the closest to Mars , on the basis of orbit data measured over a period of one week and taking into account the gravitational pull of the various celestial bodies desired orbits, with an accuracy of only a few dozen kilometers, in the best case a few kilometers. It is hoped to be able to reduce this in the future to an error margin of 1 km by improving the interferometry measurements by the ground stations and by including small track disturbances due to outgassing etc. in the computer model.

Landing maneuvers

The most critical phase in a Mars mission is the landing maneuver. The probe enters the atmosphere at 17,280 km / h, i.e. 14 times the speed of sound , where it slows down the atmosphere , the parachute opens, and only 8 minutes after the start of the maneuver, the lander is on the surface of the planet - within a short time there is a strong change in speed. For telemetry this means that the Doppler effect , which shifts the frequency of the carrier wave in the X band , changes by up to 200 kHz; when the parachute opens, this frequency shift takes place at a speed of up to 3.5 kHz / s. In addition to the X-band antennas in the Goldstone Deep Space Communications Complex , NASA therefore uses decimeter wave antennas in the Green Bank Observatory and Parkes Observatory on its Mars missions , while ESA uses the ExoMars mission for communication used the Indian Giant Metrewave Radio Telescope in the UHF range . With the Five Hundred Meter Aperture Spherical Radio Telescope of the Chinese Academy of Sciences in Guizhou , better known under the abbreviation FAST, China has a corresponding facility with a receiver for 70 MHz to 3 GHz. The main department responsible for satellite launches, orbit tracking and control (卫星 发射 测控 系统 部) of the People’s Liberation Army's Strategic Combat Support Force is currently planning to use FAST for the Tianwen-1 Mission 2020 for auxiliary services, similar to the radio telescopes of the Chinese VLBI network for track tracking.

Ground segment

The ground segment of the Mars program, led by astrochemist Li Chunlai since 2016, is, like that of the lunar program , in the headquarters of the National Astronomical Observatories of the Chinese Academy of Sciences in Beijing, Datun Str. 20a located. It is organized in a similar way, but requires, for example, a data storage capacity of 60 TB in order to be able to make the data supplied by the 13 payloads of the Tianwen-1 Mission available online to the scientific community. In principle, the ground segment consists of five sectors:

  • Payload Operation Department (works with the People's Liberation Army deep space network to transmit control commands)
  • Data reception department (has the 40 m and 50 m antennas in Miyun , the 40 m antenna in Kunming and the 70 m antenna in Wuqing available)
  • Department for initial treatment of the data (converts the raw data into standard formats)
  • Department for data management (makes the data categorized in three "confidentiality levels" available to science after consultation with the China National Space Administration )
  • Department for Scientific Application and Research (determines, among other things, the final landing site of the probe)

As long as perfect demodulation is possible with at least two of the antennas , these two antennas are used simultaneously but separately in individual operation. After receiving the data, encrypted with a convolutional code and sent via the X-band , it is decrypted and converted into an image-like data packet that provides a snapshot. On the one hand, these “images” are sent to the head office in Beijing, and on the other hand, they are stored on local hard drives, directly at the ground stations. If the transmission conditions do not allow this, the new 70 m antenna in Wuqing will be linked together with the two antennas in Miyun to form a group. If the signal attenuation still turns out to be too high, the antenna in Kunming, 2000 km further south, can also be added. You then work with a group of four very widely spaced antennas. The ground stations send the received raw data directly to Beijing, where the data sets are superimposed and transmission errors are compensated for. Only then is it demodulated, decoded and the image data packets saved.

Individual evidence

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