Exploring the heliopause

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The exploration of the heliopause ( Chinese  太陽系 邊際 探測  /  太阳系 边际 探测 , Pinyin Tàiyángxì Biānjì Tàncè ) is a project to explore the heliosphere , especially its limit, the so-called heliopause, presented for the first time on May 2, 2018 . It is under the scientific direction of the Faculty of Earth and Space Sciences of Beijing University (Zong Qiugang) in cooperation with the Center for Lunar Exploration and Space Projects of the National Space Agency of China ( Wu Weiren ), the China Aerospace Science and Technology Corporation ( Yu Dengyun ) , the Chinese Academy of Space Technology (Huang Jiangchuan), the Beijing Institute for Spacecraft Design (Meng Linzhi) and the National Center for Space Science ( Wang Chi ).

The project relies to a large extent on three probes which are to be sent to different areas of the heliosphere . The first probe is to set off to the Template: future / in 3 yearsedge of the heliosphere near the sun in May 2024 , the second at a later point in time, but also in the opposite direction in 2024, to explore the "Helios tail". The third, nuclear-powered probe is scheduled Template: future / in 5 yearsto break away perpendicular to the plane of the ecliptic to the upper or lower edge of the heliosphere in 2030 . All three probes would then penetrate into interstellar space .

Scientific goals

Area 1 (up to 100 AU)

Exploring the spatial distribution of interstellar energetically neutral atoms and interstellar dust

The heliosphere in the previous assumption. The solar system moves to the left, the tail to the right.

The heliosphere is a large area around the sun in which the solar wind of electrically charged particles displaces the interstellar medium and forms a kind of "bubble" around the sun. The limit of this area, which extends far beyond the planetary orbits, where the solar wind meets the interstellar medium, is called the "heliopause". Since the solar system moves through the interstellar medium at a speed of 23.2 km / s or 84,000 km / h, the previous assumption is that the heliosphere is deformed by the "airstream" and has a comet-like shape, with a head , where the boundary line to interstellar space with 100 AU is relatively close to the sun, and a tail pointing in the direction opposite to the direction of travel. After evaluating the data supplied by the deep space probes Voyager 1 and Voyager 2 , the Saturn probe Cassini and the IBEX satellite , it seems that the heliosphere does not have a comet-like tail, but is actually spherical, i.e. rather round.

The space physicist Wang Chi, responsible for the analysis of the data from the plasma spectrometer and the development of a theoretical model of the heliosphere on NASA's Voyager missions , had already worked with John D. Richardson from the Massachusetts Institute of Technology in 2003 on the decrease in the speed of the solar wind as it approaches the outer edge of the heliosphere.

In 2015, the national space agency of China then initiated preliminary planning for an exploration of the heliopause using deep space probes. The Chinese Academy of Space Technology, which would build the probes, and the Institute of Space Physics and Applied Technology (空间 物理 与 应用 技术 研究所) at the Faculty of Earth and Space Sciences (地球 与 空间 科学) were entrusted with the specific planning work for the project 学院) of Beijing University and the National Center for Space Science of the Chinese Academy of Sciences , where Wang Chi was now director of the National Specialized Laboratory for Space Weather (空间 天气 学 国家 重点 实验室).

The ENA area on the left

NASA's previous deep space probes, from Pioneer 10 (1972) to New Horizons (2006) were primarily designed for exploring the outer planets and only collected data from the heliosphere as a by-product. Neither the measuring instruments nor the mission profile were designed for this purpose. For example, Voyager 1, diverted north by the flyby of Saturn's moon Titan , and Voyager 2, diverted to the south by the flyby of Neptune , flew to the side or above and below the zone of energetically neutral atoms (ENA), which in The direction of travel of the solar system lies between the edge shock wave and the heliopause. The Chinese mission should now be designed in such a way that a probe would fly precisely through the “tip of the nose” of the ENA area.

According to the current state of research, the plasma of the solar wind hits neutral hydrogen atoms from the interstellar medium about 84 to 94 AU away from the sun, which penetrated through the heliopause into the heliosphere at a speed of 25 km / s. When such a hydrogen atom collides with an ultraviolet photon from the solar wind, it loses its electron, which is picked up by an ionized atom from the solar wind. During this process, the solar wind slows down from around 350 km / s to 130 km / s. About 70% of its kinetic energy is consumed during the ionization of the hydrogen atoms. By slowing down and the subsequent flow of additional material from the direction of the sun, the plasma of the solar wind compressed and heated from approximately 11,000 K to 180,000 K. Meanwhile, the hydrogen ions by the magnetic field of the sun are carried out, it is why a pick-ion , ie "Collected ions" are called. According to the original assumption, the pick-up ions repeatedly collide with the edge shock wave, gaining energy until they finally escape the edge shock wave and diffuse into the inner heliosphere. These accelerated ions then form what is known as “ anomalous cosmic radiation ”. However, the Voyager probes were able to determine a further increasing strength of the anomalous cosmic radiation even after crossing the edge shock wave and penetrating the helio hull , until it suddenly disappeared beyond the heliopause. This suggests that the anomalous cosmic rays actually originate in the helio envelope, which is now to be confirmed by further measurements on site.

By observing the radial distribution of the pickup ions or the anomalous cosmic radiation within the heliosphere, it is hoped that a better understanding of the dynamic changes in the solar wind will be obtained. Pioneer 10 was already flying in the opposite direction to the direction of travel of the solar system, but in 2003 contact with the probe was broken after 81 AU. In 2024, two Chinese probes are expected to fly in opposite directions, one towards the heliospheric nose and one in the opposite direction. Around 2030, a third probe is expected to fly perpendicular to the ecliptic to the north or south pole of the heliosphere. With this method one could also confirm or refute the assumption of a Helios tail, which was previously only based on model calculations. If the neutral atoms remain neutral, they can penetrate into the interior of the heliosphere together with the dust particles of the interstellar medium without being influenced by the magnetic field of the sun and should then be collected in a tail by the gravitational pull of the sun.

Exploration of the ice giants and their moons, the centaurs, the Kuiper belt and dwarf planets

Ice volcanoes at the South Pole of Triton

While the third probe of the project is to be provided with a 10 kW nuclear drive so that it can reach a speed of 6 AU / year under its own power, the first two probes, since they move in the plane of the ecliptic, can be used for Use swing-by maneuvers for acceleration . The second probe should fly past Neptune at a distance of 1000 km in January 2038 . The instruments of the probe itself are primarily designed for observing the heliosphere, but when flying past Neptune's moon Triton , which is of particular interest to researchers because of its cryovolcanism , the probe is to deploy a small sub-probe that is supposed to hit the ice crust of the moon. The first probe does not get close enough to the ice giant Uranus on its way to the heliospherical nose to use it for a swing-by maneuver, but remote sensing is to be carried out as far as possible.

Between the orbits of Jupiter and Neptune, orbits that cross the orbit of one or more gas planets , the so-called “ centaurs ” around the sun, a class of asteroids and (former) comets, some of which have a ring system consisting of ice particles and to their Origin, composition, gradual loss of speed and - in the case of comets - gas secretion, many questions remain unanswered. The researchers hope to find answers to these questions through close observation.

On the way to the edge of the heliosphere, the first two probes must cross the Kuiper Belt , an annular region that extends beyond the orbit of Neptune at a distance of 30 to 50 AU near the ecliptic. Of the estimated more than 70,000 objects with a diameter of more than 100 km that orbit the sun there, the researchers are particularly interested in the asteroid or dwarf planet (50,000) Quaoar , which will be near the heliospherical nose by 2040 and therefore can be explored by the first probe in the flyby. A three-dimensional image of the dwarf planet is to be recorded and, above all, the methane ice on its surface is to be examined, its thickness and the question of what proportion of methane has in relation to the ethane , ammonium hydroxide and nitrogen ice also present on the surface . In addition, images of the ice volcanoes on Quaoar are to be made using imaging methods and spectrographic analyzes of the mantle plumes, which are probably responsible for these, are to be carried out.

All three probes are to be equipped with dust detectors with which the radial distribution of the interplanetary dust can be measured continuously. It is hoped that this will provide information about the origin of this dust, the mechanisms that lead to its formation, its isotopic composition and the question of whether all planets or their moons are the same or different in this respect.

Observation of the extragalactic background light

The so-called “ zodiacal light ” is created by the scattering of the sunlight on the interplanetary dust, which disturbs the observation of the extragalactic background light . However, as a probe moves away from the sun, the strength of the zodiacal light rapidly decreases. The heliopause mission is therefore an opportunity to observe the background light coming from galaxies beyond the Milky Way, its intensity and its spectral course. The extragalactic background light represents a significant proportion of the electromagnetic radiation released by nuclear and gravitational processes since the age of recombination 400,000 years after the Big Bang ; From his observation, the researchers hope to gain deeper insights into the formation and development of the universe.

Area 2 (up to 200 AU)

Observation of space weather in the nearby interstellar space

Observations by the Interstellar Boundary Explorer from Earth orbit have shown that the influx of interstellar hydrogen , helium and oxygen is not uniform; sometimes helium and oxygen dominate, sometimes hydrogen. The ratio of neon to oxygen also varies greatly depending on the place and time.

The researchers hope that in-situ measurements will provide information about the density of the interstellar medium , the abundance of isotopes there, the degree of its ionization, the ratio of dust to gas and the mechanisms of its warming. A similar problem exists with the interstellar magnetic field. The Voyager probes have already taken some measurements, but there are still a number of unanswered questions. With high-resolution magnetometers , more precise information about the direction, strength, changes and the influence of the turbulent movement of the interstellar gas on the magnetic field should now be obtained on site. According to the observations made by satellites and deep-space probes in the 1980s to 2010s, it appears that the interstellar wind changes direction over the years. This could be an indication that space weather is constantly changing. Further investigations are necessary for this. The researchers hope to understand this phenomenon by measuring the composition, frequency, density and temperature of the interstellar dust clouds in situ .

Laws of the interaction between solar wind and interstellar medium

For a long time it was thought that there was a bow shock wave beyond the heliopause . According to the measurements of the Voyager probes, however, the flow speed of the solar wind outside the heliopause is below the speed of sound. That would mean that there is no bow shock wave, just a simple bow wave . The first probe is supposed to verify or refute this assumption during its flight through the heliospheric nose in order to come to a better understanding of the interaction between the heliosphere and the interstellar medium. It seems that between the heliopause and the bow wave there is a layer of heated, neutral hydrogen atoms, the so-called "hydrogen wall". On-site observations should now determine the mechanisms of their formation, their thickness and spatial distribution, their composition, density and temperature, and investigate whether these values ​​are the same or different in different directions and whether they are influenced by the respective solar activity.

Area 3 (up to 1000 AU)

Gravitational lensing effect of the sun

About 1000 astronomical units beyond the sun is the Oort cloud , a hypothetical, spherical-shell-shaped collection of more than 100 billion astronomical objects that is believed to be the origin of long-period comets . The Oort cloud is too far from both the sun and the closest stars to be sufficiently illuminated for direct observation. The researchers are now trying to get around this problem by flying the probes into the focal point of the sun's gravitational lens , which is around 550 AU away. It is hoped that the sun with its gravitational lensing effect will concentrate the weak light from objects in the Oort cloud so strongly that they can be observed directly; the lens effect of the sun would magnify by a factor of about 100 million. In addition to direct evidence of the existence of the Oort cloud, one could gain knowledge about its formation and composition, possibly also about a connection between the extinction of species on earth caused by impact events and the Oort cloud.

Technical aspects

For the first two probes is planned on the proven -Chang'e 3 - Bus resorting adapted to the scientific objectives of the mission. Since the probes are very far from the sun, a radionuclide battery with a power output of 200 W is used as power supply for the payloads and the operating systems . Two drive variants are currently under discussion:

  • A chemical drive that can work in two different ways, known in the Anglo-Saxon region as the dual mode propulsion system . The probe carries two different fuels - liquid hydrogen and a denser fuel such as RP-1 or hydrazine - and an oxidizer. At the beginning of the mission, during the acceleration phase, the hydrogen, which generates more thrust in relation to its mass, is burned, after which it is switched to the denser fuel for orbit corrections during the swing-by maneuvers etc. In this variant, the probe would have a launch weight of 3.3 t, of which 910 kg for the probe itself (50 kg of which is payload weight), the rest is fuel. As with the Chang'e-3 and Chang'e-4 lunar probes, the launch would take place with a three-stage Changzheng 3B launcher , which gives it an excessive hyperbolic speed of 20 km² / s².
  • An ion drive with an output of 0.5-5 kW, which generates a thrust of 20-200 mN; the weight-specific impulse of such an engine would be 2580 - 4000 s. Solar panels are used to power the ion engine, which runs for 20,000 hours at the beginning of the mission, i.e. a good two years . When the probe has reached Jupiter, acceleration is then only done by swing-by maneuvers. In this drive variant the probe with the heavy launcher would Changzheng 5 from the Wenchang Satellite Launch Center will not start, giving her a hyperbolic excess velocity of 77 sq km / s gives. For orbit correction maneuvers, the probe would have chemical engines with Monergol fuel in addition to the electric drive . The take-off weight in this variant would be 800 kg (including 50 kg payload weight), significantly lower than with the purely chemical drive.

In both variants have the probes at the end of the acceleration phase, a speed of 4 AE / year, which they at a minimum life of 30 years, the 100 AU away heliopause until October 1, 2049 the 100th anniversary of the founding of the People's Republic of China , reach would. The third probe, since it moves vertically out of the ecliptic plane at the beginning of the mission , cannot use swing-by maneuvers for acceleration. Here one wants to use an ion drive, which draws its electricity either from a small nuclear reactor with 10 kW power, or from a newly developed radionuclide battery with a corresponding power output. With a launch mass of 2.8 t (including 100 kg payload weight), the probe would reach a speed of 6 AU / year after the end of a three-year continuous acceleration phase and could thus cover a distance of 200 AU within its planned minimum service life of 35 years, i.e. penetrate into interstellar space.

The first two probes are pre-existing technology; an ion drive with 5 kW, 200 mN thrust and 4000 s specific impulse has been in use on the Shijian 20 technology test satellite since December 27, 2019 . The third probe, on the other hand, would be a completely new design, in which the nuclear reactor would be housed in its own, shielded unit, separated from the actual probe and connected to it only by an extendable lattice structure. The whole thing would then have the shape of a dumbbell or a shuttle . The reactor here is a fast breeder , which uses thermoelectricity to generate the electricity for the ion drive, similar to the 1965 NASA snapshot satellite . Compared to a radionuclide battery, this would have the advantage of higher performance, a better mass-performance ratio and a lower price. With a nuclear reactor, not only could the propulsion power be greatly increased, but more electricity would also be available for operating the scientific payloads and transmitting data to earth. Such a system would first have to be developed and tested on earth and in orbit. Therefore, the year 2030 is initially envisaged for the start of the third probe.

Telemetry, tracking and control

In connection with the lunar and Mars programs , China has already developed its deep space network very well. In particular, since the expansion of the Kashgar deep space station to a 4 × 35 m group antenna as part of the Mars program, the prerequisites have been given to receive signals from a distance of 100 AU, i.e. 15 billion kilometers. The K a -band should be used as the frequency band for the transmission of the payload data to earth , possibly also the X-band, which can only transmit relatively small amounts of data with the same transmission power, but is less susceptible to interference from atmospheric influences such as clouds or raindrops which is an important factor in China's humid summer monsoon climate . In order to be able to meet the scientific goals, the probes are equipped with a large number of devices that produce the data to be transmitted: magnetometers, detectors for energetically neutral atoms, anomalous cosmic rays and other particles, dust and plasma detectors, spectrometers and optical cameras . For comparison: the first two probes are supposed to carry payloads weighing 50 kg each, while the American Kuiper belt probe New Horizons has a payload weight of only 30 kg. The deep space station Kashgar and the 65-m tianma radio telescope in Shanghai already have the corresponding receiver, the 66-m antenna at Giyamusi in Manchurian takes having a K a retrofitted band receiver. For explorations in the range between 100 and 200 AU with a sensitivity of the receiver of −157 dBm, a parabolic antenna with a diameter of at least 80 m would be necessary. In the circle Qitai , Province Xinjiang one is since 2012 110 m telescope built that would be suitable with its broadband receiver (150 MHz to 115 GHz) for this purpose.

To the probes themselves a high-gain antenna is designed with an antenna gain of at least 59 dB in the K a are used which, although precisely aligned on the ground must, however, allows a relatively high data transfer rate band and if necessary 46 dB in the X band. This relatively high data rate is very low in absolute terms because of the great distances: with the first two probes with a relatively weak power supply, it should be around 160 bit / s at a distance of 100 AU, and with the third probe at 200 AU distance 200 bit / s. The probes can receive at 100 AU with 20 bit / s and with 200 AU with 10 bit / s. For comparison: Cassini transmitted from Saturn, that is, at a distance of about 8 AU, with about 50 kbit / s and received control signals from the earth with 8 kbit / s. Currently (2019) Klystron transmitters with an output of 10 kW are installed in the Chinese deep space stations . The prototype of a 50 kW transmitter for the X-band was completed and tested in 2018. For comparison: one of the X-band transmitters of the American Deep Space Network has a transmission power of 500 kW. In order to reduce data loss on the downlink, it is encrypted with a linear block code for error correction , a so-called low-density parity check code , also known as LDPC .

Since remote control of the probes is difficult due to the long signal transit times alone - around 15 hours at a distance of 100 AU - they should be given a high degree of autonomy. The probes should know where they are at all times and be able to switch on, calibrate and control the corresponding payloads themselves. They should constantly monitor their own systems and, if they notice an error, restart and configure them. Autonomous navigation, autonomous mission planning and self-repair are things that have already been tried out with the Chang'e-4 lunar probe . The engineers are currently working intensively on improving these systems.

International cooperation

When Zong Qiugang (宗 秋 刚, * 1965), the head of the Institute for Space Physics and Applied Technology at the Faculty of Earth and Space Sciences at the University of Beijing, presented the project to the public for the first time in May 2018, international guests were already present Example Elias Roussos from the Max Planck Institute for Solar System Research , Ip Wing-Huen (葉永 烜, * 1947) from the Institute for Astronomy of the National Central University of Taiwan and Dmitri Klimushkin and Anatoli Leonovich from the Institute for Solar-Terrestrial Physics of the Russian Academy of Sciences . When the scientists and engineers behind the exploration of the heliopause described the project in detail in the scientific journal Scientia Sinica in January 2019 , they again pointed out that such demanding missions should best be carried out in international cooperation. At the European Congress of Planetary Sciences in Geneva in September 2019, Zong Qiugang presented the project to a wider international audience. In addition, Wang Chi, director of the National Center for Space Science since 2017 , Ralph L. McNutt, who is working on a similar project at the Applied Physics Laboratory at Johns Hopkins University , and Robert Wimmer-Schweingruber from Christian-Albrechts-Universität zu Kiel , organized the design engineer of the dosimeter on the Chang'e-4 Lander , John D. Richardson from the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology , Li Hui (李 晖, * 1985) from the National Specialized Laboratory for Space Weather and Maurizio Falanga from the International Institute for space science in Bern and Beijing in November 2019 a workshop on this topic. In addition to Chinese and Russian scientists and the French Benoit Lavraud from the Center national de la recherche scientifique , several Americans also took part.

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