Lunar low frequency interferometry

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Lunar low-frequency interferometry is an approach currently being pursued primarily by radio astronomers from the Netherlands and the People's Republic of China in order to circumvent the shielding effect of the ionosphere on electromagnetic waves below 30 MHz. The naturalized expression “low frequency” or “long wave” (甚 低频 or 超长 波, English very low frequency or ultra-long wavelength ) is somewhat misleading; the radio astronomers mean the frequency range that is called " short wave " in radio broadcasting . So far, three test antennas have been placed behind the moon, all in connection with the Chang'e-4 mission of the China National Space Administration : a tripole antenna from Radboud University Nijmegen on the relay satellite Elsternbrücke , a microsatellite from the Harbin Polytechnic University and a tripole antenna from the Chinese Academy of Sciences on the probe lander.

principle

Reflection on the ionosphere

One of the properties of the ionosphere at around 100–200 km above the earth is to reflect electromagnetic waves below 30 MHz, an effect that radio amateurs, among other things, use to communicate over long distances. For radio astronomy, however, this means that signals from this frequency range cannot penetrate to the earth's surface. With a radio telescope in orbit, the situation is not much better because of the electromagnetic pollution . As early as 1973 NASA placed RAE-B, a satellite in lunar orbit that made radio astronomical observations in the range from 25 kHz to 13.1 MHz. In the 1990s, the ESA, among others, discussed the possibility of taking a low-frequency spectrometer as a payload when a probe landed on the back of the moon, which, together with a corresponding device , could act as an interferometer on a relay satellite , in which the mass of the moon would shield a large part of the interference radiation from the earth. The Europeans were particularly interested in observing the 21 cm line from the Dark Age 380,000 years after the Big Bang (about 13 billion years ago), which, due to the redshift due to the expansion of the universe, was less than 40 MHz arrives in the solar system.

In China at the time people were more interested in observing the sun than in the depths of space - since China relies heavily on satellites due to the size of the country, from vegetation drought monitoring / forest fire prevention to telecommunications, research into space weather has a high economic value there. Around the same time as ESA, the National Astronomical Observatories of the Chinese Academy of Sciences, together with the Institute for Electronics of the Academy of Sciences (中国科学院 电子 学 研究所), worked on a feasibility study of a solar interferometer in space or on the surface of the moon, similar to the Miyun Synthesis Radio Telescope that has been in operation at the Miyun Observatory near Beijing since 1967 . A model with two prototype antennas was built and interferometry experiments were carried out, which gave relatively good results. When China announced in 2014, after the successful landing of Chang'e-3 on the Earth-facing side of the moon, that it wanted to land on the back of the moon in a next step, one of the first payloads was a low-frequency spectrometer for radio astronomical observation of the sun firmly.

Chang'e 4 lander

The low - frequency spectrometer for the lunar probe Chang'e-4 , often abbreviated to “VLFRS” because of the English name Very Low Frequency Radio Spectrometer abroad, was developed by the Beijing Specialized Laboratory for Electromagnetic Radiation and Scouting Technology at the Institute for Electronics of the Chinese Academy of Sciences (中国科学院 电磁 辐射 与 探测 技术 重点 实验室) developed and built. Three 5 m long, vertically arranged, permanently mounted active antennas make it possible to split the electric field of incoming signals into three components that can be represented by a vector. After further processing of the individual signals, statements can be made about the position of the radio source , the polarization of the signal, its spectrum and the change over time. The scientific director of the project is Ping Jinsong (平 National, * 1968) from the National Astronomical Observatories of the Chinese Academy of Sciences .

The heart of the spectrometer is the electronic unit with a three-channel receiver for 0.1–40 MHz, power supply, reference clock with 100 MHz crystal oscillator and the modules for communication with the lander, which transmits the data with its microwave transmitter via the relay satellite Elsternbrücke and sends the large antennas in Miyun and Kunming to the ground segment of the lunar program in Beijing, where the data packets are put into tabular form and made available to the scientists. The spectrometer is also controlled via the lander's data bus: the technicians at the ground segment write command lines to set the frequency of the receiver, etc., transmit them via the Xi'an satellite control center to the deep-space network of the People's Liberation Army , which then sends them via one of its three , antennae ( Kashgar , Giyamusi , Zapala ) distributed over the whole earth to the lunar probe.

The main task of the spectrometer is to measure the low-frequency radiation of the sun during the lunar day, i.e. every two weeks for two weeks, in particular type II and III radio flashes triggered by solar flares as well as eruptions in the upper layers of the corona with the accompanying electromagnetic Waves in the 100 m and 1000 m range. In addition, the scientists can use the shielding effect that the (extremely thin) ionosphere of the moon has on the solar wind to carry out more detailed investigations into the mechanisms behind the changes in the lunar ionosphere, at least in the sky over the South Pole Aitken Basin , where the Probe landed on January 3, 2019. Apart from the antenna extension mechanism, which was used once on January 4, 2019, the low-frequency spectrometer has no moving parts that could be clogged by the lunar dust. On the 17th lunar day, the device was still working perfectly (as of April 2020), and since it is powered by the lander's radionuclide battery - the interferometer consumes less than 20 W - it could be another 30 years if there are no unforeseen incidents be used for a long time, at least as long as a relay satellite is available (see below).

A certain problem arose in May 2020 when, in preparation for the Mars Tianwen-1 mission, some of the resources of the Chinese deep space network had to be withdrawn from the lunar program . Like all other devices of the Chang'e-4 mission, with the exception of the neutron and radiation dose detector at the Christian-Albrechts-Universität zu Kiel , the low-frequency spectrometer was activated after the lander "woke up" at sunrise on the back of the moon on 17. May 2020 no longer in operation. After the completion of the renovation work at the Kashgar and Giyamusi deep-space stations on June 13, 2020, regular measurements on the moon were resumed at sunrise on June 15.

Magpie Bridge

In contrast to the previous lunar missions, the center for lunar exploration and space projects of the China National Space Administration at Chang'e-4 also invited foreign research institutes to take part in the mission with payloads. The Department of Astrophysics at the Faculty of Natural Sciences, Mathematics and Computer Science at Radboud University Nijmegen has been working with the Chinese Academy of Sciences , specifically the Shanghai Astronomical Observatory and the National Center for , since 2014 under the leadership of Heino Falcke , Professor of Astroparticle Physics and Radio Astronomy Space science , on a project that plans to set up a long-wave radio interferometer made up of a "mother ship" and 8 mini - satellites , the so-called Decametres Space Linear , based on the principle of the Low Frequency Array (LOFAR) led by Falcke at Lagrange point L 2 behind the moon Array , or "DSL" for short.

After the DSL project had been postponed in 2015 by ESA and the Chinese Academy of Sciences in favor of the Solar Wind Magnetosphere Ionosphere Link Explorer (SMILE), it was now an option to go on the relay satellite Elsternbrücke , where there was plenty of power and payload weight available to fly a single prototype of the planned space interferometer. The so-called Netherlands-China Low-Frequency Explorer , or “NCLE” for short, consists, like the version on the lander, of three 5 m long antennas arranged perpendicular to each other - the length results from the frequency of the signals the scientists are interested in the back of the satellite. While the satellite orbits around the L 2 point in a halo orbit , it always remains aligned so that the parabolic antenna points to the moon or the earth behind, in order to be able to fulfill its function as a relay station. This means that the antennas of the NCLE are always directed into the depths of space. The parabolic antenna of the Elsternbrücke, however, has a diameter of only 4.2 m, i.e. H. for calibration purposes, the interference radiation from the earth must also be received; since the orbit of the Elsternbrücke has a much larger diameter than the lunar disc, the earth is always in view.

The aim of the NCLE mission is initially to measure the interference radiation in the earth-moon system, i.e. human radio traffic, the radiation associated with auroras in the range between 50 and 500 kHz or wavelengths in the 1000 m range ( Auroral Kilometric Radiation ) and the quasi-thermal noise of the interplanetary medium . If the interference radiation is known, it can be calculated from the signals from the beginning of the universe and the result compared with the measurements of the spectrometer on the probe's lander, which is significantly better shielded by the mass of the moon. In addition, the Dutch astronomers intend to create a low-frequency map of the starry sky, to study the earth's ionosphere and to discover pulsars in the low-frequency range. A halo orbit like the one chosen for the Elsterbrücke is unstable and has to be easily corrected again and again. For this purpose, the relay satellite has fuel on board for 5 years. When the fuel is used up (probably mid-2023), that also means the end of the NCLE mission. The Chang'e-7 mission to the Moon's South Pole, planned for 2024 , will carry a new relay satellite, but it will carry different payloads.

The main task of the relay satellite Elsternbrücke is to transmit the data determined by the lander and rover of the Chang'e 4 mission from the back of the moon to the earth and the control signals from the earth to the moon. After Jadehase 2 discovered coat material from the depths of the moon in spring 2019 and broke the 300 meters mark in early November 2019, i.e. more than met its goals, the National Space Agency of China decided to take the risk and extend the antennas of the NCLE, clearly later than originally planned ( Jadehase 1 had already ceased operations after two months or 114 m). Under the supervision of Marc Klein Wolt, the head of the NCLE group at Radboud University, and Eric Bertels from Innovative Solutions In Space, the manufacturer of the antennas in Delft , the start was on November 14, 2019, i.e. while on the back of the It was a moonlit night and the Lander and Rover were in sleep mode with their antennas extended.

First of all, the relay satellite had to be rotated so that the antenna housing was illuminated by the sun as evenly as possible and reached the optimal operating temperature, then the radio command was given to extend it. The long time in space - more than a year - seems to have damaged the mechanism. After the antennas came out without any problems at first, this became more and more difficult as the work progressed. The Dutch team then decided on November 16 to first collect data with the shorter antennas, which would provide information about the state of the universe around 800 million years after the Big Bang , i.e. the end of the reionization era . It is planned to try again at a later date to extend the antennas to their full length of 5 m, which would then enable observations of events at the beginning of the Dark Age .

Longjiang-2

A further preliminary study for the DSL project was carried out by the two under the direction of Zhang Jinxiu (张 锦绣, * 1978), a specialist in group-wide satellites, at the Institute for Satellite Technology of the Faculty of Space Technology at Harbin Polytechnic University (哈尔滨 工业 大学 航天 学院卫星 技术 研究所) developed and built microsatellites Longjiang-1 and Longjiang-2 , originally called DSLWP-A1 and DSLWP-A2, where DSLWP stands for Discovering the Sky at Longest Wavelengths Pathfinder . The name "Longjiang" later chosen stands for " Heilongjiang ", the Chinese name for the Amur River and the province named after it, the capital of which is Harbin . The two satellites weighed only 47 kg each and were only 50 × 50 × 40 cm in size. They each had two tripole antennas on the top and bottom of the satellite next to the barrel-shaped platform, with which the frequency range from 1 MHz to 30 MHz could be observed, and were originally intended to fly in formation, with a variable distance of 1– 10 km, as planned for the microsatellite fleet of the DSL project. This would have made true interferometry possible.

This would have been a very demanding undertaking, even if the satellite equipment had not been reduced to a micro level. In the Russian RadioAstron project and the Japanese Highly Advanced Laboratory for Communications and Astronomy (HALCA), only one satellite in Earth orbit worked together with radio telescopes on Earth in a VLBI network, while in DSLWP two satellites were in motion and constantly were exposed to changing gravitational influences, depending on where they were in the earth-moon system. The optimum working range for lunar low frequency interferometry is in the area behind the moon where the umbra of the earth and the sun intersect, i.e. H. where remote control from Earth is absolutely impossible. Therefore, both satellites were equipped with radio equipment that enabled them to communicate with each other and to independently carry out things like synchronizing the on-board clocks and precisely adjusting their distance.

The two satellites were launched together with the Elsternbrücke on May 21, 2018. However, on May 25, Longjiang-1 was unable to brake to enter lunar orbit, flew past the moon, and Longjiang-2 had to operate as a single unit. Nevertheless, the mission is considered a success by those responsible for the People's Republic of China's lunar program . Longjiang-2 was the first microsatellite that independently swiveled into an earth-moon transfer orbit, carried out orbit correction maneuvers near the moon and thus reached an orbit around the moon. This opened up new opportunities for low-cost, deep space missions. As Chen Xuelei (陈 学 雷, * 1969), the scientific director of the project at the National Astronomical Observatories of the Chinese Academy of Sciences, put it in an interview in February 2018, DSLWP was from the start more of an experiment that you can use to plan future missions wanted to learn.

As a payload, Longjiang-2 carried, among other things, a long-wave detector developed and built by the National Center for Space Science of the Chinese Academy of Sciences under the direction of An Junshe (安 军 社), with which he - actually - during the 437 days he spent in lunar orbit was only planned for a life span of one year, but by eliminating the interferometry experiments, the satellite was able to save fuel - a continuous spectrum in the range of 1–30 MHz was recorded. Since the satellite was alternately in front of and behind the moon in its orbit, which was initially inclined by 21 ° relative to the lunar equator, it was able to carry out extensive investigations into the effect of electromagnetic interference from the earth on observations in the low frequency range. On July 31, 2019 at 10:20 pm Beijing time, Longjiang-2 was brought to a controlled crash on the far side of the moon after lowering its orbit for six months.

Lunar LOFAR

For several years now, Radboud University Nijmegen has been thinking about building something similar to the Low Frequency Array on the back of the moon. Originally it was planned to use a simple rover with an operational range of 20 km, as it should be used in the Moon NEXT project of the ESA in a first stage. This was supposed to lay out wires on the ground on a base line of around 10 km in length, which, when connected together, would form antennas made of crossed dipoles. The Moon NEXT project never got beyond the Phase A Feasibility Study of 2008, and so a full antenna array with 30-100 dipoles spread over a 100 km baseline, launched by robots or humans, remained a thought experiment.

At a press conference on January 14, 2019 on the occasion of the successful landing of Chang'e-4, however, Wu Yanhua (吴艳华, * 1962), Deputy Director of the China National Space Administration , announced that China was in talks with Russia, the USA and Europe about the establishment of a lunar base. The US did not take part in these talks in the further course, but on July 22, 2019, Wu Yanhua and Peng Zhaoyu (裴 照 vertret), the deputy head of the Center for Lunar Exploration and Space Projects at the CNSA, attended the 4. International conference for lunar and deep space exploration in Zhuhai announce that ESA, Roskosmos and the National Space Agency of China have reached a consensus to take a joint pioneering role in planning an international research base on the moon. This lunar base should not only support research on the origin and development of the moon, the environment on the lunar surface, etc., but also research on the beginning and development of the universe. With this, radio astronomy , which dealt with the Dark Ages , had come to the fore again.

Web links

Individual evidence

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