SPACE
Scientists developed a legged small celestial body landing mechanism for landing simulation and experimental test
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FIG. 1. SCHEMATIC OF THE LANDING MECHANISM.
view moreCREDIT: SPACE: SCIENCE & TECHNOLOGY
Landing stably is the precondition for exploring the small celestial body in situ. The surface of small body is weak gravity and irregular, and the surface environment is unknown and uncertain. The landing mechanism tends to rebound and turn over, and the landing stability time is long. However, there is difference on the Moon and the Mars surface while most of the landing performance researches are focused on the lunar landing so far. Therefore, it is of great important to study the landing performance in different conditions to analyze the landing stability boundary, and to propose reasonable landing suggestions to support the small celestial body exploration of China. In a research article recently published in Space: Science & Technology, researchers from Beijing Institute of Spacecraft System Engineering, Harbin Institute of Technology, and Polytechnic University of Milan establish the simulation model of the landing mechanism in different landing conditions, analyze the sensitivity of the key parameters affecting the landing performance, and verify correctness of the simulation by experimental tests, which can provide suggestions for the landing mechanism land stably on the small celestial body.
First, authors briefly repeat the landing mechanism and the landing simulation. The small celestial body landing mechanism used in the simulation contains landing foot, landing legs, cardan element, damping element, equipment base, and so on (Fig. 1). In simulation, two scenarios are taken into consideration, namely (1) The landing mechanism lands toward the landing slope with Vx > 0 and (2) The landing mechanism lands away from the landing slope with Vx < 0. In each scenario, three landing modes are classified according to the contact order between the landing foot and the landing slope, i.e. (a) 1-2 landing mode, (b) 2-1 landing mode, and (c) 1-1-1 landing mode (with 30° yaw angle). For all landing modes in both simulation scenarios, the landing mechanism turnover is prevented by the retro-rocket, and there is no sliding of the landing feet. Detailed landing performances are summarized in Table 1. The maximum overloading acceleration of the equipment base is less than 10 g, and the landing stability time is less than 4 s. It shows that the landing mechanism can land safely in different landing conditions. Additionally, when Vx > 0, it can be fund that the 2-1 mode has the best landing performance among three modes, and 1-2 and 1-1-1 modes’ landing performances are similar. When Vx < 0, the landing performance of the 2-1 mode is the best, the 1-2 mode is in general, and the 1-1-1 mode is the worst.
Secondly, key factors affecting the landing performance are analyzed. (1) Cardan element damping (c2). The landing stabilization time is significantly shortened and the overloading acceleration is weakened when c2 is variable in comparison to constant c2. The landing mechanism has better landing performance when c2 is variable. (2) Foot anchors. The foot anchors affect the friction coefficient between landing feet and the landing surface. Slipping induces the landing mechanism far away from the landing point, which would affect the anchorage of the anchoring system. Friction between the landing mechanism and the landing surface should be high to avoid sliding of the landing mechanism. Overturning of landing mechanism due to high friction can be eliminated by retro-rocket thrust. Therefore, it is helpful to design foot anchors on the landing mechanism, as it can penetrate the landing surface and prevent or weaken sliding of the landing mechanism. (3) Retro-rocket thrust. Retro-rocket thrust can prevent the landing mechanism from bouncing or turning, thus the retro-rocket thrust is helpful for landing successfully. (4) Landing slope. The larger the slope angle is, the higher the turning angular velocity of landing legs is, and the longer the landing stabilization time is. The influence of slope angle on equipment base overloading acceleration is not obvious. Therefore, the landing surface with smaller slope angle should be selected to reduce the landing stabilization time. (5) Landing Attitude. When the landing mechanism lands in different landing attitudes within the allowable landing velocity, the maximum overloading acceleration is less than 10g and the landing stabilization time is less than 5 s. Landing performance is good. When the yaw angle is 60° (that is, the 2-1 landing mode), the landing mechanism has the minimum overloading acceleration and the shortest landing stability time, and the landing performance is the best.
Then, the validity of the simulation model is verified by tests. These tests are carried out on the air-floating platform. The landing accelerations are measured by acceleration sensors. The landing attitude of the landing mechanism and the location of the sensors are shown in Fig. 2. Tests of landing on a 30° slope in the 1-2 mode, the 1-2 mode, and the 1-1-1 mode are conducted separately. These landing modes and velocities are imported into the simulation model. Landing performances between test and simulation are compared. The overloading acceleration of the equipment base obtained by simulation is close to that obtained by test, and the simulation result is slightly larger than the test. This is due to the mechanical flexibility of the landing mechanism, which will produce flexible deformation in the test and absorb part of the impact load. The changes of landing leg turnover angular velocity and turnover angle in simulation and test are relatively consistent. But at the time between about 0.7 and 2.5 s in the 1-2 mode, about 0.5 and 2 s in the 2-1 mode, and for the while course in 1-1-1 mode the landing leg turnover angle in test is less than that in simulation. The reason is that landing surface in test is hard wood and the foot anchors fail to penetrate the hard wood, which results in slight slip of the landing mechanism. In addition, it is found that the 2-1 landing mode has the shortest stability time, and there is no obvious relationship between the overloading acceleration and the landing mode.
Finally, authors come to the conclusion that the following methods are helpful to improve landing performance: (a) Three legs landing mechanism should preferentially choose the 2-1 landing mode. (b) Adjustable damping corresponding to landing conditions is helpful to improve the landing stability. (c) Foot anchors can reduce landing slip and shorten landing stabilization time. (d) Retro-rocket on top of the landing mechanism can weaken or prevent rebounding when landing. (e) The landing mechanism should preferentially land on flat areas.
Tab 1. Landing simulation results summary.
Fig. 2. Landing mechanism on the air-floating platform.
CREDIT
Space: Science & Technology
JOURNAL
Space Science & Technology
ARTICLE TITLE
A Legged Small Celestial Body Landing Mechanism: Landing Simulation and Experimental Test
Dr. Jennifer Lotz appointed Space Telescope Science Institute Director
Dr. Lotz will begin her five-year appointment as STScI Director starting February 12, 2024
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DR. JENNIFER LOTZ, NEW DIRECTOR OF THE SPACE TELESCOPE SCIENCE INSTITUTE.
view moreCREDIT: NOIRLAB
The Association of Universities for Research in Astronomy (AURA) is pleased to announce the appointment of Dr. Jennifer Lotz as the Director of the Space Telescope Science Institute (STScI). Dr. Lotz will begin her five-year appointment as STScI Director starting February 12, 2024. Previously, Dr. Lotz was the Director of the International Gemini Observatory which is operated by NSF’s NOIRLab, and managed by AURA.
“Dr. Lotz is a science driven, accomplished leader,” said Dr. Matt Mountain, President of AURA, which manages STScI on behalf of NASA. “Jen’s passion for the Institute’s mission, to enable the science community in its exploration of the ground-breaking science coming from both JWST and Hubble, and her compelling vision, will ensure an exciting future as she leads STScI into a new era of space science.”
Dr. Lotz was chosen from a pool of highly qualified candidates by a selection committee of respected leaders in the field of astronomy. Her proven leadership skills as Director of Gemini Observatory, her research experience, and her knowledge of the challenges the field of astronomy faces were some of the qualifications that led to her selection as STScI’s next Director. The Chair of AURA’s Board of Directors, Dr. Maura Hagan, added, “The AURA Board of Directors is thrilled with the selection of Jennifer Lotz as the next STScI Director. She represents a new generation of scientific leadership.”
Dr. Lotz will succeed Dr. Nancy Levenson who served since August 2022 as STScI Interim Director. “I welcome Jen’s new perspectives and look forward to working with her to advance STScI,” said Dr. Levenson, who will return to her former position as STScI Deputy Director. AURA extends thanks to Dr. Levenson for her service as Interim Director.
Dr. Lotz received her PhD in astrophysics from Johns Hopkins University in 2003 and specializes in galaxy evolution and morphology, the high-redshift Universe, and gravitational lensing. Before her appointment as Gemini Director, she was a tenured associate astronomer at STScI with a joint appointment as a research scientist at Johns Hopkins University. She was also a Leo Goldberg Fellow at the National Optical Astronomy Observatory, and a postdoctoral fellow at the University of California Santa Cruz.
“I am honored to be rejoining STScI as its next Director. The Institute's work on Hubble and JWST has been an inspiration for the world,” commented Jen Lotz. “I am also excited to partner with NASA to drive forward a new era of scientific discovery with the new generation of space telescopes - JWST, Roman, and the Habitable Worlds Observatory.”
Lotz is a leading expert in the field of galaxy mergers, and makes use of both ground-based and space telescopes to track the growth of galaxies over cosmic time. She led the Hubble Frontier Fields program, one of the largest programs undertaken with Hubble to detect the faintest distant galaxies yet seen. She continues her study of galaxies at the edge of the universe as part of the JWST Cosmic Evolution Early Release Science team.
STScI, in Baltimore, Maryland, is managed by AURA. For NASA, STScI leads the science operations for the Hubble Space Telescope and the science and mission operations for the James Webb Space Telescope. The Institute will also perform the science operations for the upcoming Nancy Grace Roman Space Telescope. STScI staff conducts world-class scientific research and runs the Barbara A. Mikulski Archive for Space Telescopes (MAST) that curates and disseminates data from over 20 astronomical missions. STScI brings science to the world through internationally recognized news, and public outreach and engagement programs.
NOIRLab’s Director Patrick McCarthy has nominated Gemini Deputy Director, Scott Dahm, as the International Gemini Observatory’s Interim Director pending review by governing bodies and funding agencies. NOIRLab will now begin the search for a new permanent Director to lead Gemini Observatory through its next phase.
AURA congratulates Dr. Lotz on her appointment!
Black holes are messy eaters
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CARBON MONOXIDE (CO; INDICATING THE PRESENCE OF MEDIUM-DENSITY MOLECULAR GAS) IS SHOWN IN RED; ATOMIC CARBON (C; INDICATING THE PRESENCE OF ATOMIC GAS) IN BLUE; HYDROGEN CYANIDE (HCN; INDICATING THE PRESENCE OF HIGH DENSITY MOLECULAR GAS) IN GREEN; AND THE HYDROGEN RECOMBINATION LINE (H36Α; INDICATING THE PRESENCE OF IONIZED GAS) IN PINK. THE SIZE OF THE CENTRAL DENSE GAS DISK (GREEN) IS APPROXIMATELY 6 LIGHT-YEARS. THE PLASMA OUTFLOW TRAVELS ALMOST PERPENDICULAR TO THE DISK.
view moreCREDIT: ALMA (ESO/NAOJ/NRAO), T. IZUMI ET AL.
New observations down to light-year scale of the gas flows around a supermassive black hole have successfully detected dense gas inflows and shown that only a small portion (about 3 percent) of the gas flowing towards the black hole is eaten by the black hole. The remainder is ejected and recycled back into the host galaxy.
Not all of the matter which falls towards a black hole is absorbed, some of it is ejected as outflows. But the ratio of the matter that the black hole “eats,” and the amount “dropped” has been difficult to measure.
An international research team led by Takuma Izumi, an assistant professor at the National Astronomical Observatory of Japan, used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the supermassive black hole in the Circinus Galaxy, located 14 million light-years away in the direction of the constellation Circinus. This black hole is known to be actively feeding.
Thanks to ALMA’s high resolution, the team was the first in the world to measure the amount of inflow and outflow down to a scale of a few light-years around the black hole. By measuring the flows of gasses in different states (molecular, atomic, and plasma) the team was able to determine the overall efficiency of black hole feeding, and found that it was only about 3 precent. The team also confirmed that gravitational instability is driving the inflow. Analysis also showed that the bulk of the expelled outflows are not fast enough to escape the galaxy and be lost. They are recycled back into the circumnuclear regions around the black hole, and start to slowly fall towards the black hole again.
JOURNAL
Science
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Supermassive black hole feeding and feedback observed on sub-parsec scales
ARTICLE PUBLICATION DATE
3-Nov-2023
New research shows quasars can be buried in their host galaxies
-With pictures-
A new study reveals that supermassive black holes at the centres of galaxies, known as quasars, can sometimes be obscured by dense clouds of gas and dust in their host galaxies.
This challenges the prevailing idea that quasars are only obscured by donut-shaped rings of dust in the close vicinity of the black hole.
Quasars are extremely bright objects powered by black holes gorging on surrounding material.
Their powerful radiation can be blocked if thick clouds come between us and the quasar.
Astronomers have long thought this obscuring material only exists in the quasar's immediate surroundings, in a "dusty torus" (or donut) encircling it.
Now, a team of scientists led by Durham University have found evidence that in some quasars, the obscuration is entirely caused by the host galaxy in which the quasar resides.
Using the Atacama Large Millimeter Array (ALMA) in Chile, they observed a sample of very dusty quasars with intense rates of star formation.
They found that many of these quasars live in very compact galaxies, known as “starburst galaxies”, no more than 3000 light-years across.
These starburst galaxies can form more than 1000 stars like the Sun per year.
To form such a large number of stars, the galaxy needs a huge amount of gas and dust, which are essentially the building blocks of stars.
In such galaxies, clouds of gas and dust stirred up by rapid star formation can pile up and completely hide the quasar.
The full study has been published in the journal MNRAS.
Lead author of the study Carolina Andonie, PhD student in the Centre for Extragalactic Astronomy at Durham University, said: "It's like the quasar is buried in its host galaxy.
“In some cases, the surrounding galaxy is so stuffed with gas and dust, not even X-rays can escape.
"We always thought the dusty donut around the black hole was the only thing hiding the quasar from view.
“Now we realise the entire galaxy can join in.
“This phenomenon only seems to happen when the quasar is undergoing an intense growth spurt."
The team estimates that in about 10-30% of very rapidly star-forming quasars, the host galaxy is solely responsible for obscuring the quasar.
The findings provide new insights into the link between galaxy growth and black hole activity.
Obscured quasars may represent an early evolutionary stage, when young galaxies are rich with cold gas and dust, fuelling high rates of star formation and black hole growth.
Study co-author Professor David Alexander of Durham University said: “It's a turbulent, messy phase of evolution, when gas and stars collide and cluster in the galaxy’s centre.
“The cosmic food fight cloaks the baby quasar in its natal cocoon of dust.”
Unveiling these buried quasars will help scientists understand the connection between galaxies and the supermassive black holes at their hearts.
ENDS
Media Information
Carolina Andonie from Durham University is available for interview and can be contacted on carolina.p.andonie@durham.ac.uk.
Alternatively, please contact Durham University Communications Office for interview requests on communications.team@durham.ac.uk or +44 (0)191 334 8623.
Graphics
Associated images are available via the following link: https://www.dropbox.com/scl/fo/8206r75p5b7fofc1l6x39/h?rlkey=apmuc3o6xzz0axoip5lggzgvo&dl=0
Image 1 - Artistic illustration of the thick dust torus thought to surround supermassive black holes and their accretion disks. [ESA / V. Beckmann (NASA-GSFC)]
Image 2 - Illustration of the sources of obscuration. Orange clouds represent the dust and gas close to the central black hole, and blue clouds with stars represent the dust and gas in the galaxy forming the stars. The gradient in the blue colour represents the amount of gas and dust in the galaxy, from little (transparent) to large (opaque) amount of gas and dust.
Source Information
‘Obscuration beyond the nucleus: infrared quasars can be buried in extreme compact starbursts’, (2023), Carolina Andonie et al., MNRAS.
A full copy of the paper can be viewed here - https://arxiv.org/pdf/2310.02330.pdf
About Durham University
Durham University is a globally outstanding centre of teaching and research based in historic Durham City in the UK.
We are a collegiate university committed to inspiring our people to do outstanding things at Durham and in the world.
We conduct research that improves lives globally and we are ranked as a world top 100 university with an international reputation in research and education (QS World University Rankings 2024).
We are a member of the Russell Group of leading research-intensive UK universities and we are consistently ranked as a top 10 university in national league tables (Times and Sunday Times Good University Guide, Guardian University Guide and The Complete University Guide).
For more information about Durham University visit: www.durham.ac.uk/about/
END OF MEDIA RELEASE – issued by Durham University Communications Office.
JOURNAL
Monthly Notices of the Royal Astronomical Society
Researchers find gravitational lensing has significant effect on cosmic birefringence
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COSMIC MICROWAVE BACKGROUND (CMB) POLARIZED LIGHT SUBJECTED TO GRAVITATIONAL LENSING EFFECTS, IN ADDITION TO COSMIC BIREFRINGENCE. ON THE FAR LEFT, THE WHITE LINES SHOW THE POLARIZATION PATTERN OF THE CMB LIGHT GENERATED IN THE EARLY UNIVERSE. THESE ROTATE DUE TO COSMIC BIREFRINGENCE, RESULTING IN THE CURRENTLY OBSERVED CMB DEPICTED BY THE BLACK LINES ON THE RIGHT SIDE OF THE IMAGE. HOWEVER, THE PATH OF LIGHT IS BENT BY THE GRAVITATIONAL DISTORTION OF SPACE-TIME CREATED BY THE LARGE-SCALE STRUCTURE IN THE MIDDLE, AND SO THE WHITE LINES SHOWING THE POLARIZATION PATTERN ON THE RIGHT SIDE OF THE IMAGE SHOWS WHAT IS OBSERVED.
view moreCREDIT: NAOKAWA AND NAMIKAWA, HTTPS://DOI.ORG/10.1103/PHYSREVD.108.063525
Future missions will be able to find signatures of violating the parity-symmetry in the cosmic microwave background polarization more accurately after a pair of researchers has managed to take into account the gravitational lensing effect, reports a new study in Physical Review D, selected as an Editors' Suggestion.
How far does the universe extend? When and how did the universe begin? Cosmology has made progress in addressing these questions by providing observational evidence for theoretical models of the universe based on fundamental physics. The Standard Model of Cosmology is widely accepted by researchers today. However, it still cannot explain fundamental questions in cosmology , including dark matter and dark energy.
In 2020, an interesting new phenomenon called cosmic birefringence was reported from the cosmic microwave background (CMB) polarization data. Polarization describes light waves oscillating perpendicularly to the direction it is traveling. In general, the direction of polarization plane remains constant, but can be rotated under special circumstances. A reanalysis of the CMB data showed the polarization plane of the CMB light may have slightly rotated between the time it was emitted in the early universe and today. This phenomenon violates the parity symmetry and is called the cosmic birefringence.
Because cosmic birefringence is challenging to explain with the well-known physical laws, there is a strong possibility that yet to be discovered physics, such as the axionlike particles (ALPs), lies behind it. A discovery of cosmic birefringence could lead the way to revealing the nature of dark matter and dark energy, and so future missions are focused on making more precise observations of the CMB.
To do this, it is important to improve the accuracy of current theoretical calculations, but these calculations so far have not been sufficiently accurate because they do not take gravitational lensing into account.
A new study by a pair of researchers, led by The University of Tokyo Department of Physics and Research Center for Early Universe doctoral student Fumihiro Naokawa, and Center for Data-Driven Discovery and Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Project Assistant Professor Toshiya Namikawa, established a theoretical calculation of cosmic birefringence that incorporates gravitational lensing effects, and worked on the development of a numerical code for cosmic birefringence that includes gravitational lensing effects, which will be indispensable for future analyses.
First, Naokawa and Namikawa derived an analytical equation describing how the gravitational lensing effect changes the cosmic birefringence signal. Based on the equation, the researchers implemented a new program to an existing code to compute the gravitational lensing correction, and then looked at the difference in signals with and without the gravitational lensing correction.
As a result, the researchers found that if gravitational lensing is ignored, the observed cosmic birefringence signal cannot be fitted well by the theoretical prediction, which would statistically reject the true theory.
In addition, the pair created simulated observational data that will be obtained in future observations to see the effect of gravitational lensing in the search for ALPs. They found that if the gravitational lensing effect is not considered, there would be statistically significant systematic biases in the model parameters of ALPs estimated from the observed data, which would not accurately reflect the ALPs model.
The gravitational lensing correction tool developed in this study is already being used in observational studies today, and Naokawa and Namikawa will continue to use it to analyze data for future missions.
Details of their study were published in Physical Review D on September 27 as an Editors’ Suggestion.
JOURNAL
Physical Review D
ARTICLE TITLE
Gravitational lensing effect on cosmic birefringence
Simons Observatory in Chile.
CREDIT
Debra Kellner
The difference in the cosmic birefringence signal with and without gravitational lensing. The blue dots show the signals when the gravitational lensing effect is ignored, and the red dots are the signals when the gravitational lensing effect is considered. The red error bars show the expected observation errors when the Simons Observatory will be used. The difference with and without gravitational lensing is not negligible.
CREDIT
F. Naokawa & T. Namikawa “Gravitational lensing effect on cosmic birefringence”, Phys. Rev. D 108, 063525, Copyright (2023) the American Physical Society https://doi.org/10.1103/PhysRevD.108.063525
Scientists discussed the key questions of solar wind–moon interaction
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FIG. 1. THE COMPLICATED LUNAR SURFACE ENVIRONMENT THAT IS AFFECTED BY FACTORS BOTH FROM OUTSIDE AND INSIDE OF THE MOON.
view moreCREDIT: SPACE: SCIENCE & TECHNOLOGY
As the nearest celestial body to Earth, Moon’s space environment is distinctive to Earth’s mainly because of lack of a significant atmosphere/ionosphere and a global magnetic field. From a global perspective, solar wind can bombard its surface, and the solar wind materials cumulated in the soil record the evolution of the Solar System. Many small-scale remanent magnetic fields are scattered over the lunar surface and, just as planetary magnetic fields protect planets, they are believed to divert the incident solar wind and shield the local lunar surface beneath, thus producing unique local surface environment that is critical to activities of human beings/facilities, thus providing unique landing sites to explore the origins of lunar swirls and remanent magnetic fields. Evidences have hinted that this local interaction, however, may be also distinct with the interacting scenario on planets, and the specific process has not been revealed because of lack of in situ observations in the near-Moon space or on the lunar surface. The global and local solar wind interactions of the Moon represent 2 types of characteristic interaction of celestial bodies with stellar wind in deep space, i.e., the interactions of nonmagnetized bodies and of small-scale magnetized bodies, both of which may occur on asteroids and Mars. These deep-space celestial bodies, either difficult or impossible to reach for human beings or artificial satellites, are hard to measure, and the exploration of the Moon can reveal the mystery of stellar wind interaction on these bodies. In a review article recently published in Space: Science & Technology, scholars from Chinese Academy of Sciences and Beihang University reviewed Key questions on solar wind–Moon interaction.
First, authors introduce the background environment and properties of the Moon (schemed in Fig. 1). The Sun continually releases energy into interplanetary space in two forms of ejecting particles and emitting radiations, which stirs up the space environment of the entire Solar System. The Sun ejects the magnetized plasma flow, which is solar wind composed of the oppositely charged particles, i.e., ions and electrons. The difference in mass between protons and electrons is huge, which makes the solar wind a complicated material with multiple scales. The solar wind interaction of the Moon consists of processes across multiple plasma scales, thus resulting in the complicated Moon’s space environment and solar wind interaction. The Sun also emits wide frequency-range radiations. About half of the solar radiation energy is confined within the visible waveband; however, the other radiations are key to the space environment around all kind of Solar System objects. The properties of the Moon itself also influence the solar wind interaction, and they are mainly the electrical conductivity and magnetism, where interdisciplinary studies between space physics, geophysics, and even geology are involved. As for internal structures, evidence shows that the Moon as a whole is not a good conductor. As for lunar magnetic fields, the Moon has been thought to have no global intrinsic magnetic field and, thus, have no lunar magnetosphere; but many small-scale remanent magnetic fields scattered over the lunar surface. As for the lunar atmosphere, a very thin atmosphere is present, and there is no a significant lunar ionosphere. As for the lunar surface, the interior of the Moon is covered the regolith layer, the outmost layer with a depth that can be as large as tens of kilometers. Lunar dust is another term frequently occurring when discussing the lunar surface environment, whose dynamics is controlled by the electric and magnetic fields.
Then, authors propose key questions of solar wind–moon interaction.
(1) Do the lunar mini-magnetospheres exist?
1.1. Do the mini-magnetosphere have the same structures as those of the huge planetary magnetosphere? In particular, does it have a bow shock ahead? Why?
1.2. How do the lunar magnetic anomalies affect the motion of the solar wind electrons and ions inside and outside of the local magnetic structures? And how do these influences (waves and particles) propagate in the space outside of the anomalies?
1.3. How do the lunar swirls form? Do they form because of different solar wind implantation or because of local immigration of fine soil grains? Why there is not one to one correspondence between the lunar swirls and the magnetic anomalies?
1.4. Is the granularity of the lunar soil grains abnormal under the magnetic anomalies compared with those on the surface without remanent magnetic field?
1.5. What is the difference in cumulation of solar wind materials and water (hydroxyl) in the lunar surface soil? What is the relation of this difference to the geometry or magnitude of magnetic fields near the lunar surface?
(2) How do the nonconductive lunar regolith layer and the purely conductive solar wind couple at the lunar surface boundary?
2.1. How do the key physical parameters, including the distribution functions of electrons (from solar wind or photoelectrons), ions (from solar wind or scattered/reflected ions) and neutrals (spattered from the lunar surface), and the electric field and the magnetic field, vary with altitudes in the near-Moon space, solar zenith angles, and solar conditions (solar wind and solar EUV radiations)?
2.2. Is there a boundary layer above the unmagnetized lunar surface where kinetic plasma physics dominates and IMFs are slightly piled-up?
2.3. How are the lunar soil grains ejected from the lunar surface to be dusts in space?
2.4. Is there any evidence of local transport of lunar grains within a region with complicated terrains?
2.5. How do the lunar dusts reach altitudes as large as several hundreds of kilometers?
2.6. Why have not the finest grains in the surface soil been exhausted during the long evolution history of the Moon? After all, the lunar dust is not a very rare phenomenon, indicating that there are always fine grains on the lunar surface. What is the endless source of these finest grains? Is there any circulation process of these finest soil grains between the nearby space and the ground?
(3) Do we really know the magnetic field of the Moon itself?
3.1. Is there a weak global intrinsic magnetic field on the Moon?
3.2. Is there any local induced magnetic field on the lunar surface?
3.3. How can we obtain a real map of magnetic fields of the Moon itself?
3.4. How many types of origin do the lunar magnetic anomalies have?
3.5. Is there any special distribution pattern of the magnetic anomalies that formed in similar geological ages, in terms of strength or direction of their magnetic momenta?
3.6. How to determine the formation ages of the lunar magnetic anomalies?
To answer these questions, the extensive measurements in the near-Moon space and on the ground are necessary.
Finally, authors review previous lunar missions and put up concepts of future lunar exploration missions. Although high-quality instruments have been equipped onboard the modern satellite missions, the detail space physical structures/processes have not been fully understood because most satellites do not tend to descend to such altitudes as low as 30 km to avoid impacting on lunar surface or lunar mountains (Fig. 3). Authors create a new term “near-Moon space” to refer to the space surrounding the Moon with heights below 30 km. In future, the near-Moon space and the lunar ground will be the hot target regions for lunar space missions. Extensive space physics and geology exploration done by low-altitude lunar orbiters in the near-Moon space or done by manned or unmanned lunar rovers along long-distance traces on the lunar surface are desired, which can give answers to the key questions mentioned above. The surface of magnetic anomaly is the most valuable candidate landing site, where the interdisciplinary studies can be performed among space physics, geophysics, and geology on the subjects such as the origin of lunar swirls, the origin of remanent magnetic fields (magnetic anomalies), and the mechanism of lifting of lunar dust, finding evidence for evolution of Solar System and even finding suitable place for human habitation.
Reference
Article Title: Key Questions of Solar Wind–Moon Interaction
Corresponding Author Information: HUI ZHANG, JINBIN CAO* , YANGTING LIN, YONG WEI, LEI LI, XIANGUO ZHANG, HONGLEI LIN, AND LIANGHAI XIE
Author Affiliations: School of Space and Environment, Beihang University, Beijing 100191, China.
Link to the Article: https://spj.science.org/doi/10.34133/space.0060
Fig. 2. The different scenario of solar wind interaction of magnetic anomalies (the spatial scale is exaggerated), in both of which there is no bow shock and the ion kinetic effects dominate. The top (bottom) shows the interaction of magnetic field with a horizontal (vertical) magnetic momentum.
Fig. 3. The topography of the lunar surface (top) and the sketch showing the near-Moon space (bottom) where few of lunar satellites orbit the Moon regularly.
CREDIT
Space: Science & Technology
JOURNAL
Space Science & Technology
ARTICLE TITLE
Key Questions of Solar Wind–Moon Interaction
Scientists designed the deployment of three-body chain-type tethered satellites in low-eccentricity orbits using only tether
IMAGE:
FIG. 1. MODEL OF 3-BODY CHAIN-TYPE TETHERED SATELLITE SYSTEM.
view moreCREDIT: SPACE: SCIENCE & TECHNOLOGY
Recently, the tethered satellite system (TSS) has been used in Earth observations, space interferometry and other space missions, due to potential merits of TSS. The tethered TSAR (tomographic synthetic aperture radar) system is a group of tethered SAR satellites that can be rapidly deployed and provide a stable baseline for 3-dimensional topographic mapping and moving target detection. Successful deployment is critical for TSAR tethered system. Several control methods, including length, length rate, tension, and thrust-aided control, have been proposed over the years. Among them, adjusting tension is a viable yet challenging approach due to tether's strong nonlinearity and underactuated traits. Current tether deployment schemes focus on two-body TSS, with little emphasis on multi-TSSs. In a research article recently published in Space: Science & Technology, the team led by Zhongjie Meng from Northwestern Polytechnical University develops a new deployment strategy for a 3-body chain-type tethered satellite system in a low-eccentric elliptical orbit.
First, authors establishe the motion model of a 3-body chain-type TSS in a low-eccentric elliptical orbit. Two assumptions are made: (a) the tethers are massless; (b) only the planar motion is considered. The proposed model consists of 3 point masses (m1, m2, and m3) and 2 massless tethers (L1 and L2), as illustrated in Fig. 1. The orbit of m1 is defined by its orbital geocentric distance r and true anomaly α; the position of m2 relative to m1 is determined by tether L1 and in-plane libration angle θ1; the position of m3 relative to m2 is determined by L2 and θ2. The dynamic model of 3-body TSS is derived using Lagrangian formulation, and the motion equations are expressed in the Euler–Lagrange form as M(q)q̈ + C(q,q̇)q̇ + G(q) = Q with generalized coordinates q = (r, α, θ1, θ2, L1, L2)T. Since the TSS model in is a typical underactuated systems, the generalized coordinates are decomposed into 2parts, i.e., the actuated configuration vectors (qa = (L1, L2)T) and the unactuated configuration vectors (qua = (r, α, θ1, θ2)T).
Then, authors introduce a novel deployment scheme for the 3-body chain-type TSS. Sequential deployment strategy, ejecting satellites one by one, is employed to avoid collisions, this method utilizes the deployment techniques for a 2-body system directly; Poincaré’s recurrence theorem, Poisson stability, and the Lie algebra rank condition (LARC) are used to analyze the controllability of underactuated TSS system. A combination of exponential and uniform deployment law yields a simple and efficient deployment scheme, providing the requisite reference trajectory for satellite deployment. During the deployment process, positive tension must be guaranteed due to the characteristic tether, and to avoid tether rupture, tension could not exceed the given boundaries. So, deployment process can be simplified to a underactuated control with constrained control inputs. To address this limitation, a hierarchical sliding mode controller (HSMC) was designed for accurate trajectory tracking. The controller framework is shown in Fig. 2. In the controller, an auxiliary system is introduced to mitigate the input saturation caused by tether tension constraint. A 3-layer sliding surface for the whole TSS is constructed. A disturbance observer (DO) was introduced to estimate second derivative signal q̈. The uncertainty of sliding surface and its time derivative for orbit motion (r,α) are estimated by a sliding mode-based robust differentiator.
Finally, authors present the numerical simulation and draw the conclusion. To verify the effectiveness of the proposed deployment scheme (marked as Scheme 3), 2 alternative deployment schemes were used for comparison. In Scheme 1, the system is regarded as 2 independent 2-body, in which the tether length L2 remains constant, and only tension T1 is adjustable. In Scheme 2, the system is regarded as two 2-body, but the coupling between adjacent tethers is neglected. That is to say, tether L1 only affects angle θ1 and L2 only affects θ2. In Schemes 1 and 2, the deployment controller in the literature (Murugathasan L, Zhu ZH. Deployment control of tethered space systems with explicit velocity constraint and invariance principle. Acta Astronaut. 2019;157:390–396.) is adopted. The results show that the tether deployment error and libration angle converge to zero asymptotically in 3 h (a little more than one orbital period) under Scheme 3, and the deployment error under Schemes 1 and 2 is quite larger than that under the proposed Scheme 3. A comparison is made between Schemes 2 and 3 based on the integration of tracking error and tether tension, and the normalized results are illustrated in Fig. 3. Compared to Scheme 2, the proposed HSMC explicitly takes the 3-body TSS couple into account, resulting in faster and more accurate tether deployment with a smaller in-plane angle, which further shows that a fairly better deployment process is achieved under the proposed scheme, and confirm the effectiveness of the proposed deployment scheme.
Fig. 2. Schematic of the deployment control framework. (IMAGE)
Fig. 2. Schematic of the deployment control framework.
Fig. 3. Error and tension integral in Schemes 2 and 3.
CREDIT
Space: Science & Technology
Reference
Article Title: Deployment of Three-Body Chain-Type Tethered Satellites in Low-Eccentricity Orbits Using Only Tether
Journal: Space: Science & Technology
Authors: Cheng Jia, Zhongjie Meng*, and Bingheng Wang
Corresponding Author Affiliation: School of Astronautics, Northwestern Polytechnical University, Xi'an, China.
Link: https://spj.science.org/doi/10.34133/space.0070
JOURNAL
Space Science & Technology
ARTICLE TITLE
Deployment of Three-Body Chain-Type Tethered Satellites in Low-Eccentricity Orbits Using Only Tether
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