Modeling of the very high velocity impact process with respect to in-situ ionization measurements

This overview deals with very high impact velocities, where complete vaporization of an impacting cosmic dust particle is to be expected upon expansion from the high pressure high temperature state behind the stopping shock (v > 15 km/s). The topics discussed are the mechanics and thermodynamics of compression, adiabatic release, equation of state and nonequilibrium states upon expansion. The case of very high particle porosity (ρ 1 g/cm³) and the case of very small dust masses (m < 10⁻¹⁷ g) are discussed from what one presently knows. The possibility of three body collisions in the expanding gas phase is discussed briefly. The effect of oblique impact is discussed with respect to its relevance to the ionization process. The numbers communicated are up to the highest “experimental” impact velocities (80 km/s, Halley mission). As one goes to lower impact velocities (20 < v < 30 km/s) there is still complete vaporization of the dust particle but ionization out of the bulk of the particle becomes low. Other than thermal processes may become important. Ideas are outlined to understand their physical nature.     

Interplanetary medium – A dusty plasma

The average mass of dust per volume in space equals that of the solar wind so that the interplanetary medium should provide an obvious region to study dust plasma interactions. While dust collective behavior is typically not observed in the interplanetary medium, the dust component rather consists of isolated grains screened by and interacting with the plasma. Space measurements have revealed several phenomena possibly resulting from dust plasma interactions, but most of the dust plasma interactions are at present not quantified. Examples are the production of neutrals and pick-up ions from the dust, dust impact generated field variations at spacecraft and magnetic field variations possibly caused by solar wind interacting with dust trails. Since dust particles carry a surface charge, they are exposed to the Lorentz force in the interplanetary magnetic field and for grains of sub-micrometer sizes acceleration can be substantial.     

Ion formation by high- and medium-velocities dust impacts from laboratory measurements and Halley results

The ion formation processes by dust impacts have been studied qualitatively as well as quantitatively by dust accelerator laboratory measurements. Iron, carbon and metallized glass particles in the femto- to nano-gram mass range had been impacted on various metal targets in a velocity regime of v = 2 - 64 km/s. In the high velocity regime as relevant for the (retrograde) Halley encounter more than 99% of the ions produced are singly charged atomic, the rest molecular ones. The ion/atom ratios are apparently modified SIMS yields, the modification parameter being impact velocity dependent. A semiempirical formula was deduced for the determination of mass and density of the impacting particle from target and projectile ion yields. When evaluating the Halley encounter results, the elemental distribution of p/Halley dust appeared nearly to be solar; the organic fraction (CHON) could be characterized in a rough manner as fairly unsaturated. Oligomers of the monomers C₂H₂ (65%), CH₂O (25%), and HCN (10%) are probable.

With medium velocities (for prograde comet encounter), i.e. v = 15-30 km/s molecular ion types govern the mass spectra. Consequently, more chemical information of the projectile can be expected in this case, additional to the elemental distribution. Mass and density of the impinging dust particles can be determined as well.     

Impact induced plasma during a cometary fly-by

GIOTTO, the probe which is presently developed by the European Space Agency, will encounter comet Halley in March 1986 with a relative velocity of 69 km/s. The fore section of the surface will be submitted to the bombardment of dust grains and neutral molecules in the final phase of the mission, like that of an Earth orbiter during atmospheric re-entry. These particles have a kinetic energy of 24 eV per a.m.u.; they produce secondary ions and electrons which form a plasma cloud around the body and control the electric potential of its surface. This paper is a review of the work which has been performed on the subject by dedicated study groups; the purpose of their action was to gather information and produce new findings which might have an influence on the design of the spacecraft and help in the interpretation of the data collected by the scientific payload.

The effect of impact induced plasma may already be significant at 10⁵ km from the comet nucleus; at a distance of 1000 km the flux of ions and electrons produced by cometary dust and neutrals will possibly exceed that of the ambient plasma by more than three orders of magnitude. It is expected that the spacecraft surface potential will be positive and will reach at least a few tens of volts; coating the leading surface of the spacecraft with a thin layer of gold or silver will help reducing the emission of ions from neutral gas. Computer simulation models are used to predict the structure of the charged particle density distribution in the vicinity of the surface. Effects associated with the wake and differential charging are also discussed. The significance of these results is conditioned by the validity of the models and the largest source of uncertainty seems to be associated with the plasma generated by dust impact.     

IRAS observations of the zodiacal background

The Infrared Astronomical Satellite (IRAS) sky survey measurements are used to constrain a model for the interplanetary dust emission. Efforts to date, although hampered somewhat by calibration uncertainties, indicate that the cloud is inclined to the ecliptic by approximately 1.7° with its line of ascending nodes at 70° ecliptic longitude. The data is well fit by a dust density distribution described in cloud symmetry plane coordinates by p = p_o (r_o/r)^1.1 exp [−4.2 (z/r)^1.2], where the dust has a gray emissivity and an equilibrium temperature at 1 AU of 280 K.     

Physical properties of cometary dust: Relation of DIDSY data to grain properties

The Dust Impact Detection System (DIDSY) for the Giotto Halley Mission consists of two types of sensors for the detection of cometary dust particles: two impact plasma sensors and five piezo-electric momentum sensors. One sensor of each type is covered by a penetration film. A 1 μm thick aluminum film covers an impact plasma sensor. One momentum sensor is mounted onto the rear shield behind the 1 mm front shield made from aluminum. The parameters measured are the total charge released upon impact and the amplitude of the acoustic signal generated by the impact. Both quantities depend on the mass and speed of the impacting particles. At the impact speed of 68 km/sec the mass of cometary dust particles can be determined in the mass range from 10⁻¹⁷ g to 10⁻³ g. From the difference in the countrates measured by the sensors with and without penetration film the average bulk density of dust particles of masses 10⁻¹⁴ g and 10⁻⁶ g can be determined. With appropriate calibration an accuracy of a factor of 2 for both the mass and density determination can be obtained.     

Laboratory calibration measurements of a piezoelectric lead zirconate titanate cosmic dust detector at low velocities

A cosmic dust monitor for use onboard a spacecraft is currently being developed using a piezoelectric lead zirconate titanate element (PZT). Its characteristics of the PZT sensor is studied by ground-based laboratory impact experiments using hypervelocity particles supplied by a Van de Graaff accelerator. The output signals obtained from the sensor just after the impact appeared to have a waveform that was explicitly related to the particle’s impact velocity. For velocities less than ∼6 km/s, the signal showed an oscillation pattern and the amplitude was proportional to the momentum of the impacting particle. For higher velocities, the signal gradually changed to a single waveform. The rise time of this single waveform was proportional to the particle’s velocity for velocities above ∼6 km/s. The present paper reports on results for the low velocity case and especially discusses the effect of an outer coating of the sensor with a paint, which is used to reduce heating by solar radiation.     

发动机羽流作用下的实时月尘运动仿真

月尘运动是月球探测器软着陆过程中不可或缺的重要环节,针对发动机羽流作用下月尘运动真实感不强和月尘颗粒运动模型过于简单的问题,提出了一种逼真的实时月尘运动仿真方法.通过计算流体动力学(CFD, Computational Fluid Dynamics)和二次谢别德插值(Quadratic Shepard)方法,分析和计算单个月尘颗粒的运动学模型,得到一定初始条件下粒子运动的二维轨迹曲线;通过分析粒子的数量、初始位置、初始速度、生命周期等参数对粒子运动学的影响和变化规律,抽象出月尘系统的粒子集;建立基于月尘粒子集的月尘运动模型.实验结果显示:该运动模型逼真的模拟了发动机羽流作用下月尘腾起、飞溅、弥漫、消散等运动过程,视觉真实感和实时性能良好,对研究真空环境中的月尘运动及月球软着陆等相关领域具有一定的参考意义.目前该方法已应用于北航虚拟现实国家重点实验室月球软着陆仿真系统.     

基于SIMION软件的空间等离子体探测器的数值仿真

空间等离子体探测器采用带偏转板的柱形静电分析器作为传感器,具有大于320°探测视场,可探测约10eV～30keV的热等离子体。利用SIMION软件采用均匀随机抽样建立粒子源的方法仿真探测器的基本特性参数:静电分析器因子、能量分辨率、偏转板因子和偏转角。仿真结果与实测结果吻合很好。静电分析器因子的仿真与实测结果偏差为3.2%,能量分辨率仿真与实测结果偏差为1.5%,偏转角-偏转板因子的关系式与实测结果的相关系数为0.999 4。仿真结果可以有效地应用于该类仪器的设计和定标过程中。     

Experimental investigation on silica dust lofting due to charging within micro-cavities and surface electric field in the vacuum chamber

The charged dust particles can be mobilized electrostatically by the repulsion between the adjacent grains and the surface electric field due to the incoming electron current and the charge accumulation within the micro-cavities. In this study, the experimental results of the initial vertical launching velocities and the maximum dust heights are compared with the estimated values for the lofted spherical dust grains by the patch surface charging equations. Silica particles with the sizes between <6 and 45?µm in radius are loaded on a graphite plate, and they are exposed to the electron beam with 450?eV energy under 4?×?10^?3?Pa vacuum chamber pressure. During the first set of the experiments, the dust samples are tested without an initial compression process and an additional horizontal electric field. Second, the dust samples are compressed by two different weights in order to increase the packing density under approximately 780.7?Pa and 3780?Pa. Finally, the dust grains are placed between the two parallel aluminum plates to apply approximately 2000?V/m and 4800?V/m horizontal electric field. A high-speed camera is used to record the transportation of the dust grains together with a microscopic telescope, and the results point out that the patch surface dust-charging model estimations are in agreement with the first experiments. On the other hand, the dust particles from the compressed samples are lofted with higher velocities than the estimations, and the number of the dust lofting observations decreases significantly, which demonstrates the importance of the micro-cavities and the increased charging requirement to overcome the contact forces. When the horizontal electric field is present, the initial vertical launching velocities are measured to be lower than the other experiments, which can be attributed to the decreased charging requirement for the dust lofting as a result of inter-particle collisions and rolling motion. According to the experimental results, the electrostatic dust transportation can be controlled not only by the ambient plasma and the solar irradiation on the airless planetary bodies, but also by the surface properties such as the contact surfaces between the dust grains, the number of the micro-cavities related to the packing density, and the presence of the horizontal electric field contributing to the external forces by other particle motions.     

Electric antenna as a dust detector

Electric antenna responds to three effects caused by a dust impact-induced plasma cloud: -change separation electric fields, -charging of the antenna, -pulse of the spacecraft potential. Each effect, being a function of the induced charge (particle mass), depends on the ambient plasma conditions, including photo- and secondary emissions. The first two effects are also strongly dependent on the impact geometry. In the paper an attempt is made to consider systematically the dependence on the charge and ambient conditions for two main types of antenna (probe, wire/cylinder) and for both configurations (monopole, dipole). A conclusion is drawn that for a plasma wave/radio experiment to be utilized as a dust detector, the most advantageous option is a monopole, whose probe (wire/cylinder) is not exposed to dust-induced plasma.     

The detection of energetic cometary and solar particles by the EPONA instrument on the Giotto mission

EPONA is an energetic particle detector system incorporating totally depleted silicon surface barrier layer detectors. Active and passive background shielding will be employed and, by applying various techniques, particles of different species, including electrons, protons, alpha particles and pick-up ions of cometary origin may be detected over a wide spectrum of energies extending from the tens of KeV into the MeV range.

The instrument can operate in two modes namely (a) in a cruise phase or storage mode and (b) in a real time mode. During the real time mode, observations at high spatial (octosectoring) and temporal (0.5s) resolution in the cometary environment permit studies to be made of accelerated particles at the bow shock and/or in the tail of the comet. In conjunction with magnetic field measurements on board Giotto, observations of energetic electrons and their anisotropies can determine whether the magnetic field lines in the cometary tail are open or closed. Further, the absorption of low energy solar particles in the cometary atmosphere can be measured and such data would provide an integral value of the pertaining gas and dust distribution. Solar particle background measurements during encounter may also be used to correct the measurements of other spacecraft borne instruments potentially vulnerable to such radiation.

Solar particle flux measurements, obtained during the cruise phase will, when combined with simultaneous observations made by other spacecraft at different heliographic longitudes, provide information concerning solar particle propagation in the corona and in interplanetary space.     

悬浮月尘的遮挡模型

探月活动中,月表的尘埃会对太阳翼等光电器件的工作效率造成严重影响.基于附着月尘的遮挡模型,从立体空间的角度分析了悬浮月尘对工作器件的遮挡,提出了悬浮月尘的单粒度遮挡模型,利用月尘的平均粒度可计算在指定立体空间内,定量月尘造成的遮挡量;引入月尘的粒度分布函数,提出了悬浮月尘的多粒度遮挡模型,能够计算已知粒度分布函数的月尘在指定立体空间内造成的遮挡量;通过设计悬浮月尘遮挡的仿真实验,分别利用3种不同粒度大小的月尘进行实验,实验数据与悬浮月尘的单粒度、多粒度遮挡模型的理论计算值都较好吻合,实验结果证实了悬浮月尘遮挡模型的正确性与可靠性.      

The Giotto mission to Halley''s comet

ESA's Giotto mission to Halley's comet is a fast flyby in March 1986, about four weeks after the comet's perihelion passage when it is most active. The scientific payload comprises 10 experiments with a total mass of about 60 kg: a camera for imaging the comet nucleus, three mass spectrometers for analysis of the elemental and isotopic composition of the cometary gas and dust environment, various dust impact detectors, a photopolarimeter for measurements of the coma brightness, and a set of plasma instruments for studies of the solar wind/comet interaction. In view of the high flyby velocity of 68 km/s the experiment active time is very short (only 4 hours) and all data are transmitted back to Earth in real time at a rate of 40 kbps. The Giotto spacecraft is spin-stabilised with a despun high gain parabolic dish antenna inclined at 44.3° to point at the Earth during the encounter while a specially designed dual-sheet bumper shield at the other end protects the spacecraft from being destroyed by hypervelocity dust impacts. The mission will probably end near the point of closest approach to the nucleus when the spacecraft attitude will be severely perturbed by impacting dust particles leading to a loss of the telecommunications link.     

Collection of cometary dust

Rendezvous Missions to Comets lead to low velocities at the nucleus of the comet. The resulting impact velocity of the cometary dust on a target will range between 10 and 400 m/s. The dust particle which impacts on a target can be collected for a subsequent in-situ analysis.

The collection efficiency of a target depends in addition to obvious geometrical conditions upon the surface of the target. The surface characteristics can be divided into two groups:

• “dirty” surfaces, covered with silicate or hydrocarbon compounds (for example vacuum grease),

• “clean” surfaces, like gold (with additional sputtering).

This paper deals with the experimental and theoretical investigation of the collection efficiency of “clean” targets. Laboratory experiments are described which were conducted at the Technische Universität München, Lehrstuhl für Raumfahrttechnik, and the Max-Planck-Institut für Kernphysik, Heidelberg. In both experiments an electromagnetic accelerator is used to accelerate different types of dust in vacuum to velocities between 10 and 400 m/s.

The target is then examined under the microscope and a secondary ion mass spectrometer (which is a model of the laboratory carried on board of the spacecraft for “in situ” analysis). The adhesion of the dust grains at the target is evaluated experimentally in an ultracentrifuge.     

Cosmic dust measurements in lunar orbit

On 15th February 1992, ISAS space engineering satellite HITEN was successfully inserted into an elliptical orbit around the moon with perilune between some 100 km and 8000 km and apolune of about 50.000 km. On board was a small scientific experiment designed to detect cosmic dust particles, MDC - Munich Dust Counter. During a period of more than one year, until Hiten's hard landing on the moon surface at 10th of April 1993 (UTC), measurements of impact velocity, mass and crude flight direction of micrometeoroid particles have been performed. In total 150 cosmic dust impacts were detected and evaluated. From these measurements, the impact rate versus time and the dust flux versus distance from the moon are derived. The evidence of moon ejecta and some indications of particles which are orbiting the moon will be discussed. The spatial distribution of the measured particles is shown in lunarcentric as well as in heliocentric coordinate systems. The directional distribution is also given, showing the different populations of cosmic dust particles. Finally, the gathered data will be compared with previous results from measurements in the vicinity of the Earth and in the geomagnetic tail region.     

Thin film surface treatments for lowering dust adhesion on Mars Rover calibration targets

The current generation of calibration targets on Mars Rover serve as a color and radiometric reference for the panoramic camera. They consist of a transparent silicon-based polymer tinted with either color or grey-scale pigments and cast with a microscopically rough Lambertian surface for a diffuse reflectance pattern. This material has successfully withstood the harsh conditions existent on Mars. However, the inherent roughness of the Lambertian surface (relative to the particle size of the Martian airborne dust) and the tackiness of the polymer in the calibration targets has led to a serious dust accumulation problem. In this work, non-invasive thin film technology was successfully implemented in the design of future generation calibration targets leading to significant reduction of dust adhesion and capture. The new design consists of a μm-thick interfacial layer capped with a nm-thick optically transparent layer of pure metal. The combination of these two additional layers is effective in burying the relatively rough Lambertian surface while maintaining diffuse properties of the samples which is central to the correct operation as calibration targets. A set of these targets are scheduled for flight on the Mars Phoenix mission.     

On the evolution of dust in the solar vicinity.

The analysis of interplanetary dust shows that the majority of particles in out-of-ecliptic regions comes from comets and also that near solar dust, in the ecliptic regions, results most probably largely from comets. The intense radiation flux in the solar vicinity is expected to cause strong modifications in the material composition and surface structure of interplanetary dust particles and hence the analysis of near solar dust provides interesting insights into the evolution of meteoritic, especially cometary materials. Because of the lack of in-situ measurements our present knowledge concerning these processes derives from remote sensing, i.e. observations of the solar F-corona. In particular these are observations of albedo, polarization and colour temperature given in terms of average particle properties. For example the analysis of near infra-red F-corona data points to the existence of a strong component of irregularly structured silicate particles, most probably of cometary origin. The data may indicate a subsequent sublimation of different particles or different constituents of the particles. Here we compare particle properties derived from F-corona observations with model calculations of single particle properties and discuss perspectives of future analysis of cometary dust in the interplanetary cloud.     

The effect of aerosols on climate and aerosol climatology on the basis of observations from space

Principal aspects of the effect of aerosols on climate are discussed and the possibilities of obtaining a climatic data set of global aerosols are analyzed. Based on the analysis of space images, new data have been obtained on gigantic dust outbreaks in various regions of the Earth. It has been shown that dust outbreaks can propagate over hundreds and sometimes thousands of kilometers. The western Sahara - Atlantic Ocean is the major region of propagation of these outbreaks. The continent-to-continent trajectories of dust clouds have been discovered (from Africa to the coast of America, from Central Asia to the Pacific Ocean). Maps of the sources of strong dust transformations have been studied and drawn. In particular, an anthropogenic dust source has been found out on the northeastern coast of the Aral Sea. A striped mesostructure of dust formations has been analyzed, determined by both the inhomogeneous surface and peculiarities of the eddy dust transport. The techniques have been discussed in detail for retrieving the parameters of aerosol size distribution and the vertical profiles of the coefficients of aerosol extinction in the stratosphere and lower mesosphere from the data on the brightness of the twilight and daytime horizon as well as occultation measurements of solar radiation attenuation by the atmosphere.The difficulty of reliably predicting possible environmental changes arises both from the problems of estimating complex interactions of numerous processes and from a lack of information concerning various environmental parameters. For example, an important factor in present day climatic changes is the increased dust content of the atmosphere due to man's activities. However, a reliable estimate of this influence is found to be impossible due to the absence of definitive data on the global distribution of atmospheric dust and the properties of dust in various parts of the world [4,5,13–15]. The impact of aerosols on climate has been discussed in detail in a number of monographs [12–15].Observations from space have opened up new possibilities for studying atmospheric dust. For this purpose, both the imagery and spectrometry of the Earth's atmosphere from space are used. Rather attractive are the prospects for laser sounding [1].     

Sensor calibration impacts on dust detection based on MODIS and VIIRS thermal emissive bands

Dust detection using remotely sensed measurements has been one of the challenging problems encountered by atmospheric scientists. MODerate Resolution Imaging Spectroradiometer (MODIS) on the Terra (T) and Aqua (A) platforms have been a versatile sensor for well over 21 and 18 years respectively, and have been extremely useful in the retrieval of aerosol information over the entire globe. The MODIS radiances from the Level 1B in general are expected to be within 5% accuracy in the reflective wavelengths and within 1% in the thermal emissive wavelengths. In this paper, we evaluate the sensitivity of previously developed dust detection technique based on thermal emissive wavelengths, which correspond to MODIS bands 20, 29, 31, and 32 respectively. The Thermal Emissive Dust Index (TEDI) performed very comparably to the traditional Aerosol Optical Thickness (AOT) retrievals by MODIS reflective channels. Since the MODIS Thermal Emissive Bands (TEB) are well calibrated on-orbit using a BlackBody (BB) source, the calibration of these long wave infrared bands is quite robust. As A-MODIS continues to perform well beyond its designed lifetime of 6 years, the instrument has undergone various levels of degradation during its mission time. As a consequence, it is imperative to check the impacts of calibration on the higher-level retrievals. In this paper, we rigorously analyze the sensitivity of TEDI due to the impact of calibration by the afore-mentioned TEB. The perturbation of the dominant (linear) calibration term demonstrated the following: first, there was a correlation in the sensitivity of the TEDI due to the uncertainty in the linear calibration term. Based on a perturbation in the linear calibration term for all aforementioned bands over a range of ±5% yielded the TEDI sensitivity to vary from approximately ?3.2% to about ?3.6%. When considering the uncertainty in each individual band significant changes were observed. The least change was observed for the perturbation in the calibration of band 20 with the TEDI sensitivity and the largest sensitivity in TEDI was observed in the perturbation of band 31 calibration. Thus, in the case of TEDI, noticeable sensitivity due to calibration uncertainty was observed in bands 29, 31, and 32, reiterating the importance of the TEB calibration in these bands. Also, the dust detection scheme based on A-MODIS was successfully transferred to the follow-on sensors such as Suomi (SNPP) and NOAA 20 (N20) VIIRS. The results presented in this paper would be extremely helpful in understanding impacts of calibration on the higher-level products for both current and future missions based on the MODIS heritage. Finally, the work also identifies the importance of radiometric fidelity in maintaining the accuracy of the dust detection. Results presented will show drastic improvement of the Saharan dust detection after the reduction of the electronic crosstalk in the 8.5 µm channel of T-MODIS.