The Cassini Cosmic Dust Analyzer

The Cassini-Huygens Cosmic Dust Analyzer (CDA) is intended to provide direct observations of dust grains with masses between 10⁻¹⁹ and 10⁻⁹ kg in interplanetary space and in the jovian and saturnian systems, to investigate their physical, chemical and dynamical properties as functions of the distances to the Sun, to Jupiter and to Saturn and its satellites and rings, to study their interaction with the saturnian rings, satellites and magnetosphere. Chemical composition of interplanetary meteoroids will be compared with asteroidal and cometary dust, as well as with Saturn dust, ejecta from rings and satellites. Ring and satellites phenomena which might be effects of meteoroid impacts will be compared with the interplanetary dust environment. Electrical charges of particulate matter in the magnetosphere and its consequences will be studied, e.g. the effects of the ambient plasma and the magnetic field on the trajectories of dust particles as well as fragmentation of particles due to electrostatic disruption.The investigation will be performed with an instrument that measures the mass, composition, electric charge, speed, and flight direction of individual dust particles. It is a highly reliable and versatile instrument with a mass sensitivity 10⁶ times higher than that of the Pioneer 10 and 11 dust detectors which measured dust in the saturnian system. The Cosmic Dust Analyzer has significant inheritance from former space instrumentation developed for the VEGA, Giotto, Galileo, and Ulysses missions. It will reliably measure impacts from as low as 1 impact per month up to 10⁴ impacts per second. The instrument weighs 17 kg and consumes 12 W, the integrated time-of-flight mass spectrometer has a mass resolution of up to 50. The nominal data transmission rate is 524 bits/s and varies between 50 and 4192 bps.This revised version was published online in July 2005 with a corrected cover date.     

Heterogeneous grain morphologies and acceleration mechanisms in cometary coma dust dynamics: Mass envelope dispersion

Modelling of the cometary coma with respect to the distribution of dust particles within the coma and tail have been performed by a number of authors /1,2,3/. Applications of the Divine model using a program coded for the Giotto DIDSY sensors have also been made to calculate expected sensor response of the instrument and spacecraft impact rates /4/. For a chosen mass of ～ 10^?10g we use the Divine Reference model /1/ to investigate the effect on the mass envelope of i) a velocity spread in dust particle ejection; and ii) a variation in the particle type. The results show that effects i) and ii) lead to a smoothing-out of the anticipated peak flux at an envelope boundary. A conceptual model to follow the formation and development of dust jets is also presented and effects illustrated for various nucleus rotation periods.     

Atmospheric drag modelling for orbital micro-debris at LEO altitudes

Data from satellite impact experiments and the scanning of recovered spacecraft offers an extended timebase to examine, using a consistent methodology, the microparticle fluxes. New penetration data from the TiCCE experiment on Eureca /1, 2/ adds to this database and shows that - despite an expected growth in the micro-debris flux - the observed flux is not greater than either LDEF or SMM. The question arises: “is this consistent with the micro particle flux being dominated by space debris or by meteoroids”.

To assist this assessment, numerical modelling using the Gear method /3/ of explicit time integration of the atmospheric drag lifetime of micron dimensioned orbital debris in both circular (LEO) and eccentric (GTO) orbits has been performed for the relevant space exposures. Results are applied to the data to examine whether the recent variations in flux can be attributed to varying levels of, orbital micro-debris caused by atmospheric drag and its changes during the solar cycle.     

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.     

The Characterisation of Titan's Atmospheric Physical Properties by the Huygens Atmospheric Structure Instrument (Hasi)

The Huygens Atmospheric Structure Instrument (HASI) is a multi-sensor package which has been designed to measure the physical quantities characterising the atmosphere of Titan during the Huygens probe descent on Titan and at the surface. HASI sensors are devoted to the study of Titan's atmospheric structure and electric properties, and to provide information on its surface, whether solid or liquid. This revised version was published online in August 2006 with corrections to the Cover Date.     

Micro-particle impact flux on the timeband capture cell experiment of the Eureca spacecraft

Measurements of hypervelocity impact fluxes (in both thick and thin targets) detected by the University of Kent at Canterbury's Timeband Capture Cell Experiment (TiCCE) (flown on ESA's Eureca spacecraft) are presented. The foil perforations are used to derive the ballistic limit values, or the maximum thickness of A1 perforated, for the impacting particles. This data is then combined with the thick target data to derive a unified ballistic limit flux. A significant enhancement in the observed large particle flux compared with LDEF is found, possibly due to the pointing history of Eureca compared to the Earth's orbital direction. Comparisons are also made to predictions from ESABASE modelling. Preliminary results of a study of perforation morphology are also presented, providing insight into particle shape, density and directionality.     

Application of hydrocode modelling to the study of hypervelocity impact crater morphology

In order to obtain a better understanding and model of the natural and artificial particulate environment from measurements of impact damage features on returned spacecraft materials, it is necessary to be able to determine how the size and shape of an impact feature are related to the parameters of the impacting particle. The AUTODYN-3D hydrocode has been used to study the effects of projectile density, velocity and impact angle on the depth, diameter and ellipticity of the impact craters. The results are used to determine the distributions of crater depth to crater diameter ratios and of crater ellipticities to be expected on an aluminium surface exposed to an isotropic distribution of incident particles of given densities and velocities. Comparison of these calculated distributions with those observed for craters on aluminium clamps on various faces of the Long Duration Exposure Facility shows that particles with a wide range of densities, including significant proportions both greater and smaller than that of aluminium, were responsible for these craters.     

Microscopic and chemical analyses of major impact sites on timeband capture cell experiment of the eureca spacecraft

Extensive studies of over 100 impact sites on aluminium foils and mesh supports of the Timeband Capture Cell Experiment (TiCCE) on the European Retrievable Carrier (EuReCa) spacecraft were conducted with scanning electron microscope and energy dispersive X-ray spectrum analyser. Chemical elements of residues in and around the perforations and craters were examined to identify the origin of impactors. 73 % of the impacts were classified; the minimum of 15 % was due to natural particle impacts and the rest indicated high silicon presence. Possible origins of these silicon profiles were discussed. For micrometeoroid craters, the depths to diameter ratios were compared with those of meteoroid and orbital debris impacts on the Solar Maximum Mission satellite.     

Meteoroids and space debris hypervelocity impact penetrations in LDEF MAP foils compared with hydrocode simulations

The continued analyses of penetrating impacts on MAP foils of Aluminium and Brass have produced data for several LDEF faces, i.e., Space, West, and East. These data have immediate bearing on the interpretation and design of devices to detect the penetration of a thin metallic film by a dust grain which have been tested both in the laboratory and in space. A crucial component of the analysis has been the theoretical calculation utilizing CTH, a Sandia National Laboratory Hydrodynamic computer code /1/ to assess the parameters of the hypervelocity penetration event. In particular theoretical hydrodynamic calculations have been conducted to simulate the hypervelocity impact event where various cosmic dust grain candidates, e.g., density = 0.998, 2.700, 7.870 (gm/cm³), and velocities, i.e., 7 - 16 km/s, have been utilized to reproduce the events. Theoretical analyses of hypervelocity impact events will be reported which span an extensive matrix of values for velocity, density and size. Through a comparison between LDEF MAP foil measurements and CTH hydrocode calculations these analyses will provide an interpretation of the most critical parameters measured for space returned materials, i.e., for thin films, the diameter of the penetration hole, D_h, and for semi-infinite targets, the depth-to-diameter ratio of craters, . An immediate consequence of a comparison of CTH calculations with space exposed materials will be an enhancement of the coherent model developed by UKC-USS researchers to describe penetration dynamics associated with LDEF MAP foils.