Cometary particle impact simulation using pulsed lasers

Calibration of the DIDSY experiment momentum sensors for the GIOTTO Mission to Comet Halley requires laboratory simulation of impacts at 68 km s⁻¹ for particle mass values in the range 10⁻⁶ g to 10⁻¹⁰ g. Existing techniques for particle acceleration cannot simultaneously attain these extreme values of velocity and particle mass, making it necessary to adopt some less direct method of impact simulation. This paper considers the application of high power pulsed lasers for laboratory simulation of the momentum impulse produced by a cometary dust particle impact on the GIOTTO spacecraft.     

A capacitor impact sensor (CIS) on board Giotto for detection of cometary dust particles

A low power high reliability impact sensor based on the discharge of a parallel plate capacitor is described. The choice of a surface area of about 1000 cm² and a penetration thickness of 50 micrometers will provide data on the flux density of cometary dust particles in the 5 micrometers diameter range (10⁻¹⁰g). A high noise immunity promotes excellent reliability under conditions of heavy spacecraft bombardment and high plasma densities in the late stages of the 500 km approach distance. Self-limiting of the event rate compression system also provides flux data at arbitrarily high impact rates. The capacitor sensor will be located on the external face of the outer dust shield of Giotto Spacecraft and it will be a part of the DIDSY experiment.     

Hypervelocity impact on the GIOTTO Halley Mission dust shield: Momentum exchange and measurement

The two layer dust shield on the GIOTTO Halley Mission is constructed in a meteoroid bumper configuration. The dust shield is instrumented so that parameters associated with the hypervelocity collision of cometary particles on the exposed surface can be determined. A multisensor detector array provides simultaneous sensing of the momentum exchange of particles impacting and subsequently penetrating the outer layer of the dust shield. Current knowledge of momentum exchange during hypervelocity impact relative to the GIOTTO Halley Mission and the dust shield experiment is reviewed. The sensors used for determination of momentum exchange exhibit a functional dependence on projectile velocity leading to an enhancement of the sensor signal as the relative impact velocity increases. The GIOTTO Mission provides a very unique opportunity to obtain hypervelocity momentum exchange information at a known impact velocity. Therefore, with the dust experiment, a determination of the velocity index for both momentum and multilayered penetration sensor is possible. Results of analysis of analytical and laboratory studies indicate that the velocity index for hypervelocity impact is approximately 2.0 at the 68 km/sec encounter impact velocity of the GIOTTO Mission. A clear determination of the size and mass distribution of the cometary dust near the comet will be possible from the in-situ measurement of the DIDSY GIOTTO experiment.     

Earth encounter velocities for interplanetary meteoroids

The speed distribution of meteoroids encountering the Earth is shown to be similar for all meteoroid masses in the range 1 g to 10⁻¹² g. The speed distribution of interplanetary meteoroids encountering the Earth has usually been inferred from meteor observations. This paper reviews commonly quoted distributions and introduces more recent estimates. The influence quoted measurement uncertainties have on the distribution of Earth encounter velocities presented by Sekanina and Southworth (1975) and Erickson (1968) is presented. The Divine (1993) model of interplanetary meteoroids fits a set of orbital distributions to a wide range of spacecraft and ground based dust detector observations. By ‘flying’ the Earth through this model the distribution of geocentric encounter velocities has been obtained for typical particle masses, 10⁻⁹ and 10⁻¹² g while those at 10⁻⁴ and 10⁻⁵ g are shown to be in error.     

LDEF: Chemical and isotopic measurements of micrometeoroids by SIMS

Described is a passive experiment for LDEF (Long Duration Exposure Facility) to measure the chemical and isotopic composition of interplanetary dust particles >10⁻¹⁰g for most of the major elements expected to be present. The detector consists of Ge targets covered with a metallized plastic film. During impact micrometeoroid vapor and melt are deposited on the underside of the foil which can be analyzed be secondary ion mass spectroscopy (SIMS) after the return of LDEF. Additional information on projectile mass, velocity and density can be obtained from the study of the penetration hole and the impact crater. Criteria for the choice of materials are given and first results of impact simulation experiments are reported which demonstrate the viability of the basic concept and show that isotopic data can be obtained from the deposits.     

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.     

S3 and S4 absorption cross sections in the range of 340 to 600 nm and evaluation of the S3 abundance in the lower atmosphere of Venus

S₃ absorption cross section equals 6×10⁻¹⁷ cm² at 400 nm, 6 × 10⁻¹⁹ cm² at 500 nm (less by a factor of 4 than that given by Sanko), 4×10⁻²⁰ cm² at 600 nm. That of S₄ equals 1.5 × 10⁻¹⁷ cm² at 450 nm, 8 × 10⁻¹⁷ cm² at 500 nm, and 4.7 × 10⁻¹⁷ cm² at 600 nm. Preliminary evaluation of the S₃ mixing ratio in the lower atmosphere of Venus is (8±3)×10⁻¹¹ at 5 to 25km according to the Venera 14 measurements and several times lower at the locations of the Veneras-11 and -13.     

Use of combined light flash and plasma measurements to study hypervelocity impact processes

We present new measurements concerning generation of light flash during hypervelocity impacts. We use iron particles (10⁻¹³ to 10⁻¹⁷ kg) with velocities over the range 1 to 42 km/s impacting semi-infinite targets (aluminium and molybdenum). The main results of previous work in the field are found to be reproduced with some slight deviations. For iron projectiles with given mass and velocity the energy of the flash (normalized to mass) is proportional to velocity to the power of 3.5 for aluminium targets and 3.9 for molybdenum targets. The duration of the flash is of order 1 microsecond. Simultaneous measurements of the generation of impact plasma do not change this. The onset of plasma generation of the bulk target material does not affect the total light flash energy. We discuss the duration of the flash compared to a simple calculation of temperature in the target and plasma vs time.     

Experimental drop tube of the metallurgy department of Grenoble

The drop tube which will be available in the “Centre d'Etudes Nucléaires de Grenoble” is described. Its main features are the following: - Dimensions : Drop height : 47.1 m Drop time : 3.1 s Tube inside diameter : 0.2 m - Experimental atmosphere : 1 Ultra-vacuum : 10⁻⁶ to 10⁻⁷ Pa - Residual gravity level : 10⁻⁸ to 10⁻⁹ g according to the vacuum level and drop diameter.

This facility is unique insofar as it enables experiments to be performed under ultra-vacuum conditions which, by delaying the formation of surface oxides, should contribute to improving maximum undercooling values.

The techniques used for obtaining small metallic drops (0.5 to 3 mm) are described. The availability of this instrument for the scientific community is also foreseen by the french sponsoring organizations (CEA, CNES, CNRS) ; some practicle informations will be given to potential experimenters.     

Fluctuation analyses of the X-ray background

We review recent progress in the use of analyses of fluctuations in the cosmic X-ray background (XRB) to determine the source counts below the detection thresholds. Three flux domains are discussed: the range around 10⁻¹²erg cm⁻² s⁻¹ where the Ginga and Einstein Observatory results remain inconsistent unless the sources at these fluxes (mainly Seyfert galaxies) are highly absorbed at low energies, the 10⁻¹⁴erg cm⁻² s⁻¹ zone where the flattening of the source counts predicted by fluctuation analyses of Einstein Observatory images is now confirmed by Rosat, and the 10⁻¹⁵erg cm⁻² s⁻¹ flux domain, where fluctuation analyses of Rosat images show that source counts remain subeuclidean with very little contribution to the XRB coming from these sources.     

First calibration measurements with the dust impact detector DIDSY-IPM

The IPM detector consists of two separate impact ionization detectors, one of them covered by a 2.5 μm thick plastic film and a piezoelectric sensor mounted to the back of the joint impact plate. First impact tests, with iron projectiles in the mass range 10^?15 to 10^?9 g and in the speed range 1 km/s to 70 km/s, were performed with the calibration (FS) and the flight (F) model of this detector. The charge yield at 69 km/s impact speed (flyby speed of GIOTTO) has been extrapolated from the data and amounts to 400 Coulombs per gram. This corresponds to a preliminary sensitivity threshold for the impact ionization detector of about 6×10^?17 g. The penetration limit introduced by the plastic film is about 10^?14 g for iron particles. Only the biggest particles used for the test produced signals at the piezoelectric sensor. If one assumes an energy dependence of the piozoelectric signal, a preliminary sensitivity threshold of about 10^?13 g at 69 km/s can be established.     

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.     

First results of plasma and neutral gas measurements from Vega-1/2 near comet Halley

Based on the ion, electron and neutral gas observations, performed by five of the six sensors comprising the PLASMAG-1 experiment on board VEGA-1 and -2, the following results are discussed: (1) the existence of the bow shock and its location at 1.1×10⁶ km for VEGA-1 inbound; (2) the existence of a cometopause and its location at 1.6×10⁵ km for VEGA-2 inbound; (3) the plasma dynamical processes occurring inside the cometosheath; (4) the phenomena taking place within the cometary plasma region including mass-spectroscopy of cometary ions at distances 1.5×10⁴ km; (5) the existence of keV electrons near closest approach to the nucleus; and (6) the radial dependence of the cometary neutral gas and the comparison with model calculations, yielding a mean ionization scale length of 2×10⁶ km and an overall production rate of 1.3×10³⁰ molecules s⁻¹ for VEGA-1 inbound. The results are also discussed in the context of the other, both remote and in-situ, observations, performed on board the VEGA- and GIOTTO-spacecraft.     

Geminga pulsar observations with gamma-telescope Gamma-1

The Geminga light curve obtained with the “Gamma-1” telescope features two peaks separated by 0.5 ± 0.03 period. The light curve is pronounced for γ-quanta energies higher than 400 MeV. The pulsed flux upper limit (1σ) in the energy interval 50 – 300 MeV is 6·10⁻⁷ cm⁻²sec⁻¹. For energies >300 MeV the pulsed component power law spectrum has an exponent 1.1 _−0.3^+1.1 and an integral flux (1.1±0.3)·10⁻⁶ cm⁻²sec⁻¹.     

Taurid complex meteoroids detected near aphelion with Ulysses

Between 3.4 and 4.0 AU the dust detection system aboard the Ulysses spacecraft showed an increase in detection rate for particles with masses greater than 5 × 10⁻¹³ g. The spacecraft meteoroid encounter geometry indicates highly eccentric orbits detected near aphelion. The outer limit of the enhanced flux is imposed as meteoroids on such orbits move outside the aperture of the dust detector. The inner edge of the enhanced flux would be consistent with the aphelion distance acquired by 50-200 μm particles evolving for 10-20 kyr under Poynting-Robertson drag from an Encke type orbit. We propose such meteoroids provide a source population from which collisional fragmentation produces particles in the mass range to which the Ulysses detector is sensitive. Daughter fragments produced away from the aphelia of the parent orbits, a 2.2 AU, e 0.85, enter hyperbolic orbits which are not evident in the Ulysses data. The spatial density of fragments from collisions very near aphelion drops off rapidly as they evolve inward under Poynting-Robertson drag while collisions closer to 3.4 AU leave the subsequent peak density outside that radius for a significant fraction of the fragment's subsequent lifetime. The rapid orbital evolution for these collision fragments implies a recent breakup and probably a large reservoir of parent meteoroids.     

CMB anisotropy testing from small satellites

We review the advantages and possibilities of small satellites. New results of data reduction of the satellite-borne experiment RELICT-1 are presented. For the inflation spectrum of primordial perturbations we obtained the estimate for quadrupole component 6·10⁻⁶ <ΔT₂/T<3.3·10⁻⁶. The RELICT-2 mission will provide a possibility of measurement of CBR anisotropy down to the level less than ΔT₂/T = 10⁻⁸. We present the results of engineering testing of RELICT-2 measurement system and discuss ways of improving of the radiometers sensitivities.     

Hypervelocity acceleration techniques: A review of exisiting capabilities and prospects for future developments

During the last few decades various techniques have made it possible to accelerate microparticles (10⁻⁶ – 10⁻¹⁵ gr) up to tens of km/sec and macroparticles (1 gr or so) up to 10 km/sec, thus furthering our understanding of many impact related phenomena occurring on the surfaces of celestial bodies.

This review will deal with existing techniques for the acceleration of hypervelocity projectiles. The performance of electrostatic accelerators, electromagnetic rail guns and related systems, plasma drag accelerators, light gas guns and explosive accelerating techniques is reviewed, and the capabilities and limitations of each type are briefly discussed. An attempt is made to assess the future promise of existing techniques and the realism of some current suggestions.     

Radiative forces and LAGEOS' orbit

The orbit analysis of LAGEOS satellite has resulted into the discovery and/or reassessment of several very small sources of perturbation on satellite orbits. The analysis of orbital arcs of duration ranging from one month to several years has revealed that perturbative effects are present, having unpredicted long-term or secular components down to the 10⁻¹² m/s² acceleration level. It was soon realized that those perturbations have a non-gravitational origin.

In recent years, we have devoted some effort to the physical modelling of radiative perturbations, caused by momentum exchanges with an appropriate radiative field, and have considered their potential role in the analysis of LAGEOS orbit residuals. These perturbations include: (i) direct solar radiation pressure; (ii) radiation pressure from the Earth's emitted/reflected/diffused radiation flux; (iii) the so-called thermal thrust force.

The main results of this work are reviewed, discussing its relationships with models developed by other research groups. In particular, we present a list of the physical processes which still appear to need more detailed and realistic modelling to reach a better understanding of LAGEOS dynamics at the 10⁻¹² m/s² level.     

The critical energy density and the inelasticity coefficient for asteroidal catastrophic collisions

The experimental results on high-velocity impacts reported in the literature are analyzed in detail, with the purpose: (a) to check the possibility of applying to asteroidal collisions, without a size-dependent scaling, the critical energy densities associated with various degrees of fragmentation; (b) to ascertain which fraction of the projectile's energy is converted into kinetic energy of fragments after a catastrophic breakup.

The results of our analysis on these two problems indicate that: (a) the critical energy density is independent on size only for super-catastrophic events, when no massive core survives, while it slowly decreases for larger targets when the fragmentation is only partial (barely-catastrophic collisions). Therefore the “energy gap” between cratering and complete distruction is probably wider for the asteroids than for the small experimental targets. (b) the inelasticity coefficient (i.e., the resulting fraction of kinetic energy) depends both on the impact velocity and on the projectile-to-target mass ratio; for asteroidal catastrophic collisions, it probably ranges from 10⁻² to 10⁻¹.     

Near infrared observations of comet Halley from Vega 2

Spatial distribution of the continuum radiation in the range of 0.95–1.9 μm presumes total dust production rate of the comet of 10ρ tonne s⁻¹ (ρ is the dust material density) and its angular distribution proportional cos . Observations of the water vapor band at 1.38 μ m reveal strong jets, their time shift from the dust jet measured in situ is consistent with gas velocity of 0.82±0.1 km s⁻¹ and dust velocity of 0.55±0.08 km s⁻¹. The OH vibrational-rotational bands observed are excided directly via photolysis of water vapor. Water vapor production rate deduced from the H₂O band and OH band intensities is 8×10²⁹ s⁻¹. Intensity of the CN(0,0) band result in the CN column density of 9×10¹² cm⁻², i.e. larger by a factor of 3 than given by the violet band.