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.     

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 European probe to comet Halley

A European probe to comet Halley is proposed. The probe's model payload consists of 8 scientific instruments, viz. neutral, ion and dust impact mass spectrometers, magnetometer, medium energy ion and electron analyzer, camera, dust impact detectors and plasma wave experiment. Fly-by of the comet Halley nucleus will take place on November 28th, 1985, at about 500 km miss distance. The main spacecraft serves as relay link to transmit the observed data to Earth. As probe, a modified ISEE 2 design is proposed. Because of the cometary dust hazard expected in the coma a heavy dust shield (27 kg) is required, consisting of a thin front sheet and a 3 layer rear sheet. The probe is spin-stabilized (12 rpm), has no active attitude and orbit control capability and uses battery power only to provide about 1000 Wh for a measuring phase. A despun antenna transmits up to 20 kbit/s, in X-band. The total probe mass is estimated at 250 kg. The 3 model development programme should start in mid 1981 with Phase B.     

Cometary constraints on the planet forming environment.

Molecular elemental and isotopic abundances of comets provide sensitive diagnostics for models of the primitive solar nebula. New measurements of the N2, NH and NH2 abundances in comets together with the in situ Giotto mass spectrometer and dust analyzer data provide new constraints for models of the comet forming environment in the solar nebula. An inventory of nitrogen-containing species in comet Halley indicates that NH3 and CN are the dominant N carriers observed in the coma gas. The elemental nitrogen abundance in the gas component of the coma is found to be depleted by a factor approximately 75 relative to the solar photosphere. Combined with the Giotto dust analyzer results for the coma dust component, we find for comet Halley Ngas + dust approximately 1/6 the solar value. The measurement of the CN carbon isotope ratio from the bulk coma gas and dust in comet Halley indicates a significantly lower value, 12C/13C = 65 +/- 9 than the solar system value of 89 +/- 2. Because the dominant CN carrier species in comets remains unidentified, it is not yet possible to attribute the low isotope ratio predominantly to the bulk gas or dust components. The large chemical and isotopic inhomogeneities discovered in the Halley dust particles on 1 mu scales are indicative of preserved circumstellar grains which survived processing in the interstellar clouds, and may be related to the presolar silicon carbide, diamond and graphite grains recently discovered in carbonaceous chondrites. Less than 0.1% of the bulk mass in the primitive meteorites studied consists of these cosmically important grains. A larger mass fraction (approximately 5%) of chemically heterogeneous organic grains is found in the nucleus of comet Halley. The isotopic anomalies discovered in the PUMA 1 Giotto data in comet Halley are probably also attributable to preserved circumstellar grains. Thus the extent of grain processing in the interstellar environment is much less than predicted by interstellar grain models, and a significant fraction of comet nuclei (approximately 5%) may be in the form of preserved circumstellar matter. Comet nuclei probably formed in much more benign environments than primitive meteorites.     

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.     

Giotto extended mission

The navigation of the ESA spacecraft Giotto to its encounter with comet P/Halley on 14 March 1986 required just 10% of the fuel available. Although the spacecraft was damaged by dust impacts during its close flyby at the nucleus of P/Halley it was retargeted to return close to Earth to maintain the option to extend the mission to encounter another comet, P/Grigg-Skjellerup on 10 July 1992.

On 2 April 1986 the spacecraft was put into hibernation configuration and had been orbiting the Sun in the ecliptic with an orbital period of 10 months. On 19 February 1990 it was reactivated, spacecraft subsystems and the payload checked out to determine its health status.

On 2 July 1990 Giotto performed succesfully the first-ever Earth gravity assist manoeuvre of a spacecraft approaching the Earth from deep space and was retargeted for comet P/Grigg--Skjellerup. It was concluded that the spacecraft is ready to provide valuable data during a potential encounter with a second comet.     

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.     

Auroral signatures of transient processes in the outer magnetosphere

We present an analysis of sporadic and recurrent injections of magnetospheric ions in the midnight auroral oval during substorms and of the associated ionospheric ion outflows. The source of plasma sheet precipitating ions is determined using a simple method, based on the measured relation between the ion inverse velocity and time (l = v × t). This method is applied here to two typical passes of the Interball-Auroral (IA) satellite at distances of 3 R_E above the auroral regions. Substorm related ion injections are shown to be mainly due to time of flight effects. In contrast with particle trajectory computations (Sauvaud et al., 1999), the inverse velocity method does not require magnetic and electric field models and can thus be used systematically for the detection of time of flight dispersed ion structures (TDIS). This allowed us to build a large database of TDIS events and to perform a statistical analysis of their spatial distribution. For the cases presented here the source region of the injected ions is found at radial distances from 18 to 30 R_E near the equatorial magnetosphere. At Interball altitudes ( 3 R_E), ion injections detected at the poleward boundary of the nighside auroral oval are associated with shear Alfvén waves superimposed over large-scale quasi-static current structures. We show that the most poleward TDIS are collocated with a large outflow of ionospheric H⁺ and O⁺ displaying pitch-angle distributions peaked in the pitch-angle range 90°–120°. These ions are thus accelerated perpendicularly to the magnetic field not only in the main auroral acceleration region but also up to at least 3 R_E. The expanding auroral bulge thus constitutes a significant source of H⁺ and O⁺ ions for the mid-tail magnetosphere.     

Interplanetary scintillations of PKS 2314+03 during occultation by comet Halley

Strong interplanetary scintillations (IPS) of the quasar 2314+03 were recorded at 103 MHz at Thaltej-Ahmedabad, India with a transit type correlation interferometer on 18, 19 and 20 December 1985, as the radio source was predicted to be occulted by the ion tail of the comet Halley.

On 18th through 20th very strong scintillations, with periodicities of 1 sec average were observed, their amplitude progressively decreasing as the source approached the tail-end. The rms variations of scintillating flux of the source on 18, 19 & 20 were about 18, 11 & 4.7 Jy, as against 3.3 Jy on control days 17 and 21 December for solar elongation of 87°.

Assuming Gaussian irregularities with weak scattering, the rms density variations, ΔN, of 10, 6, 3 and 1 elec./cm³ on 18 through 21 December, from the comet nucleus towards its tail-end, varied as (ΔN) ∝ r^−3.3 as against (ΔN) ∝ r⁻² in the solar plasma.

Quasi-periodic modulations of the enhanced scintillating flux possibly imply 10⁴ km scale-size ion condensations and mean electron density of 10³ to 10⁴ electrons/cm³ in the Halley's plasma tail.     

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.     

The plasma regime at comet Giacobini-Zinner

This review of the plasma regime sampled by the encounter of the International Cometary Explorer spacecraft (ICE) with the comet Giacobini-Zinner, discusses the shock, or bow wave, ion pickup, ionization mechanisms, and the cometary plasma tail.

The observations are consistent with the existence of a weak shock, which may be pulsating, but do not exclude the suggestion by Wallis and Dryer that the shock, though present around the sub-solar point, is in process of decaying to a wave on the flanks.

Pickup of cometary ions provokes, by means of several mechanisms, ion cyclotron, mirror, beam and electrostatic instabilities which cause strong turbulence in the inner coma, as indicated in the power spectra of the magnetic field in the coma and the surrounding volume. Heavy mass loading and consequent slowing down of the solar wind is observed. Acceleration of ions by a stochastic mechanism is indicated.

Ionization of cometary neutrals occurs principally by photoionization and charge exchange. Alfvens critical velocity mechanism, likely operates only in the inner coma not visited by ICE. A steep increase of nearly two orders of magnitude in electron density occurs in the tail, where electron velocity distributions show evidence of entry of electrons from the solar wind. The turbulence there is damped by the high ion density and low temperature.

In general, the vicinity of the comet is filled with plasma phenomena and a rich variety of corresponding atomic and molecular processes can be studied there. Comparison between the ICE, Giotto, and Vega observations forms a most valuable future study.     

Intensity profiles of the multiple scattering light near the cometary nucleus

The multiple scattering of solar radiation in the cometary atmosphere is treated with the method of successive scattering. Referring to in situ measurements of comet Halley about the size and spatial distributions of dust, the optical thickness τ₁ of dust has been estimated, i.e. τ₁=0.03 at wavelength λ=0.62μm in a quiet time, but τ₁=0.3 when the outbursts/jets occur. In the derivation of τ₁, optical properties of dust including a mixing ratio of absorbing to silicate grains, are determined based on the polarimetry of P/Halley at λ=0.62μm observed during the phase angles over Nov. 1985 to May 1986 at the Dodaira Station of Tokyo Astronomical Observatory.

It is found that a temporary enhancement of τ₁ leads an increase of the upward reflected intensity when the surface albedo A of the nucleus is less than 0.04, but the reverse is true when A>0.04. On the other hand, the intensity of the downward radiation at the surface of the nucleus always decreases as an increase of τ₁.     

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.     

Cometary ion observations at and within the cometopause-region of comet Halley

Three distinct boundaries are identified from the PICCA cometary ion observations within the innermost part of the coma of comet Halley: (1) the 'cometopause' at a cometocentric distance R_c 1.5×10⁵ km, characterized by the appearance of water-group ions well above background; (2) the 'cold cometary plasma boundary' at R_c 3×10⁴ km, characterized by a sudden and simultaneous decrease in the temperatures of all cometary ions, and (3) the 'ionopause' at R_c 6000 km, characterized by a fast decrease in the intensity of all cometary ions by a factor 3–5. Between the first two boundaries only ions with masses less than 50 amu are present, showing distinct maximum intensities at 18, 32 and 44 amu at the second boundary. Downstream of the second boundary also ions of mass 12, 64, 76, 86 and 100 amu are detected.     

Applications of PVDF dust sensor systems in space

The advantages of ruggedness, no bias requirement, ease of large area sensor construction, high counting rate capability, and space reliability inherent in the Polyvinylidene Fluoride (PVDF) dust sensors which have been under development at the University of Chicago over the last decade have led to PVDF flux/mass/velocity/trajectory systems which have advantages over other systems and are well suited for a variety of dust studies in space. The thermal stability characteristics and flux/mass/velocity/trajectory determining characteristics of PVDF and Vinylidene Fluoride/Trifluoroethylene (PVDF copolymer) dust sensors are described. We summarize the objectives and designs of our earlier VEGA- comet Halley instruments, a PVDF velocity/trajectory dust instrument for launch on the Advanced Research and Global Observation Satellite (ARGOS) in January 1996, and a PVDF high flux dust instrument for launch on the CASSINI spacecraft to Saturn in October 1997.     

Hydrogen coma of comet P/Halley

The main molecular processes to produce the hydrogen comae of comets are now well known: Water, the main constituent of cometary atmospheres, is photodissociated by the solar ultraviolet radiation to form the high (20 km s⁻¹) and low (8 km s⁻¹) velocity components of the atomic hydrogen. The hydrogen clouds of various fresh comets have been observed in 1216Å by a number of spacecrafts. Ultraviolet observations of short period comets are, however, rather rare. Consequently Comet P/Halley in this apparition is a good object to obtain new physics of the hydrogen coma. Strong breathing of the hydrogen coma of this comet found by “Suisei” provides just such an example. The rotational period of Comet Halley's nucleus, its activity in the form of outbursts alone, and the position of jet sources etc. are determined from the breathing phenomena. Atomic hydrogen from organic compounds with a velocity of 11 km s⁻¹ play an important role in that analysis. The time variations of the water production rate of Comet Halley during this apparition observed by various spacecrafts appear to be in agreement with each other and are about 1.5–2 times larger than the standard model. The difficulty of the calibration problem was emphasized.     

Cometary atmosphere (gas and dust)

There is important progress now in the identifications and measurements of primary (parent) molecules in the inner coma of Comet Halley. H₂O, CO₂ and CO are definitely in the list, CH and some complicate organic molecules are suspected. Gas production rate for water vapor is Q_H2O 10³⁰ s⁻¹. The bulk of data doesn't contradict to the Whipple model of nucleus (with clathrate modification). Pronounced spatial structure of gaseous flow in the coma was observed, but in general measured properties of neutral gas in the coma of Comet Halley are not very different from predicted. Situation for dust is different. In situ dust measurements show that size spectrum and optical properties of particles in coma are substantively declining from predicted on the base of groundbased photometry. However there are discrepancies between Vega and Giotto dust counter data. Dust in the inner coma didn't prevent the succesful imaging of nucleus by TV on Vega 1 and 2.     

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.     

Dirty ice grains in comets

Based on the computed equilibrium temperature of evaporating dirty water-ice grains, dirty water-ice halo is examined, taking into account of a size dependence of terminal velocity of dust at P/Halley. It is found that due to an enhanced grain's temperature caused by dirtiness, icy halo cannot extend over 100 km from the nucleus when comet approaches inside a solar distance r of 1 AU. Therefore, it is unlikely that the ice bands in the near infrared wavelengths could be detected in the cometary coma at r<1 AU.     

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.