Interpretation of the measurements of secondary electron currents induced by impacts during the flyby of comet Halley

The Giotto, Vega-1 and Vega-2 spacecraft flew through the environment of comet Halley at a relatively close range with velocities of the order of 70–80 km/s. The fore sections of their surface were bombarded by neutral molecules and dust grains which caused the emission of secondary electrons and sputtered ions. This paper makes use of the secondary electron current measurements performed on Vega-1 to infer some characteristic features of the cometary atmosphere. The total gas production rate is estimated to be of the order of 10³⁰ molecules/s and is found to vary with time; the presence of a major jet is also detected at closest approach.     

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.     

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.     

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.     

Project overview and highlights of Suisei and Sakigake

ISAS's (Institute of Space and Astronautical Science) project for the exploration of comet Halley consists of two spacecraft, Sakigake and Suisei, launched on 7 January 1985 and 18 August, respectively.

Sakigake passed the sunward side of the comet on 11 March 1986 with a miss distance of 6.99 million km. Three experiments, a plasma wave probe with dipole and search-coil antennae, a magnetometer with three axis ring core sensor on an extended boom and a four-grid Faraday cup attached to the inner side of the wall of the spacecraft, detected various phenomena caused by the comet at a distance as far as 7 million km.

The other spacecraft, Suisei, flew by the comet on its sunward side with a miss distance of 151 thousand km on 8 March 1986. It carried two experiments, an ultraviolet imager and an energy analyzer for ions. The UV imager was able to take the first image of the hydrogen cloud of comet Halley on 26 November 1985. With this experiment, the spin period of the cometary nucleus, location of jets, amount of water evaporation, distribution of hydrogen density inside cloud, etc. were clarified. The energy analyser experiment provided information on the intensive interaction between cometary and solar wind ions.     

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.     

Interaction between a body and its environment during a cometary fly-by

The surface of a spacecraft is submitted to the bombardment of dust grains and neutral molecules during an atmospheric re-entry or a cometary fly-by. These particles create secondary ions and electrons which form a plasma cloud around the body and control the electric potential of its surface. Computer simulation models are used to predict the structure and dynamic behaviour of the charged particle density distribution for the cases of planar and cylindrical bodies. It is found that an ion and an electron layer form in the vicinity of the surface at distances of the order of the ion and electron Debye lengths, respectively. The potential of the surface is positive on the average and is a function of the electron mean kinetic energy. A positive potential barrier develops at the location of the ion layer and its height is governed by the sum of the electron and ion mean kinetic energies. The threat caused by this interaction to the spacecraft and its instrumentation is discussed and an in-situ observation of this phenomenon is proposed as a possible diagnostic technique of the environment.     

Energetic particles in the comet Halley environment

An overview is presented of electrons, protons and heavier ions (E > 20 keV) recorded by the energetic particle detector EPONA in the Comet Halley environment, 12–15 March, 1986. Pick-up ions were detected at distances of up to at least 7.5 × 10⁶ km from the nucleus. Estimates of the energies that typical cometary ions may be expected to acquire from the solar wind pertaining at Encounter show that the pick-up process is insufficient to account for the energies of the particles detected. An additional mechanism must thus be postulated to account for the observed particle signatures. Preliminary correlations with magnetic and plasma wave data from other instruments suggest that the presence of MHD turbulence at several million kilometers upstream of the bowshock may have contributed to the acceleration of the first pick-up ions observed. The bowshock boundary (inbound) does not appear to have constituted a location where particle acceleration to high energies took place. Downstream of the shock boundary, hardening of the energy spectrum and the development of less anisotropic particle streaming was observed to occur when the spacecraft was in a turbulent environment 1 × 10⁶ km from the nucleus. The waxing influence of mass loading as a mechanism for reducing energetic particle fluxes as well as the depletion of energetic ions due to their escape along open field lines and to charge exchange collision with neutrals in a progressively more stagnant solar wind, may be inferred in a regime (seen on the magnetometer data to be largely non-turbulent) traversed by the spacecraft from 5 × 10⁵ km from the nucleus to within the magnetic pile-up region. A major burst of ions and electrons (not yet established to be of cometary origin) occurred when the spacecraft was close to the Contact Surface. A population of high energy electrons (from 180 keV to at least 300 keV) was detected for about one hour before Closest Approach and for several hours thereafter. Also an energetic beam of electrons was identified exiting from a location at about 1 × 10⁶ km from the nucleus (outbound). Finally, differences between inbound and outbound particle signatures are described.     

Numerical simulations of the radiance from the comet 46P/Wirtanen in the various configurations of the measurements during “Rosetta” mission

The work we present deals with the spectrometric measurements of VIRTIS instrument of the Comet P/Wirtanen planned for the Rosetta mission. This spectrometer can monitor (VIRTIS M channel: 0.250μm – 0.980μm; Δκ=20cm⁻¹; 0.980 – 5.0 μm; Δκ = 5cm⁻¹; VIRTIS H channel: 2.0 μm – 5.0 μm; Δκ=2cm⁻¹) the nucleus and the coma in order to provide a general picture of coma's composition, the production of gas and dust, the relationship of coma production to surface composition and the structure and variation of mineralogy of the nucleus surface. During the mission the observation conditions of the spectroscopic investigation change due to different relative positions spacecraft/comet, and to the different illumination conditions of the surface at various distances of the comet to the Sun. The nucleus surface is continuously modified by the ice sublimation accompanied by gas and dust emission. Consequently the surface also its spectrophotometric properties changes and their monitoring can give a new insight. The important role of simulations is to predict the results of measurements in various experimental condition what, in the future, can help in interpretation of the measured data.

In this paper the first results of our simulation the radiance from the comet in the 0.25–5.0μm spectral range at two distances from the Sun (1AU and 3AU) are shown. The distance between the Rosetta orbiter and the nucleus surface as well as the sun zenith angles are taken into account according to the Rosetta mission phases. In fact the surface and coma properties vary along the comet orbit, and should be taken into account in our calculations. The optical parameters of the dust on the surface (e.g. reflectance) and in the coma (e.g. Q_ext) were calculated from optical constants of possible comet analogues. The thermodynamic parameters of the comet are taken from the models of comet evolution. Through this kind of modelling it is possible to identify the surface characteristics in spectra of the radiation from the surface of nucleus transmitted through the coma loaded with dust and gases.

Even if the “Rosetta mission” is postponed, with the consequence of a target change, we think that our idea and the method used for the simulations can be useful also for the new Rosetta target - the comet 67P/Churyumov Gerasimenko.     

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.     

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.     

Symmetric model of spacecraft charging and plasma emission effects

The electrostatic charging of satellites in space and the interactions with the plasma in the near surroundings are investigated by making use of symmetric models. In this case, the Vlasov-Poisson system describing the ambient plasma disturbances and the plasma emitted from the surface can be integrated self-consistently within a numerical iteration scheme, and the current balance yields the floating potential of the probe. The spacecraft charging and the potentials in its surroundings are investigated for the following plasma and emission conditions: (1) in the ionosphere in the case of very negative surface potentials, (2) in the solar wind with regard to the HELIOS mission and (3) in the near vicinity of the comet Halley, where a very strong plasma emission due to the impact of neutral gases onto the surface must be regarded. Finally, the importance of the shielding due to the ambient plasma is discussed.     

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.     

Energetic ion emission for active spacecraft control

Future space missions aiming at the accurate measurement of cold plasmas and DC to very low frequency electric fields will require that the potential of their conductive surfaces be actively controlled to be near the ambient plasma potential. In the near-Earth space these spacecraft are usually solar-cell powered; consequently, parts of their surface are most of the time exposed to solar photons. Outside the plasmasphere, a positive surface potential due the dominance of surface-emitted photoelectrons over ambient plasma electrons is to be expected. Photo- and ambient electrons largely determine the potential and positive values between a few Volts up to 100 V have been observed. Active ion emission is the obvious solution of this problem. A liquid metal ion emitter and a saddle field ion emitter are nearing the stage of flight unit fabrication. We will attempt to clamp the spacecraft potential to values close to the plasma potential. We present first results from vacuum chamber tests and describe the emission behaviour and characteristics of emitters producing, respectively, In⁺ and N₂⁺ beams with an energy of ≥ 5 keV.     

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.     

Mass-spectrometric in situ studies of cometary organics for p/Halley and options for the future.

When the VEGA and GIOTTO spacecrafts flew by comet p/Halley in 1986 the mass-spectrometers Puma and PIA measured the composition of cometary dust particles impacting at speeds of well above 65 km/s. Ion formation upon impact lead to mostly atomic ions. However, a small fraction of the ions measured could be related to molecules. A sophisticated analysis allowed for the first time to point to the chemical nature of cometary organics based on actual mass spectra. With the instrument CoMA for the NASA-BMFT mission CRAF much higher mass-resolution and molecule masses become accessible for in situ measurement, and will yield complementary information to the gas chromatograph CIDEX also onboard CRAF.     

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.     

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.     

近月空间带电粒子环境——“嫦娥1号”“嫦娥2号”观测结果

“嫦娥1号”（CE-1）、“嫦娥2号”（CE-2）都安装了1台太阳高能粒子探测器（High-energetic ParticlesDetectors,HPD）和2台太阳风离子探测器（Solar Wind Ion Detectors,SWIDs）,进行了月球轨道200 km和100 km空间环境探测,获得了月球轨道空间高能带电粒子（质子、电子和重离子）能谱随时间的演化特征、等离子体与月球相互作用特征以及太阳风离子速度、密度和温度参量。空间环境探测数据分析结果表明：太阳活动低年、空间环境扰动水平相对较低、月球处于太阳风中时,近月空间带电粒子环境的基本特征与行星际空间相比变化不大。CE-1、CE-2在轨运行期间,发现了多起0.1～2 MeV能量电子急剧增加事件,这些事件发生在月球从太阳风运动到磁尾的所有空间区域,其中20%的事件伴随着卫星周围等离子体离子加速。模拟和统计研究表明：能量电子急剧增加使得绕月卫星和月球表面电位大幅下降导致了离子加速现象的发生;能量电子总流量大于10¹¹ cm^-2时,绕月卫星和月球表面充电电位可达负的上千伏。此外,月表溅射与反射太阳风离子、太阳风“拾起”离子等空间环境事件的发现,揭示了太阳风离子和月球存在复杂的相互作用过程。