Vel.2, Ne.]2, pp~245—249,1~9~ 0273—1177/33/120245—05$03.OO/O

Printed j~ ~ Rrit~in. Al I rights reserved. Copyright ©COSPAR

COLLECTION OF COMETARY DUST

P. LeII*, E. Igenbergs*,H. Kuczera*andN. Pailer**

* TechnischeUniversitätMunchen,LehrsluhlfurRau,nfahrttechnik,

Munchen,F.R.G.**Max,.planck1nstit Ut für Kernphysik,Heidelberg,F.R.G.

ABSTRACT

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

The collection efficiency of a target depends in addition to obvious geometr~-cal conditions upon the surface of the target The surface characteristicscan 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 thecollection efficiency of “clean” targets Laboratory experiments are describ-ed which were conducted at the Technische Universitàt Munchen, Lehrstuhl furRaumfahrttechnik, and the Max—Planck-Institut für Kernphysik, Heidelberg Inboth experiments an electromagnetic accelerator is used to accelerate differ-ent types of dust in vacuum to velocities between 10 and 400 rn/s.

The target is then examined under the microscope and a secondary ion massspectrometer (which is a model of the laboratory carried on board of thespacecraft for ‘in situ analysis) The adhesion of the dust grains at thetarget is evaluated experimentally in an ultracentrifuge.

INTRODUCTION

Currently only fly-by missions will be performed with a very high relativevelocity around 70 km/s at the retrograde Comet Halley Missions have beenproposed which involve a rendezvous at the comet (e.g. Enke). In these ren—dézvous missions the relative velocity between the cometary dust and thespacecraft will result from the motion of the dust away from the nucleusonly because the spacecraft will be at approximately the same orbital velo-city as the cometary nucleus Here a relative velocity between 10 and 400 m/scan be expected The dust experiment on board of the spacecraft will collectthe dust which has been released by the nucleus and perform an analysis ofthe dust. For this purpose the dust must be collected on surfaces, and sub-sequently be brought inside the spacecraft.

The collection efficiency of such surfaces or targets for different typesof dust can be simulated in laboratory experiments. For this purpose dustparticles with a diameter between 0 1~imand 10pm are accelerated in a vacuumtank with a preselected velocity The collection efficiency as well as theinteraction between dust and target are investigated and provide the basisfor the development of a flight experiment.

Dust particles stick to the surface of solid or liquid bodies because of theadhesion force. The history of dust collection research is therefore also thehistory of the research in adhesive forces. In 1777 De la Fond made theassumption that the adhesion force was the result of an attraction force bet-ween the basic elements of the material. He also stated that the adhesion

246 P. Cell g~al.

force was proportional to the inverse cube of the distance between the two

attracting elements. In the subsequent time, until 1920, the adhesion forcewas attributed to an adhesive material and the structure of the adhering part-ners It was only in 1956 that Lifschitz published a macroscopic theory ofadhesion. The theoretical and experimental work is collected and summarizedby Helmar Krupp in 1967: “Particle Adhesion, Theory and Experiment” /1/,where the adhesion of particles on targets was investigated and explainedtheoretically up to relative velocities around 20 rn/s. All known mechanismswhich lead to an adhesion of particles on surfaces are described by Krupp

The work described in this paper is concerned with the collecting efficiencyof targets which have a surface of solid, clean material with an atomic num-ber higher than 100, because the target material should not be found in thecometary matter in order to enable a measurement of the components of thedust particles which are collected on a target using a secondary ion—massspectrometer. The measurements of the collecting efficiency of this type oftargets are conducted at the Lehrstuhl für Raumfahrttechnik, Technische Uni-versitat Munchen, and at the Max—Planck—Institut fur Kernphysik in Heidel-berg. The collecting efficiency of greasy surfaces is investigated by B.Clarkin Denver, Colorado /2/.

EXPERIMENTS

The experiment at the Max—Planck-Institut für Kernphysik, Heidelberg, has adust source a discharge chamber for the pretreatment of the targets and asecondary ion—mass spectrometer The discharge chamber is used to clean thesurfaces of the targets prior to the impact of the dust These ultra—cleansurfaces are then impacted and brought into the mass spectrometer for theanalysis of the sticking dust These operations are conducted in one inte-grated laboratory setup which is described in a paper of N. Pailer.

The laboratory experiment at the Technische Universitat Munchen consists ofa dust source, a vacuum tank with velocity measurement equipment and an ultra—centrifuge A specific feature of the experiment in Munich is that the tar-gets and the dust can be heated to temperatures between +80 to 200°C Organicmaterials and volatile agents like grease and water will evaporate from thesurface prior to the experiment

In both experiments the dust source is art eletromagnetic accelerator whichaccelerates the dust particles that had been positioned on top of an alumini-um driver This driver is then retained by a retainer plate, whereas the dustparticles continue towards the target /3/ The velocity of the dust particleshas initially been measured using photographic techniques After calibrationonly a simple time of flight measurement is necessary because it has beenfound that this accelerator works very reproducibly (well within 1 % of thefinal velocity), when the charging voltage of the capacitor bank is adjustedto a predeterminedvalue

Fig 1 shows a schematic diagram of the experimental setup in Munich

2 window

____ita -

delay

L J Igiltron vacuum tankI Hicrophoneca~ac1tor 2 Target Holder

V 3 Peltter £lenmnt

4 TargetS Retainer6 Oust

—1’- 1 Driver8 8cce~eratorCOmoli 00

trigger

Fig.1 Experimental Setup for Dust Collection Simulation

Coliect. ion ol Cometary DiI5~ 247

The capacitor module cart be ad]usted to different ~alues pf capacity follow-ing the experimental requiremer~ts. The dii~ei~ are alumIttiuni discs of 30 mm 0and a thickness between 0.5 and 2 mm.

400 __________________________

/ /C = 23.1 liE —._ /

320 C = 15,4 liE _,~ .~ ______

C = 7,7 iF — / /‘____ // __ __

>240 ________ / ___ ___

/ / /‘b=0,Sm /

~ 7~ 15 18

Charmng Vo1ta~eu [kY]

Fig 2 Velocity of the dust as function of the chargingvoltage of the accelerator for different values of capacityof the capacitor module and driver thickness

Fig 2 shows the velocity of the dust as function of the charging voltage ofthe accelerator for different values of capacity of the capacitor module anddriver thickness

Using this diagram, the velocity that is required for a specific experimentcan easily be selected.

The first measurementsof the dust collection efficiency were madewith a mi-croscope All impacting dust particles generate a trace on the surface and acount of the traces versus the sticking dust particles gives a first value ofthe collection efficiency.

For the measurement of the adhesive force the target is installed in an ultra-centrifuge During the acceleration of the ultracentrifuge the target surfaceis photographed using a stroboscope. The time when the dust particles separa-te from the target and the velocity of the centrifuge at this time lead to adetermination of the adhesion force During these experiments it was foundthat the adhesion force is iO~ times larger than the weight of theparticles.

EXPERIMENTAL RESULTS

In the experiments that were conducted in Munich glass beadswere used to si-mulate the dust particles. The parameterswere:particle size: 0.1 to 10 innparticle velocity: 10 to 400 mispressure in vacuum tank: i~—~Torrtarget material: gold and aluminium

The following results were obtained:

a) Aluminium surfaces will have a higher collection efficiency as comparablegold surfaces. The impacting glass beads generate the same type and size ofimpacts for both materials.b) For every type of surface there is an optimum velocity for maximum collect-ing efficiency of specific dust particles For flat polished gold surfacesthis velocity will be 80 m/s, and for aluminium surfaces 100 m/s Both re-sults were obtained with glass beads of 5pm diameter. The velocity for the

24$ I. Ce I -

optimum collecting efficiency will decrease with smaller dust particles.c) Glass beads with diameters larger than 10 pm cannot be collected On solidpolished surfaces of aluminium or gold within the velocity range of the expe-riments performed here (10 — 400 m/s)

d) Any microstructure of the target will increase the collection efficiencyof the surface if the impacting particle is equal or smaller than the geomet-ric dimensions of the microstructure (distance of the grooves, distance ofthe peaks). If the particles are larger than these geometric parameters, thecollection efficiency will be reduced and will be less than the value for thepolished surface, because the area of contact between the particle and thetarget is reduced by the microstructure. A regular microstructure will coll-ect the impacting particles in a structured way. This is of advantage for theanalysis with the secondary ion—mass spectrometer.

~~—~ar: ____

Fig.3 Photograph of a dust cloud, consisting ofglass beads after separation from the driver

Fig.4 shows a mechanically structured target which has been impacted withglass beads with a diameter of 5pm. The distance of the grooves is 8 j.t, thedepth of the grooves 2pm, impact velocity 50 rn/s.

Fig.4 Regularly structured gold surface for collectingof particles. Spmglass beads, impact velocity 50 rn/s

Particles larger than 10pm in diameter can therefore be collected by structur-ed surfaces. An irreguTar structure of the surface can be obtained using agas discharqe chamber in which the surface is sputtered. The surface of sucha target is shown in Fig.5.

Co 1 le it on I c ~mci or v I). ~

~ ~Kr:.~~ç

Fig.5 Photograph of a polished, flat gold surface whichhas been structured (sputtered) in 20 hours in a gas dis-charge chamber

e) The contamination of the target surface has a great influence on the coll-ecting efficiency. Starting with an extremely clean surface (e.g. after thetreatment in the gas discharge chamber) the collecting efficiency will de-crease with increasing contamination. This is explained because free attract-ing bonds of the metallic surface are saturated and therefore not availablefor the particles which hit this element. When the contamination is increas-ed furthermore, the collection efficiency will increase again, because thereare now liquid bridges between the particles and the metallic surface.

CONCLUSIONS

The experiments conducted thus far in Munich lead to first indications to-wards an optimum target that is made of a solid, clean metal. The target istreated first mechanically, grooves with a distance around 3 jt are introduc-ed. Then the inicrostructure of the surface is altered by sputtering in a gasdischarge chamber. As material for the target aluminium, silver, gold, tantaland germaniumcan be used.

The current work is devoted towards an improvement of the experimental evalu-ation of the adhesion forces using the centrifuge. Together with the quanti-tative evaluation of the collecting efficiency in numerous experiments themeasurementof the adhesion force will determine the values of the correspond-ing parameters in the theory of adhesion of small particles on metallic tar-gets. The results of these experiments will influence the design of a targetfor a cometary rendezvousmission.

LITERATURE

/1/ H. Krupp: Particle Adhesion, Theory and Experiment, Advanc. ColloidInterface Sci., 1 (1967), p.111—239

/2/ B.C. Clark: Comet Dust Collection Experiments in Denver, Martin Mariet-ta Aerospace, Denver, Colorado 80201, Contract 955180, 1980

/3/ P. Lell, E. Igenbergs and H. Kuczera: An Electromagnetic Accelerator,to be published in Journal of Physics E