Pergamon Adv. Space Res. Vol. 17, No. 12, pp. (12)137-(12)140, 1996

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DUST FLUX ANALYSER EXPERIMENT FOR THE ROSETTA MISSION

M. R. Leese, 1 J. A. M. McDonnell, 1 S. F. Green, 1 E. Busoletti, 2 B. C. Clark, 3 L. Colan~eli, 4 J. F. Crifo, 5 P. Eberhardt, 6 F. Giovane, 7 E. Grtin, ~ B. Gustafson, 7 D. W. Hughes, 9 D. Jackson, 10 P. Lamy, 11 y . Langevin,12 I. Mann,13S. McKenna-Lawlor, 14 W. G. Tanner, I5 P. R. Weissman 16 and J. C. Zarnecki 1

J Unit for Space Sciences, UniversiO, of Kent, Canterbury, Kent CT2 7NR, U.K. 2 lstituto Universitario Navale, 80133 Napoli, Italy 3 Martin Marietta, Denver, CO 80201, U.S.A. 40sservatorio Astronomico di Capodimonte, Napoli 80131, Italy 5 CNRS, Service d'Aeronomie, 91371 Verrieres le Buisson Cedex, France 6 Physics Institute, University of Bern, CH-3012, Switzerland 7 Department of Astronomy, University of Florida, Gainesville, FL 32611, U.S.A. 8 Max-Planck-lnstitutfiir Kernphysik, Postfach 103980, D-69029 Heidelberg, Germany 9 Depat2tment of Physics, University of Sheffield, Sheffield $10 7RH, U.K. 1o Applied Optics Group, University of Kent, Canterbury, Kent CT2 7NR, U.K. 11 Laboratoire d'Astronomie Spatiale, Traverse du Siphon, 13376 Marseille Cedex 12, France 12 Universite Paris-Sud, Laboratoire Rene Bernas, 91406 Orsay Cedex, France 13 Max-Planck-lnstitut f~r Aeronomie, W-3411 Katlenburg-Lindau, Germany 14 Department of Experimental Physics, St Patricks College, Maynooth, Co. Kildare, Ireland 15 Department of Physics, Baylor University, Waco, TX 76798-7303, U.S.A. 16 Jet Propulsion Laboratory, Pasadena, CA 91109, U.S.A.

ABSTRACT

We present the description of a design for a proposed Dust Flux Analyser for the Rosetta cometary mission. A concept first developed for the NASA/ESA Tempel II Rendezvous and Halley Intercept Mission /1/, the instrument is able to measure dust particle parameters and fluxes over a velocity range typical of emission from cometary surfaces. It would be mounted on the Rosetta Orbiter and would measure the variation in flux rate throughout all mission phases at the comet. The instrument would measure particle flux, velocity, momentum and density, shape and scattering properties.

INTRODUCTION

The Rosetta cometary mission has been selected as the third ESA Cornerstone Mission. The Dust Flux Analyser (DFA) forms part of the Orbiter Model Payload along with the Remote Imaging System (RIS), Visible and IR Spectral and Thermal Mapper (VIRSTM), Gas and Ion Mass Spectrometer (NGIMS), Cometary Mass Analyser (COMA), Scanning Electron Microprobe (SEMPRA) and Plasma Package. The instrument would measure dust flux, size distribution and velocity, as part of the Rosetta Prime Science Objective to measure the physical properties and interrelation of volatiles and refractories in the context of the comet nucleus.

The DFA instrument development and proposal is led by the Unit for Space Sciences at the University of Kent at Canterbury (UKC) in collaboration with several UK and international Co-Is. Currently the following resources are allocated to DFA in the Orbiter Model Payload: Mass 2.5kg, Power 1W average (the peak power requirement will be higher than this, but the 1W average will be achieved by using a low power mode during "low flux" parts of the mission). The data rate will be variable according to the flux rate and mission phase, but is generally low.

(12)137

(12)138 M.R. Leese etal.

INSTRUMENT CONCEPT- THE DUST FLUX ANALYSER

(x~(xl,yl) ~ L a s e r Curtain 1

100 : / ~t¢_(x2,y2) / t (~ ~-tm / ~ e r C u r t a i n 2

~ Momentum Sensor

Position in 2 planes => VELOCITY + Time of Flight, t(s) and RADIANT

Momentum + Velocity => MASS

Detected => SCATTERING FUNCTION and Pulse SPECTRAL CHARACTERISTICS

Combined Parameters => PARTICLE DENSITY

Fig. 1. Measurement Concept of the Rosetta DFA instrument

The concept of the Rosetta Dust Flux Analyser is shown in Figure 1. Incident particles pass through two laser curtains where their x and y coordinates and time of arrival are measured. The position in two planes plus the time of flight yields the particle velocity and radiant. The third stage is a momentum sensor, the momentum and velocity giving the particle mass. From the detected pulse shape at the laser curtain stage it should be possible to determine the particle scattering function, which is related to size, and from its mass estimate the density, in addition to its spectral characteristics.

SCIENCE REQUIREMENTS

Scientific Aims

The scientific aims are to determine, for a typical low-activity periodic comet, the following dust coma properties from aphelion to perihelion:

1) Dust size and mass distributions 2) Dust dynamics in the coma 3) Dust production rate 4) Particle densities, shapes and scattering properties and provide a precursive monitor of contamination of spacecraft surfaces (e.g. solar cell arrays).

Measurements

1) 2) 3) 4) 5)

Individual dust particle sizes from scattered light (diameters > 10tam). Mass of some or all observed particles from impact momentum (mass > 10"12kg). Speed of detected particles (10 - 2000ms -1) from time-of-flight in instrument. Direction of detected particles (to +/- 1 o for cone of half angle -25 o) from x-y measurement. Shape and scattering properties of subset of panicles from variation of signal with time and scattering angle.

Detection Rates from Fountain Model

The expected "baseline" detection rates have been estimated (Table 1) using the modified fountain model /2/, which was initially developed for the Comet Halley environment/3/but later extended to short period comets, with the following assumptions:

Comet orbit q (perihelion) = 1.0 AU, Q (aphelion) = 6.0 AU Gas production rate (IAU) 1029 mols -1 Dust to gas mass ratio 2.5 Detection of all 10~rn and larger particles Instrument aperture 0.01 m 2

Nucleus radius 2.5km Nucleus density 500kgrn -3 Smooth emission function Giotto DIDSY mass distribution Instrument pointing at nucleus

Detection rates are found to depend critically on the nucleus temperature, and to a lesser extent other assumed quantities in the model. The results give an indication of average detection rates however, and especially the variability in different phases of the mission. Increased activity or jets increase the maximum liftable mass. Smaller particles will contribute most to the surface coverage, and may be detectable as scattered light background.

Rosetta's Dust Flux Analyser (DFA) Table 1. Predicted Mean Time Between Events (for mass >10-12kg).

(12)1"39

[ m 6.0 3.5 1.7 1.0

. < . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distance from the comet . . . . . . . . . . . . . . . . . . . . . . . . . . . > • I000 km I 100 km I I0 km

. < . . . . . . . . . . . . . . . . Maximum liftable mass < 10-1Zkl~ . . . . . . . . . . . . . . . . . . . . . . . . > • 1 count in 20 rain 10 sec 0.1 sec

1 min 1 sec 1 ms 20 sec 0.2 sec 2 ms

DFA BASELINE DESIGN

Figure 2 shows a schematic of the baseline DFA design. There are two main measurement light curtains each with an x-y detector plane. An additional light curtain has been introduced above these measurement planes to act as a trigger plane. When a panicle transition is detected at this plane by one of several detectors (Avalanche Photodiodes) then the measurement light curtains and the CCD electronics will be powered up. This low-power mode allows the average power consumption of the instrument to be vastly reduced during periods when the flux is very low. The DFA system is represented in Figure 3.

The velocity measurement would be made with Avalanche Photodiodes (APDs) between the upper trigger plane and the lower measurement plane. The positional and particle scattering information will be sensed at the two x-y detector planes. The scattered light will be imaged by Graded Index (GRIN) lenses onto Linear CCD or photodiode arrays. This allows the x-y positional information and possibly particle size, shape and scattering properties to be determined. The final stage is a momentum sensor. This comprises a flat aluminium plate with either piezoelectric transducers (similar to the DIDSY experiment on Giotto)/4/or novel fibre-optic interferometric sensors being developed in the Applied Optics group at UKC/5L

An additional detection stage measuring scattering at various angles (using APDs) would provide detailed information about the types of particles. This stage may be provided as a sub-system by internal partners within the DFA consortium [e.g.P. Lamy (LAS-Marseilles (F)), B. Gustafson and F. Giovane (University of Florida (USA))]; this could possibly be combined with the initial trigger stage.

X-YGRiNdetector plane =Trigger ~ t -- JB and C ON.

Fibre L e n s ~ ~Timing Circuits I

t rimiogi °

landaeadout ~ Card ] [ Electr°nies I [~ S ~ t ~

Linear ) ~ ( / ~ d / f 1 ~ +15V"lml I ' Detector ¢ ~ _ ] ) ~ ( ~ G R I N -15V.,~ PSU [ Array ~ f / ~ x ~ ~ L e n s +5V.~ CARD [,ql. +28V

HV

Light Source: Linear Laser Diodes Detector

Array

Fig. 2. Schematic of the Rosetta DFA instrument. A single channel laboratory breadboard test facility is shown in Figure 4 and results in Figure 5.

EXPERIMENTAL RESULTS

Fig. 3. Proposed DFA System Measurement.

Architecture for

Experimental work at breadboarding level commenced with a HeNe laser based system with a mechanical chopper but has now developed to a flight level concept proving stage using a laser diode based system with electronic chopping of the source (Fig 4). This arrangement allowed particles of diameter down to

(12)140 M.R. Leese etal.

100~tm to be dropped through the scattering volume and their scattered signal to be detected. Results have so far proven the detection of glass balotini (small solid glass spheres) in the range above 100 I.tm at low particle velocities (1-3 ms -1) using a simple photodiode detector with a current amplifier. (See Figure 5 for results with a 250 ~tm balotini at 3ms-l). The limitation at present is electrical noise (the dark current from the photodiode after amplification). This sensitivity has been achieved with diodes of only 3roW output power compared with the proposed >300mW for flight diodes. Scaling the power leads to a sensitivity down to 10grn diameter for a non-absorbing dielectric (e.g. a silicate grain).

SCOPE

LAser Diode [ / Driv~ @2.5kHz Synch

_ _ _

3roW @ 670nm Beam Expmder

~ e-Amp V= 10E8 x I

Photodiode Trigger

C ~ t o scope

--'~ /-'-.-.-. Panicle dropper \ I../ and ~igg~

I ~ Scattering 45de i //Particle

i Scattering Slit 4xl0mm Volume

Fig. 4. DFA Laboratory Test Facility. Demonstrating the performance of one channel. The scattered light from a particle within the scattering volume is detected at 135 °. A number of similar channels provides position sensitive measurements.

C H ~ ~ . . L . ] ~1 I..~ k~ L.J ~ I L~ LASER DRIVE (at 2.Skl-lzl

• i iii i IO~V~v ~ £ 1 t e

0.5 ms/ally

Fig. 5. DFA Experimental Results. Scattered light from a grain passing through the light curtain at 3ms -1. The peak envelope amplitude yields the scattered light and its width the velocity.

FUTURE DEVELOPMENTS

Future development will include an increase in laser power by two orders of magnitude and the addition of a phase sensitive detector amplifier to achieve detection of particles as small as 10lxm and with lower albedos than the balotini. Methods for handling small particles (< 100~tm) and for increasing the velocity through the scattering volume (probably using compressed air) are under development. Some aspects of the particle size and velocity ranges may be achieved using an in-house light gas gun facility. A two stage system experiment is to be constructed shortly to demonstrate the practicality of velocity measurement.

Later development will include GRIN lenses focussing onto linear CCD (or photodiode) arrays, this requires higher laser power levels than are currently in use. Another feature which may be included in the DFA experiment is a type of deposition sensor on the external surface of DFA, this would measure the build up of particulates on the Orbiter. This could have an engineering function (protection of the solar arrays) in addition to its science interest. Early experimental work has started on deposition sensor concepts.

ACKNOWLEDGEMENTS

We wish to acknowledge the financial support of PPARC (UK) and the contributions of the DFA team to the manufacture and testing of the prototype DFA models.

REFERENCES

1. J.A.M.McDonnell, W.M.Alexander, D.G.Ashworth, D.W.Hughes, A Dust Detection System (DUDSY) for the Tempel II Rendezvous, submitted to NASA and ESA, (1980) 2. J.A.M.McDonnell, R.Beard, S.F.Green, G.H.Schwehm, Cometary coma particulate modelling for the ROSETI'A mission aphelion rendezvous, Ann. Geophys. 10, 150-156, (1992) 3. N. Divine, Numerical models for Halley dust environments, ESA SP 174, 25-30, (1981) 4. J.A.M.McDonneU et al, The Giotto Dust Impact Detection System, ESA SP 1077, 85-108, (1986) 5. R.D.Pechstedt and D.A.Jackson, Performance analysis of an optical fibre accelerometer based on a compliant cylinder design, submitted to Journal of Lightwave Technolo~', (1994)