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Physical aspect of an “impact sensor” for the detection of cometary dust momentum onboard the “Rosetta” space mission
Pergamon www.elsevier.com/locate/asr
Adv. Space Res. Vol. 29, No. 8, pp. 1159-1163,2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/02 $22.00 + 0.00 PII: SO273-1177(02)00132-l
PHYSICAL ASPECT OF AN “IMPACT SENSOR” FOR THE DETECTION OF COMETARY DUST MOMENTUM ONBOARD THE “ROSETTA” SPACE MISSION F. Esposito’.2, L. Colangeli2, V. Della Carte’, P. Palumbo3, and the International
GIADA Team
‘Universit& degli Studi di Napoli “Federico II”, Space Science and Engineering Department, Piazzale Tecchio, Napoli, Italy 20sservatorio Astronomic0 di Capodimonte, Via Moiariello 16, 80131, Napoli, Italy “Istituto Universitario Navale, Via de Gasperi 5, 80133, Napoli, Italy
ABSTRACT The Impact Sensor (IS) is a subsystem of the GIADA experiment onboard the Rosetta space mission and is aimed at measuring the momentum of cometary grains in the range from 3.10-” to 3.10m5 N.s. This sensing device is formed by an aluminium plate equipped with five piezoelectric elements, which are mounted below each comer and its center. Tests have been performed on a laboratory model of the IS, fully representative of the flight unit, to demonstrate that the performances are compatible with the design specifications and to address the dependence of the output signal on some of the most relevant parameters of impacting grains, i.e. shape, composition, momentum and position of the impact. The results reported in the present paper summarize the capabilities of such an impact momentum detector. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
THE IMPACT SENSOR OF THE GIADA EXPERIMENT FOR THE ROSETTA MISSION The ESA Rosetta space mission is aimed at a rendezvous, lasting about one year, with comet 46P/Wirtanen to study nucleus and coma properties (Bar-Nun et al., 1993; Schwehm and Schulz, 1999). Comets are very interesting objects of the Solar System due to their connection to the primordial material forming the proto-solar nebula. Comet nuclei probably preserve some unaltered material that participated to the gravitational collapse of our nebula and still retain a substantial fraction of volatiles. A detailed in situ study of these objects can shed light on the origin of the solar system. Among the various measurements, the dynamical study of the dust ejected by the nucleus will be performed by the GIADA (Grain Impact Analyzer and Dust Accumulator) experiment (Bussoletti et al., 1999). The instrument includes different sensors. A Grain Detection System (GDS), which reveals the grain passage by optical detection (Mazzotta Epifani et al., 2000) and the Impact Sensor (IS), which measures its momentum, are placed in cascade to determine also velocity and mass of each grain entering the instrument. Moreover, five micro-balance sensors, pointing in different directions, monitor the cumulative dust flux in time (Palomba et al., 2000). The GIADA field of view angle of 40” and the Rosetta pointing direction towards the cometary nucleus will allow us to detect direct grains, i.e. grains coming from the nucleus and not reflected by the radiation pressure of the Sun (Bussoletti et al., 1999). The detection of such particles give important information on the coma evolution and the nucleus activity. The IS is formed by a square aluminium diaphragm (thickness of 0.5 mm) whose sensitive area is 100 mm x 100 mm. Five lead zirconate titanate ceramic PZT’s, with resonant frequency f = 200 kHz, are the sensors placed below the center and each comer of the plate. An impact event generates acoustical waves which propagate along the plate. When a wave reaches the sensor, this begins to vibrate at the resonant frequency and generates a voltage proportional to the incident grain momentum, that can be measured after calibration. The proportionality factor is (l+e), where e is the coefficient of restitution of the particle. The signal from each PZT is processed by a narrow
F. Esposito et al.
1160
bandwidth filter and amplified. A typical output signal is shown in Figure 1. The maximum wave (the first wave packet) is proportional to the normal component of the grain momentum.
Fig. 1. Typical output signal from a piezoelectric
peak of the principal
sensor after an impact event.
The momentum sensitive range required for application to comet environment is very wide. Theoretical models (Fulle et al., 1992; Fulle et al., 1999; Muller and Grtin, 1998) based also on the analysis of data from the DIDSY dust experiment onboard the GIOTTO mission to lP/Halley (Zamecki et al., 1986), provide the following reference limits for grains ejected from the nucleus: momentum from 1~10~‘~ to 2~10~~ Ns for the lower grain mass (lo“* kg) and from 1~10.~ to 5x10-’ Ns for the upper grain mass ( 10m6kg). The design of the IS must guarantee to cover the range from 3.10.” N.s (nominal detection limit) up to 3.10-’ N.s (saturation limit). In the present work we report the results of the tests and calibration performed on an IS laboratory model (Development Model, DM). This system has been assembled in the laboratory with technical characteristics based on the nominal instrument design to check if the actual performances of the IS are compatible with the scientific requirements and to demonstrate that, by calibration of the instrument, it is possible to deduce impact position and actual momentum of the impacting grains. The experimental work has been, then, oriented to study the dependence of the output signal on grain impact parameters, i.e. momentum and position, and on grain intrinsic properties, i.e. shape and composition. The calibration results are, in principle, applicable to derive dynamic properties of any impacting grain with velocity between -1 and 100 ms-‘. The positive results confirm that the Impact Sensor is a system suitable for momentum monitoring of dust in space environments, such as comets. The work has been performed in collaboration with OfSicine Galileo, the industrial partner for the GIADA experiment realization. WORKING PRINCIPLE Previous studies (Clapp, 1963; McDonnell, 1969) have shown that, after a grain impact on a thin diaphragm, multiple modes of wave propagation occur, but the transverse bending wave (or flexural wave) is dominant. The velocity associated with this mode is frequency dependent and so dispersive. The theoretical group velocity (for a free diaphragm) is (Landau and LifSits, 1995):
u
=
f
167r2E
J
o.shos4
3p(l-02)
(1)
where f = the wave frequency, h = diaphragm thickness, E = Young’s modulus, p = diaphragm density and u = Poisson’s ratio. In the case of our Impact Sensor, E = 6.7.10’0Nm2, p = 2750 kg.mm3, cr= 0.33, h = 0.5.10e3 m and f = 200 kHz is just the piezo resonant frequency. Thus, the theoretical group velocity is U = 1948 ms~’ for an unbound plate without any piezoelectric sensor. The experimental value for U is (1711f60) ms-i. This value has been determined by measuring the time delay between impact in a know position and detection of the signal at one of the piezoelectric sensors. From the experimental group velocity we derive a spatial resolution of the IS system of 8.5 mm, considering that the proximity electronics samples the piezo output signal at frequency f.
An Impact Sensor to Measure Dust Momentum Onboard Rosetta
1161
MEASUREMENTS ON IS RESPONSE Dependence on impact positions The IS diaphragm has been stimulated with a constant impulse to obtain a map of the IS response to a fixed momentum. This has been generated by using a metallic tip, free moving perpendicularly to the diaphragm and stimulated by a piezoelectric device piloted by the rising ramp of a 10 V square wave. In Figure 2 a map of the diaphragm at a step of 5 mm is plotted as viewed from the central (A) and a comer (C) piezoelectric sensors.
, ‘. i .’
,
Fig. 2. 3-D plots of the output of the central (a) and a comer (c) piezo-sensors for a constant stimulus (step 5 mm) on the plate. Panels b) and d) represent the plane projections of the surfaces plotted respectively in a) and c). The plots show that the intensity of the piezo A response decreases rather symmetrically moving away from the center, as expected. The response of the comer piezos decreases moving out of the diagonal relevant to the considered piezo. This behavior has to be accounted for when interpreting the output data generated by grain impacts. Dependence on particle shape and phvsical and chemical properties These tests are aimed at studying the sensitivity of the IS system to variations of structural/morphological properties of the impinging grains. The used particles are spherical and irregularly shaped silicate grains, which can be considered cometary “analogue” materials. For the irregular particles, the grain sizes have been calibrated by grinding and sieving bulk minerals. Spherical particles give an output about 5 times higher than the corresponding irregularly shaped particles. This difference could be due to the higher restitution coefficient e for spheres with respect to irregular particles. Moreover, tests have shown that grains of different materials, with density ranging from 0.12 gemm3 (porous glass) to -3 gem-” (silicates such as caolinite, nontronite, diopside, quartz and a synthetic porous glass; carbon and a siliceous rock: andesite) give rise to similar output signals within 10%.
F. Esposito et al.
1162
Dependence on momentum intensity The IS response vs. momentum intensity of grains impinging onto a fixed position follows a linear behavior in the analyzed range (Fig. 3). To obtain the linear parameter of the best fitting line we have considered only the results of the more realistic (in cometary environment) irregular particles, neglecting the spheres. Of course, since the IS response also depends on the impact position (Fig. 2), its determination is mandatory to apply the proper correction factors to the output signal, Vout. To this aim, the differences in arrival time of the acoustical wave produced by the impact for at least three different piezoelectric sensors must be measured in order to derive the impact position. Based on geometrical considerations, it is possible to demonstrate that the best detection conditions, which always bring to unambiguous solutions, are when the impact is detected by at least three sensors placed at the corners of the plate. An example of position reconstruction for grains of andesite, with similar mass, shot approximately in the same region of the diaphragm, is shown in Figure 4.
Fig. 3. Output signal from the central piezoelectric sensor for particle impacts of known momentum. Each point is obtained from a test of 100 drops of the same particle on the sensitive plate. The measurements are independent of the impact position as they have been corrected for the position calibration factors. The tests performed on the available IS system give a sensitivity limit of about 10.’ Ns, mainly due to electronic noise. The electronics design for the final Flight Model (FM) has been improved in order to achieve the required sensitivity. Tests on the IS electronics for the FM have shown a significant reduction of the noise.
om
200
4m
em
803
1om
1200
x (cm)
Fig. 4. Reconstruction 7.3.10-’ N.s.
of the impact position for andesite grains with diameter = (189f3)
pm and momentum
=
An Impact Sensor to Measure Dust Momentum Onboard Rosetta
1163
DISCUSSION AND CONCLUSIONS The results obtained on the development model of the Impact Sensor demonstrate the capability of this instrument to detect grain impacts on the sensing plate at a space resolution better than 1 cm and with momentum ranging from 10m9to 10” Ns. From the identification of the impact position, the signals detected by at least three of the five piezoelectric sensors, can be converted into the actual grain impact momentum, by using relative calibration maps, as those reported in Figure 2, and absolute calibration curves as that reported in Figure 3. The non-uniformity in the response varying the position of the impacts has been corrected by using calibration in position. As our experimental analysis demonstrates, the momentum derivation is not significantly affected by structure/morphology of impinging irregular grains, in a wide range of densities (from 0.12 gem” to -3 gcme3) and chemical composition (from crystalline silicate and rocks to amorphous glass). Unfortunately, electronic noise in the used model could not allow us to determine the actual lower sensitivity limit of the IS and only the range down to 10e9N.s could be explored. However, preliminary tests on the final version of the electronic board confirm that a better minimum detection limit can be actually achieved in the FM. This ensures us that the Impact Sensor of the GIADA experiment will be able, for the first time, to measure momentum of single grains ejected by the comet nucleus and to derive physical quantities related to the dynamical properties of cometary grains.
ACKNOWLEDGMENTS The work is supported under ASI, MURST and CNR contracts. Officine Galileo (Florence) are the Italian in the instrument industrial partner for the GIADA project and are warmly thanked for the collaboration realization. REFERENCES Bar-Nun A., Barucci A., Bussoletti E., Coradini A, Coradini M., et al., Rosetta Comet Rendezvous Mission, ESA SC1 (93) 7, 1993. Bussoletti E., Colangeli L., Lopez Moreno J.J., Epifani E., Mennella V., Palomba E., Palumbo P., Rotundi A., Vergara S., Girela F., Herranz M., Jeronimo J.M., Lopez-Jimenez A. C., Molina A., Moreno F., Olivares I., Rodrigo R., Rodriguez-Gomez J.F., Sanchez J., MC Donnel J.A.M., Leese M., Lamy P., Perruchot S., Crifo J.F., Fulle M., Perrin J.M., Angrilli F., Benini E., Casini L., Cherubini G., Coradini A., Giovane F., Gruen E., Gustafson B., Maag C., Weissman P.R.: The GIADA Experiment for Rosetta Mission to Comet 46PfWirtanen: Design and Performances, Adv. Space Res., 24, 1139, 1999. Clapp S. S., Air Force Cambridge Research Laboratories Report, AFCRL-64-98, 1963. Fulle M., Cremonese G., Jockers K., and Rauer H., The Dust Tail of Comet Liller 1988 V, Astron. & Astroph,, 253,615,1992. Fulle M., Crifo J.F., Rodionov A.V., Numerical Simulation of the Dust Flux on a Spacecraft in Orbit around an Aspherical Cometary Nucleus - I, Astron. & Astroph,, 347, 1009, 1999. Landau L. D., Lifiits E. M., Theory of elasticity (Theoretical Physics Vol. 7), 1995. Mazzotta Epifani E., Bussoletti E., Colangeli L., Palumbo P., Rotundi A., Vergara S., Perrin J. M., Lopez Moreno J. J., Olivares I., The Grain Detection System for the GIADA Instrument Design and Expected Performances, Adv. Space Res., submitted, 2000. McDonnell, J. A. M., Lamy, P.L., Pankiewicz G.S., Physical Properties of Cometary Dust, in Comets in the postHallev Era, Volume 2, Newbum, Jr., Neugebauer M., Rahe J., eds., Kluwer Academic Publishers,l043, 1991. McDonnell, J. A. M. Calibration studies on a piezoelectric sensing diaphragm for the detection of micrometeorites in space, Journal of Scientific Instruments (Journal of Physics E), Series 2 Volume 2, 1026, 1969. Muller M. and Grtin E., An Engineering Model of the Dust and Gas Environment of the lnner Coma of Comet 46/Pwirtanen, Part l&2, ESA-document RO-ESC-TA-5501,1998. Palomba E., Colangeli L., Palumbo P., Rotundi A., Perrin J.M., Bussoletti E., Performance of Micro-Balances for Dust Flux Measurement, Adv. Space Rex, submitted, 2000. Schwehm G., and Schulz R., ROSETTA Goes to Comet Wirtanen, in Comnosition and Origin of Cometary Materials, Space Science Series of ISSI, Altwegg K., Ehrenfreund P., Geiss J. And Huebner W. Eds., Kluwer Acad. Pub., Space Science Reviews, 90,3 13, 1999.
Adv. Space Res. Vol. 29, No. 8, pp. 1159-1163,2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/02 $22.00 + 0.00 PII: SO273-1177(02)00132-l
PHYSICAL ASPECT OF AN “IMPACT SENSOR” FOR THE DETECTION OF COMETARY DUST MOMENTUM ONBOARD THE “ROSETTA” SPACE MISSION F. Esposito’.2, L. Colangeli2, V. Della Carte’, P. Palumbo3, and the International
GIADA Team
‘Universit& degli Studi di Napoli “Federico II”, Space Science and Engineering Department, Piazzale Tecchio, Napoli, Italy 20sservatorio Astronomic0 di Capodimonte, Via Moiariello 16, 80131, Napoli, Italy “Istituto Universitario Navale, Via de Gasperi 5, 80133, Napoli, Italy
ABSTRACT The Impact Sensor (IS) is a subsystem of the GIADA experiment onboard the Rosetta space mission and is aimed at measuring the momentum of cometary grains in the range from 3.10-” to 3.10m5 N.s. This sensing device is formed by an aluminium plate equipped with five piezoelectric elements, which are mounted below each comer and its center. Tests have been performed on a laboratory model of the IS, fully representative of the flight unit, to demonstrate that the performances are compatible with the design specifications and to address the dependence of the output signal on some of the most relevant parameters of impacting grains, i.e. shape, composition, momentum and position of the impact. The results reported in the present paper summarize the capabilities of such an impact momentum detector. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
THE IMPACT SENSOR OF THE GIADA EXPERIMENT FOR THE ROSETTA MISSION The ESA Rosetta space mission is aimed at a rendezvous, lasting about one year, with comet 46P/Wirtanen to study nucleus and coma properties (Bar-Nun et al., 1993; Schwehm and Schulz, 1999). Comets are very interesting objects of the Solar System due to their connection to the primordial material forming the proto-solar nebula. Comet nuclei probably preserve some unaltered material that participated to the gravitational collapse of our nebula and still retain a substantial fraction of volatiles. A detailed in situ study of these objects can shed light on the origin of the solar system. Among the various measurements, the dynamical study of the dust ejected by the nucleus will be performed by the GIADA (Grain Impact Analyzer and Dust Accumulator) experiment (Bussoletti et al., 1999). The instrument includes different sensors. A Grain Detection System (GDS), which reveals the grain passage by optical detection (Mazzotta Epifani et al., 2000) and the Impact Sensor (IS), which measures its momentum, are placed in cascade to determine also velocity and mass of each grain entering the instrument. Moreover, five micro-balance sensors, pointing in different directions, monitor the cumulative dust flux in time (Palomba et al., 2000). The GIADA field of view angle of 40” and the Rosetta pointing direction towards the cometary nucleus will allow us to detect direct grains, i.e. grains coming from the nucleus and not reflected by the radiation pressure of the Sun (Bussoletti et al., 1999). The detection of such particles give important information on the coma evolution and the nucleus activity. The IS is formed by a square aluminium diaphragm (thickness of 0.5 mm) whose sensitive area is 100 mm x 100 mm. Five lead zirconate titanate ceramic PZT’s, with resonant frequency f = 200 kHz, are the sensors placed below the center and each comer of the plate. An impact event generates acoustical waves which propagate along the plate. When a wave reaches the sensor, this begins to vibrate at the resonant frequency and generates a voltage proportional to the incident grain momentum, that can be measured after calibration. The proportionality factor is (l+e), where e is the coefficient of restitution of the particle. The signal from each PZT is processed by a narrow
F. Esposito et al.
1160
bandwidth filter and amplified. A typical output signal is shown in Figure 1. The maximum wave (the first wave packet) is proportional to the normal component of the grain momentum.
Fig. 1. Typical output signal from a piezoelectric
peak of the principal
sensor after an impact event.
The momentum sensitive range required for application to comet environment is very wide. Theoretical models (Fulle et al., 1992; Fulle et al., 1999; Muller and Grtin, 1998) based also on the analysis of data from the DIDSY dust experiment onboard the GIOTTO mission to lP/Halley (Zamecki et al., 1986), provide the following reference limits for grains ejected from the nucleus: momentum from 1~10~‘~ to 2~10~~ Ns for the lower grain mass (lo“* kg) and from 1~10.~ to 5x10-’ Ns for the upper grain mass ( 10m6kg). The design of the IS must guarantee to cover the range from 3.10.” N.s (nominal detection limit) up to 3.10-’ N.s (saturation limit). In the present work we report the results of the tests and calibration performed on an IS laboratory model (Development Model, DM). This system has been assembled in the laboratory with technical characteristics based on the nominal instrument design to check if the actual performances of the IS are compatible with the scientific requirements and to demonstrate that, by calibration of the instrument, it is possible to deduce impact position and actual momentum of the impacting grains. The experimental work has been, then, oriented to study the dependence of the output signal on grain impact parameters, i.e. momentum and position, and on grain intrinsic properties, i.e. shape and composition. The calibration results are, in principle, applicable to derive dynamic properties of any impacting grain with velocity between -1 and 100 ms-‘. The positive results confirm that the Impact Sensor is a system suitable for momentum monitoring of dust in space environments, such as comets. The work has been performed in collaboration with OfSicine Galileo, the industrial partner for the GIADA experiment realization. WORKING PRINCIPLE Previous studies (Clapp, 1963; McDonnell, 1969) have shown that, after a grain impact on a thin diaphragm, multiple modes of wave propagation occur, but the transverse bending wave (or flexural wave) is dominant. The velocity associated with this mode is frequency dependent and so dispersive. The theoretical group velocity (for a free diaphragm) is (Landau and LifSits, 1995):
u
=
f
167r2E
J
o.shos4
3p(l-02)
(1)
where f = the wave frequency, h = diaphragm thickness, E = Young’s modulus, p = diaphragm density and u = Poisson’s ratio. In the case of our Impact Sensor, E = 6.7.10’0Nm2, p = 2750 kg.mm3, cr= 0.33, h = 0.5.10e3 m and f = 200 kHz is just the piezo resonant frequency. Thus, the theoretical group velocity is U = 1948 ms~’ for an unbound plate without any piezoelectric sensor. The experimental value for U is (1711f60) ms-i. This value has been determined by measuring the time delay between impact in a know position and detection of the signal at one of the piezoelectric sensors. From the experimental group velocity we derive a spatial resolution of the IS system of 8.5 mm, considering that the proximity electronics samples the piezo output signal at frequency f.
An Impact Sensor to Measure Dust Momentum Onboard Rosetta
1161
MEASUREMENTS ON IS RESPONSE Dependence on impact positions The IS diaphragm has been stimulated with a constant impulse to obtain a map of the IS response to a fixed momentum. This has been generated by using a metallic tip, free moving perpendicularly to the diaphragm and stimulated by a piezoelectric device piloted by the rising ramp of a 10 V square wave. In Figure 2 a map of the diaphragm at a step of 5 mm is plotted as viewed from the central (A) and a comer (C) piezoelectric sensors.
, ‘. i .’
,
Fig. 2. 3-D plots of the output of the central (a) and a comer (c) piezo-sensors for a constant stimulus (step 5 mm) on the plate. Panels b) and d) represent the plane projections of the surfaces plotted respectively in a) and c). The plots show that the intensity of the piezo A response decreases rather symmetrically moving away from the center, as expected. The response of the comer piezos decreases moving out of the diagonal relevant to the considered piezo. This behavior has to be accounted for when interpreting the output data generated by grain impacts. Dependence on particle shape and phvsical and chemical properties These tests are aimed at studying the sensitivity of the IS system to variations of structural/morphological properties of the impinging grains. The used particles are spherical and irregularly shaped silicate grains, which can be considered cometary “analogue” materials. For the irregular particles, the grain sizes have been calibrated by grinding and sieving bulk minerals. Spherical particles give an output about 5 times higher than the corresponding irregularly shaped particles. This difference could be due to the higher restitution coefficient e for spheres with respect to irregular particles. Moreover, tests have shown that grains of different materials, with density ranging from 0.12 gemm3 (porous glass) to -3 gem-” (silicates such as caolinite, nontronite, diopside, quartz and a synthetic porous glass; carbon and a siliceous rock: andesite) give rise to similar output signals within 10%.
F. Esposito et al.
1162
Dependence on momentum intensity The IS response vs. momentum intensity of grains impinging onto a fixed position follows a linear behavior in the analyzed range (Fig. 3). To obtain the linear parameter of the best fitting line we have considered only the results of the more realistic (in cometary environment) irregular particles, neglecting the spheres. Of course, since the IS response also depends on the impact position (Fig. 2), its determination is mandatory to apply the proper correction factors to the output signal, Vout. To this aim, the differences in arrival time of the acoustical wave produced by the impact for at least three different piezoelectric sensors must be measured in order to derive the impact position. Based on geometrical considerations, it is possible to demonstrate that the best detection conditions, which always bring to unambiguous solutions, are when the impact is detected by at least three sensors placed at the corners of the plate. An example of position reconstruction for grains of andesite, with similar mass, shot approximately in the same region of the diaphragm, is shown in Figure 4.
Fig. 3. Output signal from the central piezoelectric sensor for particle impacts of known momentum. Each point is obtained from a test of 100 drops of the same particle on the sensitive plate. The measurements are independent of the impact position as they have been corrected for the position calibration factors. The tests performed on the available IS system give a sensitivity limit of about 10.’ Ns, mainly due to electronic noise. The electronics design for the final Flight Model (FM) has been improved in order to achieve the required sensitivity. Tests on the IS electronics for the FM have shown a significant reduction of the noise.
om
200
4m
em
803
1om
1200
x (cm)
Fig. 4. Reconstruction 7.3.10-’ N.s.
of the impact position for andesite grains with diameter = (189f3)
pm and momentum
=
An Impact Sensor to Measure Dust Momentum Onboard Rosetta
1163
DISCUSSION AND CONCLUSIONS The results obtained on the development model of the Impact Sensor demonstrate the capability of this instrument to detect grain impacts on the sensing plate at a space resolution better than 1 cm and with momentum ranging from 10m9to 10” Ns. From the identification of the impact position, the signals detected by at least three of the five piezoelectric sensors, can be converted into the actual grain impact momentum, by using relative calibration maps, as those reported in Figure 2, and absolute calibration curves as that reported in Figure 3. The non-uniformity in the response varying the position of the impacts has been corrected by using calibration in position. As our experimental analysis demonstrates, the momentum derivation is not significantly affected by structure/morphology of impinging irregular grains, in a wide range of densities (from 0.12 gem” to -3 gcme3) and chemical composition (from crystalline silicate and rocks to amorphous glass). Unfortunately, electronic noise in the used model could not allow us to determine the actual lower sensitivity limit of the IS and only the range down to 10e9N.s could be explored. However, preliminary tests on the final version of the electronic board confirm that a better minimum detection limit can be actually achieved in the FM. This ensures us that the Impact Sensor of the GIADA experiment will be able, for the first time, to measure momentum of single grains ejected by the comet nucleus and to derive physical quantities related to the dynamical properties of cometary grains.
ACKNOWLEDGMENTS The work is supported under ASI, MURST and CNR contracts. Officine Galileo (Florence) are the Italian in the instrument industrial partner for the GIADA project and are warmly thanked for the collaboration realization. REFERENCES Bar-Nun A., Barucci A., Bussoletti E., Coradini A, Coradini M., et al., Rosetta Comet Rendezvous Mission, ESA SC1 (93) 7, 1993. Bussoletti E., Colangeli L., Lopez Moreno J.J., Epifani E., Mennella V., Palomba E., Palumbo P., Rotundi A., Vergara S., Girela F., Herranz M., Jeronimo J.M., Lopez-Jimenez A. C., Molina A., Moreno F., Olivares I., Rodrigo R., Rodriguez-Gomez J.F., Sanchez J., MC Donnel J.A.M., Leese M., Lamy P., Perruchot S., Crifo J.F., Fulle M., Perrin J.M., Angrilli F., Benini E., Casini L., Cherubini G., Coradini A., Giovane F., Gruen E., Gustafson B., Maag C., Weissman P.R.: The GIADA Experiment for Rosetta Mission to Comet 46PfWirtanen: Design and Performances, Adv. Space Res., 24, 1139, 1999. Clapp S. S., Air Force Cambridge Research Laboratories Report, AFCRL-64-98, 1963. Fulle M., Cremonese G., Jockers K., and Rauer H., The Dust Tail of Comet Liller 1988 V, Astron. & Astroph,, 253,615,1992. Fulle M., Crifo J.F., Rodionov A.V., Numerical Simulation of the Dust Flux on a Spacecraft in Orbit around an Aspherical Cometary Nucleus - I, Astron. & Astroph,, 347, 1009, 1999. Landau L. D., Lifiits E. M., Theory of elasticity (Theoretical Physics Vol. 7), 1995. Mazzotta Epifani E., Bussoletti E., Colangeli L., Palumbo P., Rotundi A., Vergara S., Perrin J. M., Lopez Moreno J. J., Olivares I., The Grain Detection System for the GIADA Instrument Design and Expected Performances, Adv. Space Res., submitted, 2000. McDonnell, J. A. M., Lamy, P.L., Pankiewicz G.S., Physical Properties of Cometary Dust, in Comets in the postHallev Era, Volume 2, Newbum, Jr., Neugebauer M., Rahe J., eds., Kluwer Academic Publishers,l043, 1991. McDonnell, J. A. M. Calibration studies on a piezoelectric sensing diaphragm for the detection of micrometeorites in space, Journal of Scientific Instruments (Journal of Physics E), Series 2 Volume 2, 1026, 1969. Muller M. and Grtin E., An Engineering Model of the Dust and Gas Environment of the lnner Coma of Comet 46/Pwirtanen, Part l&2, ESA-document RO-ESC-TA-5501,1998. Palomba E., Colangeli L., Palumbo P., Rotundi A., Perrin J.M., Bussoletti E., Performance of Micro-Balances for Dust Flux Measurement, Adv. Space Rex, submitted, 2000. Schwehm G., and Schulz R., ROSETTA Goes to Comet Wirtanen, in Comnosition and Origin of Cometary Materials, Space Science Series of ISSI, Altwegg K., Ehrenfreund P., Geiss J. And Huebner W. Eds., Kluwer Acad. Pub., Space Science Reviews, 90,3 13, 1999.