Wirtanen: The dust distribution out of 20 nucleus radii

Adv. Space Res. Vol. 23, No. 7, pp.1329-1332, 1999 0 1999 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177199 $20.00 + 0.00 PII: SO273-1177(99)00045-9

Pergamon www.elsevier.oMocate/asr

COMET

46P/WIRTANEN:

THE

DUST

DISTRIBUTION

OUT

OF

20 NUCLEUS

RADII

M. Fulle

Osservntorio

Astronomic0

di Trieste,

Via Tiepolo 11, I-3~131

Trieste Italy

ABSTRACT

Detailed

information

of the dust, environment

around comet nuclei is fundamental

to short period comets.

All these missions

future space missions

will be strongly

to properly influenced

encountered dust environment which will determine the mission safety, the pollution of and lander experiments, the good sampling of collecting dust experiments. Although isotropic models can offer estimates of the averaged dust fluxes and fluences on the only realistic 3D models can provide reliable estimates of the variations of these dust

plan

by the

the orbiter first order spacecraft, quantities

due to the strong anisotropies of the inner comae. Within the Dust Environment Working Group established by ESA to support the Rosetta mission to comet 46P/Wirtanen, detailed 3D models of the expected dust flux are being performed starting from the outputs of 3D hydrodynamical codes able to link the surface to t,he nucleus surface. compute grain.

We review

the results

covering

of the comet nucleus to the dusty gas environment

These 3D dust flux outputs

the dust trajectories

probe orbits

properties influenced obtained

are used as input, for 3D numerical

by the solar radiation so far for selected

a wide range of nucleocentric

close

codes which

pressure acting on each sample dust

nucleus

shapes

and for several

possible

distances. 01999 COSPAR. Published by Elsevier Science Ltd.

INTRODUCTION

The target of the ESA Rosetta mission is one of the faintest short period comets, 46P/Wirtanen. Available observations, although currently increasing, are rather few, mainly regarding magnitude estimates

of its coma,

which has never shown morphological

information regarding the dust released quantity (A’Hearn and Schleicher 1984).

structures.

The

coma

magnitude

by this comet is often expressed in terms of the Afp Although this quantity is often named dust loss rate, it

is a complex function of the dust loss rate, size distribution and velocity, so that it cannot provide non-ambiguous information on the dust activity: on the other hand, it is a first quality constraint to dust environment models, which must be consistent with it. Observations of the comet close to its aphelion, when the comet is supposed to be negligibly active, have allowed to infer the effective radius of the nucleus, which should be lower than one km when we assume the typical albedo of four percent. The nucleus light curve suggests a nucleus spin period close to six hours. It is apparent that these observational constraints are consistent with a spheroid, whose dimensions can be set in order to fit available magnitude estimates. 1329

1330

M.Fulle

On the other hand, nobody can really think that, the 46P/Wirtanen’s nucleus is so simple. This problem is fundamental when we model the dust environment close to the nucleus surface, a fundamental t,ask to properly plan the Rosetta mission. The observed coma magnitudes, coupled with the inferred nucleus dimensions, suggest that most of the comet surface must be active at perihelion. In the ~~IIIII~~IIsimplistic: approach linking every coma structure to active spots on the nucleus surface: the uniform activity of the nucleus surface direct,ly explains the structureless coma of 46P/Wirt,imen. However, this approach is extremely dangerous, if we conclude that the target (:oma should br isotropic. Crifo and Rodionov (1997) have shown tha,t every dusty gas environrnent is extremely sensitive to the concavities of’ the nucleus surface, which for a fully active surface are as probable sources of the so called coma jets as actfive spots. Since it is impossible that the target nucleus has no concavities. we must wait, for anisotropies in the target coma. The fact that no st,ructure is observed in the $O;P/Wirtanen coma can be easily explained by its faintness, which cloes not allow t.o extract from it tho same structures usually observed in bright comets only. This c:onclusion is supported by recent fits of IS0 images of the 4GP/Wirtanen coma (Colangeli et al. 1998), which are able to disentangle from available observations the dust loss rate, size distribution and e,jection velocit,y, and suggest, anisotropic dust e,jection from the nucleus surface.

We can c~onc:l~~lethat, dust. flux estimates on the Rosetta spacec:raR based on isotropic dust, models consistent, with available Afp observations arc first, order ones only: they must be refined by anisotropic 3D flux models. the only ones able to estimate the uncertainties affecting the isotropic However, due to t,he poor available information provided by observations, this task estimates. is enormous, because it, is strongly dependent, on the nucleus shape: we have no observational constraint, on it (we only know that, it is not, a sphere). We need to consider many possible shapes to infer common behavioms and the possible variations. This job was started, by considering the simplest nucleus shape able to produce shock structures in the associated coma: the bean shape. The obtained results can be compared to the isotropic ones (Miiller and Griin 1997), and t,o another heuristic anisotropic model (Fulle rt al. 1995) able to fit t,he results of the DIDSYGIOTTO experiment (McDonnell rot nl. 1991) regarding the dust, fluence around Comet Halley.

THE MODEL

Among its many outputs, the 3D hydrodynamical code developed by Crifo and Rodionov (1997) provides the dust space density and vect.orial velocity of dust, grains (sampled in 23 mass bins bet,ween 10-ls aud lo-” kg) at, t,he t,erminal surface (sampled in 7200 bins), where the dust drag This output. bccaomes the input, of a dynamical model of the dust by gas becomes negligible. t,r;ijcctoritts which: out, of the gas drag region, are sensitive to the solar radiat,ion pressure force. The dust, size distribution is assumed to be a The nucleus rotation is not t,aken into accomlt. power law with index -3.5; both assumptions are consistent both with the best available fits of the DIDSY-GIOTTO results (Fulle et al. 1995) iuld with the information on the size distribution provided by IS0 observations (Colangeli rt al. 1998). The model computes the differential fluence on the space probe; which allows us to compute several observables: here we report the results regarding t,he time dependent dust, flux (m-’ s-l) integrated over all the dust masses, the mass dependent dust fiuence (m-2) integrated over a fixed time interval, and the total fluence (m-2) int,egrated over all the dust masses and a fixed t&e interval.

Comet 46P/Wirtanen Dust Distribution

1331

All these quantities, when we consider an isotropic model, are independent of the probe orbit orientation: t,hey are simply scaled as the inverse squared nucleocentric probe distance, On the contrary, when we consider an anisotropic model, the probe orbit orientation becomes fundamental in order to evaluate the possible variations of these quantities: when we change the probe orbit orientation, we obtain variations by orders of magnitude larger than those due to changes of the nucleocentric distance. We take into account three possible probe orbit shapes: circular, elliptical and petal-like orbits. The latter have the big disadvantage to be largely fuel consuming, but allow the probe to reach the same smallest and largest nucleus distance in times much shorter than the equivalent elliptical orbits. Since we are dealing with dust. velocities which are not perfectly radial, we must wait for dust, flux 011 all the probe surfaces. Therefore we consider the dust flux on all the six probe surfaces. We recall that the z probe axis points to the nucleus, the T/ probe axis is the solar panel axis (which is always perpendicular to t,he su11 direction), and the z probe axis is given by the vectorial product between the z and ?/ axes.

RESULTS

Since the fluence is integrated over a fixed time interval longer than the probe orbit around the comet, nucleus, this is the quantity showing the lowest differences with respect to the isotropic model. It is possible to study the dependence of the total fluence versus the three Euler’s angles characterizing every orbit. When we average all these total fluences taking into account the weight given by the sin of the orbit inclination, we obtain values in perfect agreement (within much less than one percent) wit#h the correspondent isotropic total fluences. This fact ensures that the 3D model normalizations are correct. However, the tot,al fluence dispersion around this average reaches a factor 5. We must conclude that, the total fluences provided by isotropic models are uncertain wit,hin a factor 5 at least, simply due to the coma anisotropicities. The variations of the dust flux along each probe orbit are by far much larger, since the dust flux remembers all the encountered coma anisotropicities. Taking into account the simplest case (a circular probe orbit), which provides a perfectly constant isotropic dust flux, we obtain a flux increase of a factor 10 when the probe crosses the coma shock, and a increase/decrease of orders of magnitude when the probe crosses the nucleus terminator. These results point out that isotropic models cannot be adopted to predict the dust flux.

Regarding the flux of reflected grains versus the direct one, Fulle et al. (1995), who considered a much more anisotropic coma than Crifo and Rodionov (1997), obtained a ratio of reflected versus direct flux ranging between 10P4 and 10P3, while present 3D hydrodynamical codes imply a ratio ranging between 10e7 and lo- 5. Thus this ratio seems to be a good indicator of the coma anisotropicity, although effects due to the dust velocity dispersion remain to be investigated. The only way to collect surely reflected grains is to point, a collecting dust experiment towards the z axis, because the antinucleus direction is forbidden (due to engineering constraints), while the y direct,ion will be always protected against reflectfed grains for small acceptance angles. Thanks to the strong coma anisotropicity they assumed, Fulle et al. (1995) obtained that, for some probe orbit orientation, the flux of reflected grains is larger than that of direct ones. In the dust environment computed by Crifo and Rodionov (1997) this never happens, due to the fact that the adopted nucleus ejects dust uniformly over all its surface.

1332

M. Fulle

However, the dust flux on the probe surfaces not facing to the nucleus is not dominated by reflected grains. This fact is due to the non radial ejection velocity of the dust in the 3D hydrodynamical code, due to the shock structures associated to the nucleus concavity. The flux of direct grains on the II: and r/ surfaces is expected to be at least one percent of the flux on the nucleus faced probe surface. This flux is ten times higher than t,he highest expected flux of reflected grains, and implies heavy pollution to surfaces that should remain completely dust free (e.g. radiators or optical surfaces). This result further points out the importance of SD hydrodynamical codes, since usual isotropic models (or heuristic improvements of them) are unable to evaluate a possible non radiality of the dust ejection from the circumnuclear coma. In the future, additional nuclei models will be used, also considering the nucleus rotation.

The mass dependent fluences result completely independent of the adopted probe orbit. The fluence of direct grains well reproduces t,he source size distribution, t,hus avoiding the dynamical bias which probably heavily affected the DIDSY GIOTTO experiment (Fulle et ul. 1995). On the contrary, these dynamical artifacts are common on the fluences collected on the other probe surfaces. but, it is planned to measure the mass dependent fluence on the nucleus faced surface only. In particular. the fluence of reflectfed grains results steeper than that, of direct grains, so that this comparison, when performed, would provide us further information on the anisotropicity of the coma the probe is crossing. Such a comparison is made unreachable by undersampling problems: for dust, collecting periods of a month, a square meter surface will collect more than a reflected grain for masses lower than lo-’ kg at, perihelion. On the contrary, no undersampling problems are expected for direct grains: a 100 cm2 surface will collect on the same period at least a direct grain for each mass lower t,han lo-’ kg.

REFERENCES

A’Hearn M.F., Schleicher D.G.! Comet, Bowel1 l%Ob, A&-on.. .I. 89, 579 (1984) Colangeli L., Bussoletti E., Cecchi Pestellini C., Fulle M., Mennella V., Palumbo P., Rotundi A., ISOCAM imaging of comets 65P/Gunn and 46P/Wirtanen, Icclrzts in press (1998) Crifo .J.F., R.odionov A.V., The dependence of the Circumnuclear Coma Structure on the Properties of the Nucleus. II. First, Investigat,ion of the Coma Surrounding an Homogeneous, Aspher&l Nucleus, Icnr~s 129, 72 (1997) Fulle M., Colangeli L., Mennella V., Rotundi A., Bussoletti E., The sensitivity of the size distribution to the grain dynamics: simulation of the dust. flux measured by GIOTTO at P/Halley, Astron. Astrophys. 304, 622 (1995) McDonnell J.A.M., Lamy P.L., Pankiewicz G.S., Physical properties of cometary dust, in Comets in the Pas--Halley Era (eds. Newburn R.L.Jr., Neugebauer M., Rahe J.), 1047, Kluwer, Dordrecht (1991) Miiller M., Griin E., An engineering model of the dust and gas environment of the inner coma of comet 46P/Wirtanen, ESA-RO-ESC-TA-5501, 1 (1997)