Comet 67P/Churyumov-Gerasimenko: the GIADA dust environment model of the Rosetta mission target

¹ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
e-mail: fulle@oats.inaf.it
² ESA – ESTEC, Keplerlaan 1, 2201AZ Noordwijk, The Netherlands
³ INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
⁴ Università Parthenope, Dip. Scienze Applicate, Centro Direzionale Isola C4, 80143 Napoli, Italy
⁵ Max-Planck-Institut fuer Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
⁶ Laboratory for Atmospheric and Space Physics, University of Colorado, 1234 Innovation Dr, Boulder, CO, 80303-7814, USA
⁷ Seoul National University, Dept. of Physics and Astronomy, Seoul 151-742, Korea
⁸ CARA – Unione Astrofili Italiani, UAI
⁹ Osservatorio di Talmassons, CAST, Italy
¹⁰ Instituto de Astrofísica de Andalucía, CSIC, PO Box, 3004 Granada, Spain

Received: 4 May 2010
Accepted: 2 July 2010

Abstract

Context. The ESA Rosetta spacecraft will reach the short-period comet 67P/Churyumov-Gerasimenko in 2014. Orbiting strategy, orbiter safety conditions, landing scenarios and expected results from dust collectors depend on models of the 67P dust environment. Many papers already tackled this matter, analysing a limited set of observations, and therefore often reaching conflicting conclusions.

Aims. We consider a set of observations representative of all ground-based and IR (thermal infrared) Spitzer data collected over the last three perihelion passages, to determine the 67P dust environment after the end of the gas drag on dust (at about 20 nucleus radii) consistent with available 67P gas and dust coma photometry, images of the dust coma, tail and trail, at optical and IR wavelengths.

Methods. In order to obtain the best fit to 67P data, we consider three independent tail and trail simulation codes (developed by three independent groups), which parametrise cometary dust by the quantity β, the ratio between solar radiation pressure and gravity forces. GIADA, the dust monitor instrument of the Rosetta orbiter, will provide an experimental determination of the β-dust mass relation.

Results. A 67P environment model based on a perihelion-symmetric dust velocity and on a perihelion-asymmetric dust size distribution, is consistent with all available data. During most Rosetta operations, the dust cross-section is dominated by mm to cm-sized grains, while the ejected dust mass is dominated by grains larger than a few mm, with a dust-to-gas ratio of 3 around perihelion.

Conclusions. 67P onsets its activity at Sun-distances r_h ≥ 3.4 AU; the dust geometric albedo is 0.04 ± 0.02; at 3.0 AU, 10 g grains escape the nucleus gravity field (10 kg grains at perihelion) with a dust mass-loss rate of 10−40 kg s^-1 (500 kg s^-1 at perihelion); 67P’s activity depends on seasons, with the northern heminucleus (rich in large grains and CN depleted) active before perihelion.