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

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 beta, 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 beta-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.