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Context.An open question in the study of comets is the so-called cohesion bottleneck, that is, how dust particles detach from the nucleus.Aims.We test whether the CO pressure buildup inside the pebbles of which cometary nuclei consist can overcome this cohesion bottleneck.Methods.A recently developed pebble-diffusion model was applied here to comet C/2017K2 PANSTARRS, assuming a CO-driven activity.Results.(i) The CO-gas pressure inside the pebbles erodes the nucleus into the observed dust, which is composed of refractories, H2O ice and CO2ice. (ii) The CO-driven activity onset occurs up to heliocentric distances of 85 au, depending on the spin orientation of the comet nucleus. (iii) The activity onset observed at ≈26 au suggests a low obliquity of the nucleus spin axis with activity in a polar summer. (iv) At 14 au, the smallest size of the ejected dust is ≈0.1 mm, consistent with observations. (v) The observed dust-loss rate of ≈200 kg s−1implies a fallout ≥30%, a nucleus surface active area ≥10 km2, a CO-gas loss rate ≥10 kg s−1, and a dust-to-gas ratio ≤20. (vi) The CO-driven activity never stops if the average refractory-to-all-ices mass ratio in the nucleus is ≤4.5 for a nucleus all-ices-to-CO mass ratio ≈4, as observed in comets Hale–Bopp and Hyakutake. These results make comet C/2017K2 similar to the Rosetta target comet 67P/Churyumov–Gerasimenko. (vii) The erosion lifetime of cometary planetesimals is a factor 103shorter than the timescale of catastrophic collisions. This means that the comets we observe today cannot be products of catastrophic collisions.

International audience; The Rosetta encounter with comet 67P/Churyumov-Gerasimenko provides a unique opportunity for an in situ, up-close investigation of ion-neutral chemistry in the coma of a weakly outgassing comet far from the Sun.Observations of primary and secondary ions and modeling are used to investigate the role of ion-neutral chemistry within the thin coma.Methods. Observations from late October through mid-December 2014 show the continuous presence of the solar wind 30 km from the comet nucleus. These and other observations indicate that there is no contact surface and the solar wind has direct access to the nucleus. On several occasions during this time period, the Rosetta/ROSINA/Double Focusing Mass Spectrometer measured the low-energy ion composition in the coma. Organic volatiles and water group ions and their breakup products (masses 14 through 19), CO2+ (masses 28 and 44) another mass peaks (at masses 26, 27 , and possibly 30) were observed. Secondary ions include H3O+ and HCO+ (masses 19 and 29). These secondary ions indicate ion-neutral chemistry in the thin coma of the comet. A relatively simple model is constructed to account for the low H3O+/H2O+ and HCO+/CO+ ratios observed in a water dominated coma. Results from this simple model are compared with results from models that include a more detailed chemical reaction network.At low outgassing rates, predictions from the simple model agree with observations and with results from more complex models that include much more chemistry. At higher outgassing rates, the ion-neutral chemistry is still limited and high HCO+/CO+ ratios are predicted and observed. However, at higher outgassing rates, the model predicts high H3O+/H2O+ ratios and the observed ratios are often low. These low ratios may be the result of the highly heterogeneous nature of the coma, where CO and CO2 number densities can exceed that of water.

Aims. We study the distant dust environment of two Long Period Comets: C/2003 O1 (LINEAR) (observed at r h = 7.39 AU) and C/2004 K1 (Catalina) (observed at r h = 3.43 AU). The case of C/2003 O1 is particularly interesting since the comet has a quite large perihelion heliocentric distance (r h = 6.85 AU).Methods. We analysed R -band images taken at the CAHA 2.2 m telescope to characterise the properties of the cometary dust coma. The images of both comets were also used as input into an inverse numerical model, to derive information about the dynamical parameters of coma dust grains until a time ~500 days before the observation.Results. Both the comets appeared to be active, and of similar shape and size despite the great difference in their heliocentric distance of observation. C/2003 O1 exhibited a tail extending to at least 3.7 105 km. It is a very active object, since its Af ρ is 552 ± 36 cm within the inner 5”. C/2004 K1 showed a similar long tail, extending to 2.6 105 km. Its Af ρ is 539 ± 35 cm within the inner 5”. An almost constant dust grain ejection velocity between 0.5 and 0.9 m/s at 1 cm dust size has been derived for C/2003 O1 and an increase from 0.5 to 2 m/s at 1 cm dust size for C/2004 K1, reflecting the different volatile dragging the dust environment (probably CO for C/2003 O1 and water for C/2004 K1). Model results allow some speculations about the comet nucleus size of C/2003 O1 (LINEAR) and its CO content: for a Q CO similar in value to those observed for other distant cometary objects, a comet radius R n from 13 to 17 km can be inferred.

Based on millimeter-wavelength continuum observations we suggest that the recent “spectacle” of comet 17P/Holmes can be explained by a thick, air-tight dust cover and the effects of H2 O sublimation, which started when the comet arrived at the heliocentric distance ≤ 2.5 AU. The porous structure inside the nucleus provided enough surface for additional sublimation, which eventually led to the break up of the dust cover and to the observed outburst. The magnitude of the particle burst can be explained by the energy provided by insolation, stored in the dust cover and the nucleus within the months before the outburst: the subliming surface within the nucleus is more than one order of magnitude larger than the geometric surface of the nucleus – possibly an indication of the latter's porous structure. Another surprise is that the abundance ratios of several molecular species with respect to H2 O are variable. During this apparition, comet Holmes lost about 3% of its mass, corresponding to a “dirty ice” layer of 20 m.

In 2014 the ESA probe Rosetta is to rendezvous with short-period comet 67P/Churyumov-Gerasimenko, where it will be inserted into a stable bound orbit. Partly by measuring the Doppler displacements of the probe’s radio carrier waves, the gravity field of the comet’s nucleus will be found. In this work, navigation strategies are developed, and important error sources for and feasibility of the comet nucleus mass determination are presented.

Aims. The non-gravitational perturbations in the orbital motion of comet 81P/Wild 2 are investigated using a two-source model of outgassing. Methods. In the process of orbit improvement the nucleus orientation and the location of two active regions are found from numerical fitting of the non-gravitational acceleration model to positional observations of the comet. Two different approaches to the lag angle of the outgassing behind the subsolar meridian are considered, assuming this angle to be either constant or varying with heliocentric distance. Results. The derived spin axis orientation (I similar to 60 degrees, phi similar to 155 degrees) as well as the location of two active regions, the northern (beta = 84 degrees) and the southern one (beta = -42 degrees), agree with determinations based on different observations by other authors. Orbital linkages of two and three successive apparitions of 81P/Wild 2 indicate an evolution of the change in the orbital period, possible time variations of the spin axis of the comet and an increase of the source areas during the last five revolutions. The non-gravitational perturbations were also used to constrain the sizes of the two active areas as well as the mass and bulk density (similar to 400 kg m(-3)) of the comet nucleus. The modeled two-source water production curve is compared with the activity data represented by the observed water production curve and the brightness curve.

Comet McNaught C/2006 P1 was the brightest comet of the last forty years when reaching its perihelion at an heliocentric distance of 0.17 AU. Two days before this perihelion, at an heliocentric distance of 0.2 AU, Themis, a French-Italian solar telescope in the Canary Islands, Spain, observed the Comet sodium emission of McNaught. The measured maximum sodium brightness of the D2 emission line peaked at 900 Mega-Rayleigh. The spatial distribution of the sodium emission with respect to the nucleus of the comet is in agreement with previous observations. It displays a clear sunward-tailward asymmetry that suggests a dichotomy of the sodium sources between a source close to the nucleus and an extended source most probably corresponding to the dust tail. The spatial distribution along the slit of the width and speed of the Doppler Na distribution also suggests such a dichotomy. The sodium ejection rate inferred from this observation agrees with the value of the ejection rate extrapolated from comet Hale-Bopp, taking into account the heliocentric distance of comet McNaught and its significantly larger dust release. If we suppose a similar concentration of sodium atoms in both comets, this observation suggests that the sodium ejection rate from comets McNaught and Hale-Bopp is proportional to the solar flux. Therefore the most probable ejection mechanisms are photo-sputtering, solar wind sputtering, or cometary ion sputtering, and not thermal desorption.

Context. Icy grains in the inner coma of a comet may play an important role in the energy balance and in the production of certain gas coma species. Their existence has therefore been assumed repeatedly to explain a variety of observed phenomena. However, owing to their extremely short life time no evidence for the presence of icy grains had been found in any active comet close to the Sun. Aims. We observed Comet 9P/Tempel 1 during the Deep Impact mission to look for phenomena induced by the impact. Methods. The comet was observed with the XMM-Newton Observatory. We used the EPIC camera for X-ray imaging and the Optical Monitor for monitoring in the ultraviolet and visible spectral range. Results. An outburst of the comet nucleus was observed as a result of the impact and the evolution of the coma was monitored in gas and dust. Our observations led to the first detection of icy grains in a comet at 1.5 AU from the Sun. Conclusions. We showed for the first time that the material ejected from the nucleus of a comet contains icy grains, even at small heliocentric distance.

Rosetta, the ESA mission dedicated to the detailed study of a comet nucleus before and alter the onset of activity, will arrive at Comet 67P/Churyurnov-Gerasimenko in 2014. At the moment, little is known about this comet. but the successful design of the ROSETTA mission requires some preliminary knowledge of'the comet physical parameters. These quantities cannot be determined only through ground-based observations, so predictive models of the thermal evolution and differentiation of the cometary nucleus are necessary. We have developed a comet nucleus thermal evolution model that helps in the estimation of the above quantities. The results have been computed with a numerical code solving simultaneously the heat conduction and gas diffusion equations through an idealized spherical comet nucleus and we show the results obtained applying this model to a comet nucleus with the characteristics of Churyumov-Gerasimcnko. Our models indicate that the peculiar dynamical history has a strong effect on the comet: during the orbital stage at large heliocentric distance the comet was covered by a dust crust that is removed when the comet arrives at the present orbit. The computed gas fluxes are in general agreement with those estimated from the observations.

The Jacobi constant of a probe under the gravitational attraction of a rotating irregular body is rederived for excited, but free rigid rotation of the central mass. A related Tisserand-like quantity is found to be sufficiently conserved for it to qualify as a pseudo-integral. The quantity's near constancy is shown to imply that certain regions in the space of the probe's cometocentric orbital elements are forbidden. In particular, based solely on this analysis, it is seen how collision with the comet nucleus is best avoided if the initial probe velocity lies in the plane normal to the nucleus' rotational spin. A conclusion which is of relevance to the Rosetta mission where the lander Philae is to be delivered by Rosetta in a possibly close prograde, and therefore probably shape unstable, orbit. At the comet's heliocentric distance when the lander is delivered, radiation and radial outgassing pressure do not significantly affect these conclusions for a nominal target nucleus in a worst-case scenario. The latter effect is seen to somewhat decrease the impact risk of prograde trajectories, making the more stable retrograde orbits slightly less safe.

Broad–band optical and infrared observations in the R , I , and J , H , K s bands of comet McNaught–Hartley (C/1999 T1) were performed from Jan. 25 to Feb. 4, 2001, shortly after the comet perihelion on Dec. 14, 2000. For the time the comet was observed, it did not show any peculiar behaviour either in the infrared or in the optical, that is, no non–spherical structures, besides the dust tail, or outburst or daily activity variations were detected. Calibration of the images in the frame provide us with values of dust production between 1000 and 3000 cm, depending on the date and on the wavelength. The general behaviour of the dust coma of C/1999 T1 is that described by a steady–state production of long–lived grains expanding radially outwards from the nucleus. The dust color does not clearly change with projected cometocentric distance. During a time span of 10 days, the cometary dust grains observed in the K s filter seemed to become slightly redder on Feb. 05 than they were on Jan. 26, 2001.

The active comet C/1995 O1 (Hale-Bopp) was target of an optical long-term monitoring program carried out at the European Southern Observatory (ESO) (Rauer et al. 1997, 2003). Longslit spectra and images were obtained at heliocentric distances from 4.6 AU to 2.9 AU preperihelion and 2.8 AU to 12.8 AU postperihelion. Based on these data, the dust activity of comet Hale-Bopp is analysed. The color of the dust coma and the Afp parameter are determined. A model for the dust release from the cometary nucleus is presented and used to compute dust production rates. The dust to gas ratio is determined.

The nongravitational motion of six \erratic" short-period comets is studied on the basis of published astrometric observations. We present the precession models which successfully link all the observed apparitions of the comets: 16P/Brooks 2, 21P/Giacobini-Zinner, 31P/Schwassmann-Wachmann 2, 32P/Comas Sol a, 37P/Forbes and 43P/Wolf-Harrington. We used the Sekanina's forced precession model of the rotating cometary nucleus to include the nongravitational terms into equations of the comet's motion. Values of six basic parameters (four connected with the rotating comet nucleus and two describing the precession of spin-axis of the nucleus) have been determined along the orbital elements from positional observations of the comets. The solutions were derived with additional assumptions which introduce instantaneous changes of modulus of reactive force, and of maximum cometary activity with respect to perihelion time. The present precession models impose some constraints on sizes and rotational periods of cometary nuclei. According to our solutions the nucleus of 21P/Giacobini-Zinner with oblateness along the spin-axis of about 0.29 (equatorial to polar radius of 1.41) is the most oblate among six investigated comets.

The Philae lander of the Rosetta mission, aimed at the in situ investigation of comet 67P/C-G, was deployed to the surface of the comet nucleus on 12 Nov 2014 at 2.99 AU heliocentric distance. The Dust Impact Monitor (DIM) as part of the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME) on the lander employed piezoelectric detectors to detect the submillimetre- and millimetre-sized dust and ice particles emitted from the nucleus. We determine the upper limit of the ambient flux of particles in the measurement range of DIM based on the measurements performed with the instrument during Philae's descent to its nominal landing site Agilkia at distances of about 22 km, 18 km, and 5 km from the nucleus barycentre and at the final landing site Abydos. The geometric factor of the DIM sensor is calculated assuming an isotropic ambient flux of the submillimetre- and millimetre-sized particles. For the measurement intervals when no particles were detected the maximum true impact rate was calculated by assuming Poisson distribution of the impacts, and it was given as the detection limit at a 95% confidence level. Based on measurements performed with DIM, the upper limit of the flux of particles in the measurement range of the instrument was of the order of $10^{-8}-10^{-7}m^{-2}s^{-1}sr^{-1}$ during descent and $1.6\cdot10^{-9}m^{-2}s^{-1}sr^{-1}$ at Abydos on 13 and 14 Nov 2014. Considering particle speeds below escape velocity, the upper limit for the volume density of particles in the measurement range of DIM was constrained to $10^{-11}m^{-3}-10^{-12}m^{-3}$. Results of the calculations performed with the GIPSI tool on the expected particle fluxes during the descent of Philae were compatible with the non-detection of compact particles by the DIM instrument., Comment: 5 figures, 3 tables

We report the results of the long-term narrowband photometry and imaging monitoring of comet C/2012 F6 (Lemmon) with the robotic TRAPPIST telescope (La Silla Observatory). Observations covered 52 nights pre- and post-perihelion between December 11, 2012, and June 11, 2013 (perihelion: 24 March, 2013). We followed the evolution of the OH, NH, CN, C3 , and C2 production rates computed with the Haser model as well as the evolution of the A (θ )fρ parameter as a proxy for the dust production. All five gas species display similar slopes for the heliocentric dependence. An asymmetry about perihelion is observed, the rate of brightening being steeper than the rate of fading. The chemical composition of the comet’s coma changes slightly along the orbit: the relative abundance of C2 to CN increases with the heliocentric distance (r ) below −1.4 au and decreases with r beyond 1.4 au while the C3 -to-CN ratio is constant during our observations. The behavior of the dust is different from that of the gas, the slope of the heliocentric dependence becoming steeper in early February, correlated to a change in the visual lightcurve slope. However, the dust color does not vary during the observations. The application of several enhancement techniques on the images revealed structures in the CN, C3 , and C2 images. These features imply the existence of one or several active zone(s) on the comet nucleus. The shape of the structures is similar in these three filters and changes from a roughly hourglass shape in December and January to a corkscrew shape in February and March. The structures in the continuum filters (sampling the dust) are not correlated to those observed for the gas. During several full nights in February, we observed changes in the CN and C2 structures that repeated periodically because of the nucleus rotation, our derived rotational period being of 9.52 ± 0.05 h.

Context. Comets are believed to hold a relatively pristine record of the physical and chemical processes that occurred during the formation and evolution of the solar system. Thorough investigations of these small bodies, such as the one that will be performed by the ESA/Rosetta cornerstone mission, are thus supposed to bring strong and unique constraints on the origins of the solar system. Aims. Because comet 67P/Churyumov-Gerasimenko was only recently selected as the target for the ESA/Rosetta mission, there has been little opportunity to study its pre-perihelion activity. This phase is, however, very important for the mission, since the global mapping of the nucleus and the choice of landing site for Philae will be performed during this pre-perihelion phase. Here, we report previously unpublished data of the last pre-perihelion passage of this comet, observed between May and September 2008. Methods. The gas and dust activity of comet 67P/Churyumov-Gerasimenko are studied through visible spectroscopy and broadband imaging, respectively, covering a range of pre-perihelion heliocentric distances between 2.99 and 2.22 AU. Results. The data we have gathered on the dust activity are consistent with trends observed by other authors and show a strong asymmetry between the pre- and post-perihelion phases of the orbit. The spectra do not show any lines due to the emission of volatiles, and upper limits on their production rates are typically one order of magnitude lower than at the equivalent post-perihelion heliocentric distances. The asymmetry in the pre- and post-perihelion phases of the activity may be due to a dusty crust quenching the activity at the surface of 67P. We estimate that this crust could be about 12 cm thick, although not uniform across the surface. Even if no gas is individually detected, the coma surface brightness profiles might indicate a possible contamination from gaseous species emitted before the comet actually reaches perihelion. © 2014 ESO.

Comet 67P/Churyumov-Gerasimenko is the main target of ESA's Rosetta mission and will be encountered in May 2014. As the spacecraft shall be in orbit the comet nucleus before and after release of the lander {\it Philae}, it is necessary necessary to know the conditions in the coma. Study the dust environment, including the dust production rate and its variations along its preperihelion orbit. The comet was observed during its approach to the Sun on four epochs between early-June 2008 and mid-January 2009, over a large range of heliocentric distances that will be covered by the mission in 2014. An anomalous enhancement of the coma dust density was measured towards the comet nucleus. The scalelength of this enhancement increased with decreasing heliocentric distance of the comet. This is interpreted as a result of an unusually slow expansion of the dust coma. Assuming a spherical symmetric coma, the average amount of dust as well as its ejection velocity have been derived. The latter increases exponentially with decreasing heliocentric distance (\rh), ranging from about 1 m/s at 3 AU to about 25-35 m/s at 1.4 AU. Based on these results we describe the dust environment at those nucleocentric distances at which the spacecraft will presumably be in orbit. Astronomy and Astrophysics, in press, 5 pages, 4 figures