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In this work, we aim to characterize the dust motion in the inner coma of comet 67P/Churyumov–Gerasimenko to provide constraints for theoretical 3D coma models. The OSIRIS camera on-board the Rosetta mission was able for the first time to acquire images of single dust particles from inside the cometary coma, very close to the nucleus. We analyse a large number of particles, performing a significant statistic of their behaviour during the post-perihelion period, when the spacecraft covered distances from the nucleus ranging between 80 and 400 km. We describe the particle trajectories, investigating their orientation and finding highly radial motion with respect to the nucleus. Then, from the particle brightness profiles, we derive a particle rotational frequency of ν < 3.6 Hz, revealing that they are slow rotators and do not undergo fragmentation. We use scattering models to compare the observed spectral radiance of the particles with the simulated ones in order to estimate their size, finding values that range from millimetres up to centimetres. The statistics performed in this paper provide useful parameters to constrain the cometary coma dynamical models. [ABSTRACT FROM AUTHOR]

We monitor the seasonal erosion and accretion of dust deposits in the Imhotep, Hatmehit, and Ma'at regions of comet 67P/Churyumov–Gerasimenko with OSIRIS Narrow Angle Camera images. The vertical accuracy of such measurements is 0.2 m and the spatial scale of the images we used is lower than 0.60 m pixel−1. We calculate the height of 21 boulders by applying a tool that allows to measure the shadow length of a boulder projected on the surrounding dust deposit, assuming that any height variation is not due to boulder intrinsic change. Any boulder height variation provides a direct measurement about the thickness variation of the surrounding dust layer due to the occurring erosion and/or accretion. The analysis concerns the period from 2014 August, inbound to perihelion, to 2016 September, outbound. We measured the erosion in the Ma'at region of 0.6 ± 0.2 m from 2014 September 12 to December 2, and an erosion of 0.4 ± 0.3 m from 2014 December 3 to 2015 February 15. Then, we measured a dust deposition of 0.7 ± 0.3 m during the following perihelion phase, until 2016 May–September. This result confirms the link between the erosion of the Southern hemisphere and the fallout in the northern regions. The Imhotep and Hatmehit regions are characterized by a negligible erosion during the inbound orbit, consistent with pebble-made nucleus models predicting no erosion when the temperature of the nucleus surface is T s < 205 K. [ABSTRACT FROM AUTHOR]

BepiColombo is the fifth cornerstone mission of the European Space Agency (ESA) dedicated to study the Mercury planet. The BepiColombo spacecraft comprises two science modules: the Mercury Planetary Orbiter (MPO) realized by ESA and the Mercury Magnetospheric Orbiter provided by the Japan Aerospace Exploration Agency. The MPO is composed by 11 instruments, including the 'Spectrometer and Imagers for MPO BepiColombo Integrated Observatory System' (SIMBIOSYS). The SIMBIOSYS suite includes three optical channels: a Stereoscopic Imaging Channel, a High Resolution Imaging Channel, and a Visible and near Infrared Hyperspectral Imager. SIMBIOSYS will characterize the hermean surface in terms of surface morphology, volcanism, global tectonics, and chemical composition. The aim of this work is to describe a tool for the radiometric response prediction of the three SIMBIOSYS channels. Given the spectral properties of the surface, the instrument characteristics, and the geometrical conditions of the observation, the realized SIMBIOSYS simulator is capable of estimating the expected signal and integration times for the entire mission lifetime. In the simulator the spectral radiance entering the instrument optical apertures has been modelled using a Hapke reflectance model implementing the parameters expected for the hermean surface. The instrument performances are simulated by means of calibrated optical and detectors responses. The simulator employs the SPICE (Spacecraft, Planet, Instrument, C-matrix, Environment) toolkit software, which allows us to know for each epoch the exact position of the MPO with respect to the planet surface and the Sun. [ABSTRACT FROM AUTHOR]

We present experimental phase function and degree of linear polarization curves for seven samples of cometary dust analogues namely: ground pieces of Allende, DaG521, FRO95002, and FRO99040 meteorites, Mg-rich olivine and pyroxene, and a sample of organic tholins. The experimental curves have been obtained at the IAA Cosmic Dust Laboratory at a wavelength of 520 nm covering a phase angle range from 3° to 175°. We also provide values of the backscattering enhancement for our cometary analogue samples. The final goal of this work is to compare our experimental curves with observational data of comets and asteroids to better constrain the nature of cometary and asteroidal dust grains. All measured phase functions present the typical behaviour for |$\mu$| m-sized cosmic dust grains. Direct comparison with data provided by the OSIRIS/Rosetta camera for comet 67P/Churyumov–Gerasimenko reveals significant differences and supports the idea of a coma dominated by big chunks, larger than one micrometer. The polarization curves are qualitatively similar to ground-based observations of comets and asteroids. The position of the inversion polarization angle seems to be dependent on the composition of the grains. We find opposite dependence of the maximum of the polarization curve for grains sizes in the Rayleigh-resonance and geometric optics domains, respectively. [ABSTRACT FROM AUTHOR]

In situ images of the 67P/Churyumov-Gerasimenko nucleus acquired by the CIVA cameras on-board PHILAE revealed a rough landscape dominated by consolidated materials. These data provide a unique view to constrain the past and present conditions prevailing at the surface of the comet. A quantitative analysis of microscopic structures (fractures and pebbles) is derived using a manual extraction from the images. Fractures/cracks are rather ubiquitous at various spatial scales with network and size (from sub-cm to 10 cm) well correlated to the texture of the landscape. The pebble size distributions are derived and compared to the size distribution of other cometary materials. The nature of the landscape is then discussed in relation to endogenic and exogenic processes of surface modification. The block seen in CIVA no. 1 is interpreted to be close-ups of fractured boulder/cliff belonging to the boulder field identified from the orbit near Abydos, this boulder field being itself the result of gravitational regressive erosion due to sublimation. The observed fractures are best explained by thermal insolation leading to thermal fatigue and/or to loss of volatile materials. This surficial fragmentation (up to >10 cm length) could generate macroscopic erosion that is also visible at larger scale from the orbit. There is at least an intriguing possibility that the pebbles are remnants of primordial accretion processes. We thus speculate that the Abydos landscape could be in favour of pebble accretion model instead of runaway coagulation model with a formation location in the outer region of the Solar system. [ABSTRACT FROM AUTHOR]

The activity of the Short Period Comets (SPCs) at large heliocentric distance ( 3 au) occurs in a region of the Solar system where the water sublimation rate is low and so the sublimation of other volatiles, for example CO or CO2, could drive the presence of a coma. The detection of distant activity in a SPC can therefore give important hints on its composition. Moreover, a complete characterization of the distant SPCs degree of activity is crucial in order to give correct estimates of the nucleus size and to obtain more reliable size-distribution curves of cometary nuclei. The aim of this paper is to present the last results of a program of CCD imaging of distant SPCs, started in 2004 December and concluded with observing runs at the 3.5-m Telescopio Nazionale Galileo at La Palma, in 2005 April, and at the 2.2-m Centro Astronómico Hispano Alemán (CAHA) telescope in Spain, in 2005 May. During the Spring 2005 campaign, 12 SPCs have been targeted in the R band (eight numbered SPCs and four still unnumbered SPCs): 61P/Schajn–Schaldach, 71P/Clark, 98P/Takamizawa, 103P/Hartley 2, 117P/Helin–Roman–Alu 1, 118P/Shoemaker–Levy 4, 121P/Shoemaker–Holt 2, 136P/Mueller 3, P/2002 T5 (LINEAR), P/2003 S1 (NEAT), P/2003 S2 (NEAT), P/2004 DO29 (Spacewatch–LINEAR). The heliocentric distance of the targets was au. Several levels of activity were detected in the sample, from stellar appearance to well-developed coma and tail. In some cases, the occurrence of cometary activity could be enhanced only with deep visible imaging (e.g. with very long exposure time). For comets with stellar appearance, it was possible to derive a value or a range for the nucleus radius rnucleus (assuming a ‘classical’ albedo value of 0.04): 98P ( km), 136P ( km), P/2003 S2 ( to 1.55 km). For the active comets, we measured dust production levels in terms of A fρ quantity, which was A f cm. Ensemble properties of the whole sample of the long-term program (a total of 17 SPCs) have been analysed in terms of the relationship among distant activity and dynamical evolution of the targets (in particular, an inward ‘jump’ of the perihelion distance): we can conclude that, even if there is some theoretical indication that this could occur, the hypothesis of distant activity triggered by a rise in perihelion temperature cannot be univocally invoked for these comets. [ABSTRACT FROM AUTHOR]

A surprisingly large number of short-period comets have been observed with significant activity (a coma and even a well-developed dust tail) at heliocentric distances greater than 3 au, where the water sublimation rate is low and thus the sublimation of other volatiles, such as for example CO, could drive the presence of a coma. As CO is not the main ice in the comet nucleus, the dust release from the nucleus surface as a result of CO drag is expected to be very different from that caused by water. In order to investigate the complexity of the transition between bare nucleus and the presence of a well-developed coma, to compare activity levels and to obtain information concerning evolutionary differences resulting from distinct dynamical histories, we started a long-term programme of CCD imaging of distant short-period comets. In this paper we present the results of the first observing run, performed at the 3.5-m Telescopio Nazionale Galileo on La Palma on 2004 December 17. Five comets were imaged in the R band (three numbered short-period comets and the two fragments of an unnumbered short-period comet): 36P/Whipple, 111P/Helin–Roman–Crockett, 159P/LONEOS, and P/2004 V5 (LINEAR–Hill) A and B. The heliocentric distance of targets was au. The targets presented several levels of activity, ranging from a stellar appearance to a well-developed coma and tail. The stellar appearance of 111P/allowed a range to be derived for the nuclear radius rnucleus from 0.46 to 1.39 km (assuming a ‘classical’ albedo value of 0.04), depending on the presence of an unresolved coma. For the active comets, we measured dust production levels in terms of the quantity Afρ, which was in the range cm. The distant activity of the target comets is analysed in relation to the target dynamical history (in terms of the perihelion heliocentric distance). A possible preliminary conclusion can be obtained, namely that the hypothesis of distant activity ‘induced’ by higher temperature owing to perihelion lowering cannot be univocally invoked for short-period comets. [ABSTRACT FROM AUTHOR]

In this work, we calculate the vapour and neutral Na production rates on the Moon, as due to the impacts of meteoroids in the radius range of 10−8–0.15 m. We limit our calculations to this size range, since meteoroids with radius larger than 0.15 m have not been found to be important for the production of the exosphere in a time interval comparable with that of the observations. We have considered a new dynamical model of the flux of meteoroids at the heliocentric distance of the Moon, regarding objects in the radius range of 10−2–0.15 m. Instead, the flux of smaller meteoroids (radius range 10−8–10−2 m) has been calculated using the two distributions adopted by Cintala and Love & Brownlee. The results of our model are that (i) the neutral Na production rate is , slightly larger than the previous estimates , and (ii) only about 6 per cent of neutral Na is produced by the impacts of meteoroids in the size range 10−3–0.15 m, whereas about 94 per cent of the Na comes from the size range. [ABSTRACT FROM AUTHOR]

Enhancements of the Na emission and temperature from the lunar atmosphere were reported during the Leonid meteor showers of 1995, 1997 and 1998. Here we report a search for similar enhancement during the 1999 Quadrantids, which have the highest mass flux of any of the major streams. No enhancements were detected. We suggest that different chemical–physical properties of the Leonid and Quadrantid streams may be responsible for the difference. [ABSTRACT FROM AUTHOR]

Using a new all-sky-imaging system, M Mendillo, C Barbieri, J Baumgardner, J Wroten, G Cremonese and G Umbriaco observed two distinctive types of aurora over London and western Europe on the night of 26-27 September 2011, including the first ground-based image of a stable auroral red arc over Europe. [ABSTRACT FROM AUTHOR]