AbstractWe are conducting an observing campaign with a sample of near-Earth asteroids (NEAs) to detect YORP-induced acceleration. Photometric lightcurves from small to medium optical telescopes are used to detect changes to spin state. Where available, radar and thermal-IR observations are used to develop physical models that are used to determine expected YORP strengths. We will present the results of an analysis of 2000 PN9 (hereafter PN9) using optical and radar observations between 2001 and 2016. A detailed physical model has been developed to describe the shape and spin-state of the asteroid. PN9 is top-shaped with an equatorial ridge, and is rotating close to the spin-breakup limit. PN9 is the largest asteroid known to have this distinctive and highly symmetrical 'YORPoid' shape, and is the fastest-rotating top-shaped body that is not part of a multiple system. Due to the size and shape of PN9, an observational detection of YORP acceleration has not been possible with the available data. The shape and rotation period indicate that PN9 is a YORP-evolved body similar to 1999 KW4 [1], and that it is a good candidate for future detection of mass-lofting events.IntroductionThe YORP effect is a thermal torque caused by the reflection, absorption and anisotropic re-emission of Solar radiation [2,3]. YORP is the main driver of physical and dynamical evolution for small bodies in the inner Solar System [4]. There are seven confirmed detections of YORP to date, all of which are in the 'spin-up' mode.The observing campaign aims to increase the number of YORP detections such that theoretical understanding of YORP can be improved. Optical and IR observations were carried out on 42 NEAs through our ESO Large Programme from 2010 to 2014, with continued monitoring conducted through associated programmes. In addition to photometric observations, thermal-IR and radar observations have been obtained for the NEA sample.PN9 was observed in 2001, 2006, 2010, 2011, 2015 and 2016 with various optical telescopes resulting in 29 lightcurves. Radar observations were also conducted at the Arecibo and Goldstone radar facilities in 2001 and 2006. Previously published lightcurves report rotational periods around 2.53 h [5,6] and initial diameter estimates range from 1.6 - 2.0 km [5,7]. PN9 has been previously classified as an Sq-type asteroid [8].Results & ConclusionsDetecting YORP requires well constrained measurement of the evolving rotation period and pole orientation. These parameters can be determined using lightcurve data alone, although it is beneficial to have a detailed radar shape model. This greatly assists with the computation of rotational phase offsets between observations and synthetic lightcurves derived from a constant-period shape model. YORP causes a linear change in rotation period, which in turn causes a quadratic increase in rotational phase offset against time. A radar shape model greatly assists in the detection of this quadratic trend, which is highly desirable for constraining YORP strength.Figure 1: Shape model for asteroid 2000 PN9 developed using both radar and lightcurve observations. The top row shows the model from the positive end of the Z, Y and X axes in the body-centric co-ordinate system. The bottom row shows the model from negative end of the Z, Y and X axes. Lightcurves of PN9 have very low amplitudes due to the highly symmetrical nature of the object. Lightcurve data alone was not sufficient to constrain the orientation of the rotational axis, so the spin-state was determined using a combination of lightcurve and radar data. Figure (2) shows the χ2 fit of pole solutions across the whole sky, with the pole-scan converging to two regions that are opposite each other on the celestial sphere. This pole solution degeneracy implies uncertainty as to whether the asteroid is a prograde or retrograde rotator. Physical models were generated for all possible pole solutions, with all solutions within 1% of the best solution having a shape and period consistent with the best solution. For the best-fit pole solution at λ=228° β=-30°, we have determined the sidereal rotation period to be 2.532972±0.000015 hours. In the body-centric coordinate system where the z-axis is aligned with the rotation axis, the maximum extents of the X, Y and Z axes are 1.885x1.888x1.780 km. As shown in Figure (1), the shape is highly symmetrical with an equatorial ridge. This shape is characteristic for a rapidly rotating rubble pile, where material is being migrated towards the equator [9] and closely resembles the convex hull model of 1917 Cuyo, a candidate for rotationally-induced mass lofting [10]. We have measured the circular polarisation ratio (SC/OC) to be 0.225±0.004, which is consistent with PN9's taxonomic type. We have been unable to measure YORP acceleration; the size and shape of PN9 results in a low expected YORP strength, and phase offsets cannot be precisely measured with relatively featureless lightcurves.Further work that will be presented at the meeting includes an updated physical model developed with newly obtained lightcurves, and an update on Spitzer thermal-IR lightcurves of PN9.Figure 2: The results of a search for the rotational pole of 2000 PN9 using radar and lightcurve data. The best solution is marked with a yellow '+'. The yellow and white lines enclose regions where χ2 is within 1% and 5% of the best solution respectively.AcknowledgementsThe authors would like to thank A. Fitzsimmons, R. Ya. Inasaridze, V. A. Ayvazian, V. V. Rumyantsev, V. G. Chiorny, I. V. Reva, M. A. Krugov, I. E. Molotov, M. W. Busch, M. C. Nolan, A. A. Hine, V. Negrón, L. A. M. Benner and M. D. Hicks for their contributions to this work.References[1] Ostro, S. J. et al. (2006) Science 314, 1276. [2] Rubincam, D. P. (2000). Icarus 148, 2. [3] Lowry, S. C. et al. (2007). Science 316, 272. [4] Bottke, W. F. et al. (2006). AREPS 34. [5] Belskaya, I. N. et al. (2009). Icarus 201, 167. [6] Warner, B. D. (2016). Minor Planet Bulletin 43, 240. [7] Busch, M. W. et al. (2006). Society for Astronomical Sciences Annual Symposium 25, 169. [8] Thomas, C. A. et al. (2014). Icarus 228, 217. [9] Walsh, K. J. et al. (2008). Nature 454, 188. [10] Rożek, A. et al. (2019) A&A 627, A172.

Primitive objects like comets hold important information on the material that formed our solar system. Several comets have been visited by spacecraft and many more have been observed through Earth- and space-based telescopes. Still our understanding remains limited. Molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. The samples returned by the Stardust mission to comet Wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. The recent Rosetta mission acquired a wealth of new data on the composition of comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) and complemented earlier observations of other comets. The isotopic, elemental, and molecular abundances of the volatile, semi-volatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. The emerging picture after Rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. Deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the Stardust results. Post-formation evolution of the material might play an important role, which further complicates the picture. This paper discusses these major findings of the Rosetta mission with respect to the origin of the material and puts them in the context of what we know from other comets and solar system objects.

One of the key goals of the Rosetta mission was to understand how, where and when comets formed in our solar system. There are two major hypotheses for the origin of comets, both pre-Rosetta: (1) hierarchical accretion of dust and ice grains in the Solar Nebula and (2) the growth of pebbles, which are then brought together by streaming instabilities in the Solar Nebula to form larger bodies. Rosetta provided a wealth of new information on comet nuclei and confirmed many past ideas on comets, e.g., high volatile content, lack of aqueous alteration of grains, and the low bulk density of the nucleus. Rosetta also provided new data on the nature of cometary activity, the active geology on the nucleus surface and the interior structure and bulk density of the nucleus. Supporters of the above-mentioned origin hypotheses each find confirmation of their ideas in the Rosetta results. But the question of which hypothesis is preferred, or if there are other, better hypotheses that could be invoked, could not be answered. Theoretical studies suggest that comet nuclei were collisionally processed in the Primordial Disk though it is not clear that the nuclei we see today display the effects of that process. Both theoretical and observational studies suggest that the major end-states for cometary nuclei are dynamical ejection, random disruption and disintegration, and/or evolution of nuclei to inactive, asteroidal-appearing objects. Rosetta has provided us with many new insights that will help to guide future cometary missions, observations, experiments and theoretical investigations that will lead to answers to the fundamental questions with regard to cometary origin.

We present photometry of the S-type near-Earth asteroid 25143 Itokawa based on both ground-based observations in the UBVRI bands and measurements from the AMICA/Hayabusa spacecraft observations with ul-, b-, v-, w-, x-, and p-filters. Hayabusa observed Itokawa around opposition during the rendezvous, thus providing a unique set of observations of this asteroid. We fit the phase curve measurements with both the Classic Hapke Model (Hapke, 1981, 1984, 1986) and Modern Hapke Model (Hapke, 2002, 2008, 2012a) and thereby extract the physical properties of Itokawa's surface regolith. The single-scattering albedo (0.57 +/- 0.05) is larger than that derived for Eros (0.43 +/- 0.02), another S-type near-Earth asteroid visited by a spacecraft. Both models indicate a regolith that is forward-scattering in nature. From the hockey stick relationship derived for the single-particle phase function (Hapke, 2012b), both modeling results suggest a regolith comprised of rough surfaced particles with a low density of internal scatterers. Application of the Modern Hapke model derives porosity parameter values from 1 to 1.1, for BVR bands, which corresponds to porosity values between 77-79%. This suggests the surface of Itokawa is very fluffy and the large boulders may be bonded with smaller size particles, typical of the particle sizes observed in Muses Sea. Both models also provide similar geometric albedo values (0.27 +/- 0.02) at the V-band wavelength, which are equivalent to Eros' geometric albedo. (C) 2018 Elsevier Inc. All rights reserved.

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun's centre, equal to half of Mercury's perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics.

GIADA (Grain Impact Analyzer and Dust Accumulator) on-board the Rosetta space probe is designed to measure the momentum, mass and speed of individual dust particles escaping the nucleus of comet 67P/Churyumov-Gerasimenko (hereafter 67P). From August 2014 to June 2016 Rosetta escorted comet 67P during its journey around the Sun. Here we focus on GIADA data taken between 2015 January and 2016 February which included 67P’s perihelion passage. To better understand cometary activity and more specifically the presence of dust structures in cometary comae, we mapped the spatial distribution of dust density in 67P’s coma. In this manner we could track the evolution of high density regions of coma dust and their connections with nucleus illumination conditions, namely tracking 67P’s seasons. We also studied the link between dust particle speeds and their masses with respect to heliocentric distance, i.e. the level of cometary activity. This allowed us to derive a global and a local correlation of the dust particles’ speed distribution with respect to the H2O production rate.

Sample return from the near-Earth asteroid known as 25143 Itokawa was conducted as part of the Hayabusa mission, with a large number of scientific findings being derived from the returned samples. Following the Hayabusa mission, Hayabusa2 was planned, targeting sample return from a primitive asteroid. The primary target body of Hayabusa2 was asteroid 162173 Ryugu; however, it was also necessary to gather physical information for backup target selection. Therefore, we examined five asteroids spectroscopically, 43 asteroids spectrophotometrically, and 41 asteroids through periodic analysis. Hence, the physical properties of 74 near-Earth asteroids were obtained, which helped the Hayabusa2 backup target search and, also, furthered understanding of the physical properties of individual asteroids and their origins.

The JAXA Hayabusa-2 mission was approved in 2010 and launched on December 3, 2014. The spacecraft will arrive at the near-Earth asteroid 162173 Ryugu in 2018 where it will perform a survey, land and obtain surface material, then depart in Dec 2019 and return to Earth in Dec 2020. We observed Ryugu with the Herschel Space Observatory in Apr 2012 at far-IR thermal wavelengths, supported by several ground-based observations to obtain optical lightcurves. We reanalysed previously published Subaru-COMICS and AKARI-IRC observations and merged them with a Spitzer-IRS data set. In addition, we used a large set of Spitzer-IRAC observations obtained in the period Jan to May, 2013. The data set includes two complete rotational lightcurves and a series of ten "point-and-shoot" observations. The almost spherical shape of the target together with the insufficient lightcurve quality forced us to combine radiometric and lightcurve inversion techniques in different ways to find the object's key physical and thermal parameters. We find that the solution which best matches our data sets leads to this C class asteroid having a retrograde rotation with a spin-axis orientation of (lambda = 310-340 deg; beta = -40+/-15 deg) in ecliptic coordinates, an effective diameter (of an equal-volume sphere) of 850 to 880 m, a geometric albedo of 0.044 to 0.050 and a thermal inertia in the range 150 to 300 Jm-2s-0.5K-1. Based on estimated thermal conductivities of the top-layer surface in the range 0.1 to 0.6 WK-1m-1, we calculated that the grain sizes are approximately equal to between 1 and 10 mm. The finely constrained values for this asteroid serve as a `design reference model', which is currently used for various planning, operational and modelling purposes by the Hayabusa2 team.

Solar System formation models predict that the building-blocks of planetesimals were mm- to cm-sized pebbles, aggregates of ices and non-volatile materials, consistent with the compact particles ejected by comet 67P/Churyumov-Gerasimenko (67P hereafter) and detected by GIADA (Grain Impact Analyzer and Dust Accumulator) on-board the Rosetta spacecraft. Planetesimals were formed by the gentle gravitational accretion of pebbles, so that they have an internal macroporosity of 40%. We measure the average dust bulk density ${\rho}D = 795 _{-65}^{+840} kg m^{-3}$ that, coupled to the 67P nucleus bulk density, provides the average dust-to-ices mass ratio δ = 8.5. We find that the measured densities of the 67P pebbles are consistent with a mixture of (15 ± 6)% of ices, (5 ± 2)% of Fe-sulfides, (28 ± 5)% of silicates, and (52 ± 12)% of hydrocarbons, in average volume abundances. This composition matches both the solar and CI-chondritic chemical abundances, thus showing that GIADA has sampled the typical non-volatile composition of the pebbles that formed all planetesimals. The GIADA data do not constrain the abundance of amorphous silicates vs. crystalline Mg,Fe- olivines and pyroxenes. We find that the pebbles have a microporosity of (52 ± 8)% (internal volume filling factor φP = 0.48±0.08), implying an average porosity for the 67P nucleus of (71 ± 8)%, lower than previously estimated.

On 2016 Feb 19, nine Rosetta instruments serendipitously observed an outburst of gas and dust from the nucleus of comet 67P/Churyumov-Gerasimenko. Among these instruments were cameras and spectrometers ranging from UV over visible to microwave wavelengths, in situ gas, dust and plasma instruments, and one dust collector. At 09: 40 a dust cloud developed at the edge of an image in the shadowed region of the nucleus. Over the next two hours the instruments recorded a signature of the outburst that significantly exceeded the background. The enhancement ranged from 50 per cent of the neutral gas density at Rosetta to factors > 100 of the brightness of the coma near the nucleus. Dust related phenomena (dust counts or brightness due to illuminated dust) showed the strongest enhancements (factors > 10). However, even the electron density at Rosetta increased by a factor 3 and consequently the spacecraft potential changed from similar to-16 V to -20 V during the outburst. A clear sequence of events was observed at the distance of Rosetta ( 34 km from the nucleus): within 15 min the Star Tracker camera detected fast particles (similar to 25 m s(-1)) while 100 mu m radius particles were detected by the GIADA dust instrument similar to 1 h later at a speed of 6 m s(-1). The slowest were individual mm to cm sized grains observed by the OSIRIS cameras. Although the outburst originated just outside the FOV of the instruments, the source region and the magnitude of the outburst could be determined.

GIADA (Grain Impact Analyzer and Dust Accumulator) on-board the Rosetta space probe is designed to measure the momentum, mass and speed of individual dust particles escaping the nucleus of comet 67P/Churyumov-Gerasimenko (hereafter 67P). From 2014 August to 2016 June, Rosetta escorted comet 67P during its journey around the Sun. Here, we focus on GIADA data taken between 2015 January and 2016 February which included 67P's perihelion passage. To better understand cometary activity and more specifically the presence of dust structures in cometary comae, we mapped the spatial distribution of dust density in 67P's coma. In this manner, we could track the evolution of high-density regions of coma dust and their connections with nucleus illumination conditions, namely tracking 67P's seasons. We also studied the link between dust particle speeds and their masses with respect to heliocentric distance, i.e. the level of cometary activity. This allowed us to derive a global and a local correlation of the dust particles' speed distribution with respect to the H2O production rate.

ESA's Rosetta Mission has followed Comet 67P/Churyumov-Gerasimenko from 3.6 au inbound to 3.6 au outbound. Many results are largely unexpected, as compared to previous models based on in situ and ground-based observations of Jupiter-family comets. The main topics discussed in this review are (1) the importance of the large concavities characterizing the 67P nucleus, that, (2) coupled to the nucleus obliquity, make seasons an unexpectedly important source of many phenomena observed in this and probably in most comets; (3) the mostly uniform distribution of ices over the nucleus surface; (4) the high dust-to-water mass ratio, which implies that much of the nucleus mass is in the form of minerals partly coming from the inner proto-solar nebula, thus making 67P very porous and less hydrated than primitive CI chondrites. 67P nucleus may have never experienced any collision at speeds larger than 1 m s(-1).

ABSTRACT The Rosetta probe, orbiting Jupiter-family comet 67P/Churyumov–Gerasimenko, has been detecting individual dust particles of mass larger than 10−10 kg by means of the GIADA dust collector and the OSIRIS Wide Angle Camera and Narrow Angle Camera since 2014 August and will continue until 2016 September. Detections of single dust particles allow us to estimate the anisotropic dust flux from 67P, infer the dust loss rate and size distribution at the surface of the sunlit nucleus, and see whether the dust size distribution of 67P evolves in time. The velocity of the Rosetta orbiter, relative to 67P, is much lower than the dust velocity measured by GIADA, thus dust counts when GIADA is nadir-pointing will directly provide the dust flux. In OSIRIS observations, the dust flux is derived from the measurement of the dust space density close to the spacecraft. Under the assumption of radial expansion of the dust, observations in the nadir direction provide the distance of the particles by measuring their trail length, with a parallax baseline determined by the motion of the spacecraft. The dust size distribution at sizes >1 mm observed by OSIRIS is consistent with a differential power index of −4, which was derived from models of 67P’s trail. At sizes <1 mm, the size distribution observed by GIADA shows a strong time evolution, with a differential power index drifting from −2 beyond 2 au to −3.7 at perihelion, in agreement with the evolution derived from coma and tail models based on ground-based data. The refractory-to-water mass ratio of the nucleus is close to six during the entire inbound orbit and at perihelion.

The Grain Impact Analyzer and Dust Accumulator (GIADA) instrument on board ESA’s Rosetta mission is constraining the origin of the dust particles detected within the coma of comet 67 P/Churyumov–Gerasimenko (67P). The collected particles belong to two families: (i) compact particles (ranging in size from 0.03 to 1 mm), witnessing the presence of materials that underwent processing within the solar nebula and (ii) fluffy aggregates (ranging in size from 0.2 to 2.5 mm) of sub-micron grains that may be a record of a primitive component, probably linked to interstellar dust. The dynamics of the fluffy aggregates constrain their equivalent bulk density to <1kg m. These aggregates are charged, fragmented, and decelerated by the spacecraft negative potential and enter GIADA in showers of fragments at speeds<1m s. The density of such optically thick aggregates is consistent with the low bulk density of the nucleus. The mass contribution of the fluffy aggregates to the refractory component of the nucleus is negligible and their coma brightness contribution is less than 15%.

omets are the best sample of primitive solar nebula material presently available to us, dating back 4.57 billion years to the origin of our planetary system.Past missions to comets have all been "fast flybys": They provided only a snapshot view of the dust and ice nucleus, the nebulous coma surrounding it, and how the solar wind interacts with both of these components.Such space-based investigations of comets began in the 1980s with a flotilla of spacecraft: the European Space Agency's (ESA's) first deep space mission, Giotto, which pursued comet 1P/Halley; Deep Space 1 at 19P/Borrelly; Stardust at 81P/ Wild 2; Deep Impact and Stardust NeXT at 9P/ Tempel; and EPOXI at 103P/Hartley 2.Rosetta is now taking a more prolonged look.The spacecraft is an ESA mission, with contributions from member states and from NASA, and it currently orbits the Jupiter family comet 67P/ Churyumov-Gerasimenko (67P).Rosetta met the comet nucleus on 6 August 2014, at 3.7 astronomical units (AU) from the Sun, and delivered the Philae lander to the nucleus surface on 12 November 2014, when the comet was 3.0 AU from the Sun.Rosetta is uniquely positioned to further the understanding of these primitive bodies, having revealed an unusual and fascinating object.After rendezvous, the Rosetta spacecraft moved from 100 km above the comet to a bound orbit only ~10 km away.

Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency’s Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10 −10 to 10 −7 kilograms, and 48 grains of mass 10 −5 to 10 −2 kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails.

The 163 comets observed during the WISE/NEOWISE prime mission represent the largest infrared survey to date of comets, providing constraints on dust, nucleus sizes, and CO+CO2 production. We present detailed analyses of the WISE/NEOWISE comet discoveries, and discuss observations of the active comets showing 4.6 μm band excess. We find a possible relation between dust and CO+CO2 production, as well as possible differences in the sizes of long and short period comet nuclei.

nalyses of images taken by the Rosetta spacecraft reveal the complex landscape of a comet in rich detail. Close-up views of the surface indicate that some dust jets are being emitted from active pits undergoing sublimation. See Letter p.63

The Grain Impact Analyzer and Dust Accumulator (GIADA) instrument on board ESA’s Rosetta mission is constraining the origin of the dust particles detected within the coma of comet 67 P/Churyumov–Gerasimenko (67P). The collected particles belong to two families: (i) compact particles (ranging in size from 0.03 to 1 mm), witnessing the presence of materials that underwent processing within the solar nebula and (ii) fluffy aggregates (ranging in size from 0.2 to 2.5 mm) of sub-micron grains that may be a record of a primitive component, probably linked to interstellar dust. The dynamics of the fluffy aggregates constrain their equivalent bulk density to < 1 ?> kg m−3. These aggregates are charged, fragmented, and decelerated by the spacecraft negative potential and enter GIADA in showers of fragments at speeds < 1 ?> m s−1. The density of such optically thick aggregates is consistent with the low bulk density of the nucleus. The mass contribution of the fluffy aggregates to the refractory component of the nucleus is negligible and their coma brightness contribution is less than 15%.