Narrow-Band Photometry of Comet C/1995 O1 (Hale-Bopp)

Photometric observations of comet C/1995 O1 (Hale-Bopp) carried out at the Stará Lesná Observatory since February to April 1997 are analyzed and discussed. Emission band fluxes and continuum fluxes are presented, from which the total numbers of molecules in the columns of the coma encircled by diaphragms are calculated. The production rates are estimated from the conventional Haser model. We found that the photometric exponent of dust contribution two months prior perihelion was n = 5.2. The photometric exponent n of the cometary magnitude solely to the C₂ emission alone equals 3.3 and that of CN equals 2.5. These values can be explained by a fact that the maximums of production rates of the gases were reached between March 2and 12 and not at the perihelion as it is valid for dust. These results are compared with the values of 1P/Halley (1986 III) under the similar conditions, obtained with the same method and instrument. C/Hale-Bopp exhibited 4.1 times more molecules radiating the CN-emission than 1P/Halley in the same column of the coma. The continuum flux of C/Hale-Bopp was also very strong. The ratios (to 1P/Halley) are 94:1 (Cont. 484.5) and 74:1 (Cont. 365.0). The cometary colour was the same as that of the Sun. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Dust in Comet C/1999 S4 (LINEAR) during Its Disintegration: Narrow-Band Images, Color Maps, and Dynamical Models

Comet C/1999 S4 was observed with the 2m-telescopes of the Bulgarian National Observatory and Pik Terskol Observatory, Northern Caucasus, Russia, at the time of its disintegration. Maps of the dust brightness and color were constructed from images obtained in red and blue continuum windows, free from cometary molecular emissions. We analyze the dust environment of Comet C/1999 S4 (LINEAR) taking into account the observed changes apparent in the brightness images and in plots of Afρ profiles as function of the projected distance ρ from the nucleus. We also make use of the syndyne-synchrone formalism and of a Monte Carlo model based on the Finson-Probstein theory of dusty comets. The brightness and color of individual dust particles, which is needed to derive theoretical brightness and color maps of the cometary dust coma from the Monte Carlo model, is determined from calculations of the light scattering properties of randomly oriented oblate spheroids. In general, the dust of Comet C/1999 S4 (LINEAR) is strongly reddened, with reddening values up to 30%/1000 Å in some locations. Often the reddening is higher in envelopes further away from the nucleus. We observed two outbursts of the comet with brightness peaks on July 14 and just before July 24, 2000, when the final disintegration of the comet started. During both outbursts an excess of small particles was released. Shortly after both outbursts the dust coma “turns blue.” After the first outburst, the whole coma was affected; after the second one only a narrow band of reduced color close to the tail axis was formed. This difference is explained by different terminal ejection speeds, which were much lower than normal in case of the second outburst. In particular in the second, final outburst the excess small particles could originate from fragmentation of “fresh” larger particles.     

The rotation period of comet 29P/Schwassmann-Wachmann 1 determined from the dust structures (Jets) in the coma

The results of the photometric observations of comet 29P/Schwassmann-Wachmann 1 are analyzed. The comet demonstrates substantial activity at heliocentric distances larger than 5 AU, i.e., outside the water ice sublimation zone. The CCD images of the comet were obtained in wideband R filters at the 6-meter telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS) and at the 2-meter Zeiss-2000 telescope of the Peak Terskol Observatory. The processing of the images with special digital filters allowed the active structures (jets) to be distinguished in the dust coma of the comet. With the cross-correlation method, the rotation period of the cometary nucleus was determined as 12.1 ± 1.2 and 11.7 ± 1.5 days for the observations taken in December 2008, and February 2009, respectively. The probable causes of the difference in the estimates of the rotation period of the cometary nucleus obtained by different authors are discussed.     

Thermal modeling of cometary nuclei

A new model of the sublimation of volatile ices from a cometary nucleus has been developed which includes the effects of diurnal heating and cooling, rotation period and pole orientation, and thermal properties of the ice and subsurface layers. The model also includes the contribution from coma opacity, scattering, and thermal emission, where the properties of the coma are derived from the integrated rate of volatile production by the nucleus. The model is applied to the specific case of the 1986 apparition of Halley's comet. It is found that the generation of a cometary dust coma actually increases the total energy reaching the Halley nucleus. This results because of the significantly greater geometrical cross section of the coma as compared with the bare nucleus, and because the coma provides an essentially isotropic source of multiply scattered sunlight and thermal emission over the entire nucleus surface. For Halley, the calculated coma opacity is approximately 0.2 at 1 AU from the Sun, and 1.2 at perihelion (0.587 AU). At 1 AU this has little effect on dayside temperatures (maximum ≈200°K) but raises nightside temperatures (minimum ≈150°K) by about 40°K. At perihelion the higher opacity results in a nearly isothermal nucleus with only small diurnal and latitudinal temperature variations. The general surface temperature is 205°K with a maximum of 209°K at local noon on the equator. Some possible consequences of the results with respect to the generation of nongravitational forces, observed volatile production rates for comets, and cometary lifetimes against sublimation are discussed.     

Dust tail streamers of a comet with rotating nucleus

Synthetic images of the dust tail are presented for a comet which has a rotating nucleus with one predominant dust source fixed to it. The images have been generated using a new computer model which, unlike similar models, allows for the study of dust tails caused by a rotating nucleus with an anisotropic distribution of sources.The dust tail is studied in the post-perihelion phase of a parabolic comet with a perihelion distance of 0.5 AU. One finds that in the case of a rotating nucleus with anisotropic emission characteristics streamers caused solely by the dynamics of the dust particles are forming in the dust tail even if there is no dependence between the solar irradiation angle of the source and the amount of dust emitted. If the dust emission depends on the solar irradiation angle of the dust source, then the brightest tail regions do not necessarily coincide with the synchrones for the times of maximum dust emission.As a consequence, a thorough analysis of streamer patterns in a cometary dust tail requires assumptions on the rotational state and the dust source distribution of the nucleus. Otherwise, it seems not possible to discern between streamers which are caused dynamically by nucleus rotation and others which reflect variations in the emission activity.     

Physico-chemical phenomena in comets—III: The continuum of comet Burnham (1960 II)

The new model of the cometary head proposed in papers I and II is developed and applied to comet Burnham. It takes into account the likely existence of a halo of large icy particles surrounding the nucleus. These particles are steadily stripped from the nucleus by evaporating gases. Their terminal velocity and their rate of evaporation set the size of the halo. The existence of the icy halo influences in two ways the photometric characteristics of the coma. This paper establishes the photometric shape of the continuum as reflected by the icy grains, and compares it to the observed continuum of comet Burnham. Paper IV will compare the predictions of the model with the photometric profile of the molecular emission bands of C₂, in the same comet.     

C/1995 O1 (Hale-Bopp): Gas Production Curves And Their Interpretation

The discovery of C/1995 O1 (Hale-Bopp) at 7 AU from the Sun provided the first opportunity to follow the activity of a bright comet over a large range of heliocentric distances r_h. Production rates of a number of parent molecules and daughter species have been monitored both pre- and postperihelion. CO was found to be the major driver of the activity far from the Sun, surpassed by water within 3 AU whose production rate reached 10³¹ s⁻¹ at perihelion. Gas production curves obtained for various species show several behaviours with r_h. Gas production curves contain important information concerning the physical state of cometary ices, the structure of the nucleus and all the processes taking place inside the nucleus leading to outgassing. They are relevant to the study of several other phenomena such as the sublimation from icy grains, dust mantling or seasonal effects. For some species, such as H₂CO or HNC, they permit to constrain their origin in the coma. We discuss models of subsurface gas production in distant comets and predictions of how such a source may vary as the comet moves along its orbit, approaching perihelion and receding again. Features in the observed gas production curves of comet Hale-Bopp are generally interpretable in terms of either subsurface production (typical example: CO at large r_h) or free sublimation (typical example: H₂O). Possible implications for the vertical stratification of the cometary ices are reviewed, and preference is found for a model with crystallization of amorphous ice close to the nuclear surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Near-Infrared Imaging Spectrophotometry Of Comet C/1995 O1 (Hale-Bopp) Near Perihelion

We present 1- to 5-μm broadband and CVF images of comet Hale-Bopp taken 1997 February 10.5 UT, 50 days before perihelion. All the images exhibit a nonspherical coma with a bright “ridge” in the direction of the dust tail approximately 10″ from the coma. Synthetic aperture spectrophotometry implies that the optically important grains are of a radius ≤0.4 μm; smallest radius for any comet seen to date. The variation of the integrated surface brightness with radial distance from the coma (ρ) in all the images closely follows the “steady state” ρ⁻¹ model for comet dust ablation (Gehrz and Ney, 1992). The near-infrared colors taken along the dust tail are not constant implying the dust grain properties vary with coma distance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Deep Space 1 photometry of the nucleus of Comet 19P/Borrelly

The NASA-JPL Deep Space 1 Mission (DS1) encountered the short-period Jupiter-family Comet 19P/Borrelly on September 22, 2001, about 8 days after perihelion. DS1's payload contained a remote-sensing package called MICAS (Miniature Integrated Camera Spectrometer) that included a 1024 square CCD and a near IR spectrometer with ∼12 nm resolution. Prior to its closest approach of 2171 km, the remote-sensing package on the spacecraft obtained 25 CCD images of the comet and 45 near-IR spectra (L. Soderblom et al., 2002, Science 296, 1087-1091). These images provided the first close-up view of a comet's nucleus sufficiently unobscured to perform quantitative photometric studies. At closest approach, corresponding to a resolution of 47 meters per pixel, the intensity of the coma was less than 1% of that of the nucleus. An unprecedented range of high solar phase angles (52-89 degrees), viewing geometries that are in general attainable only when a comet is active, enabled the first quantitative and disk resolved modeling of surface photometric physical parameters, including the single particle phase function and macroscopic roughness. The disk-integrated geometric albedo of Borrelly's nucleus is 0.029±0.006, comparable to the dark hemisphere of Iapetus, the lowest albedo C-type asteroids, and the uranian rings. The Bond albedo, 0.009±0.002, is lower than that of any Solar System object measured. Such a low value may enhance the heating of the nucleus and sublimation of volatiles, which in turn causes the albedo to decrease even further. A map of normal reflectance of Borrelly shows variations far greater than those seen on asteroids. The two main terrain types, smooth and mottled, exhibit mean normal reflectances of 0.03 and 0.022. The physical photometric parameters of Borrelly's nucleus are typical of other small dark bodies, particularly asteroids, except preliminary modeling results indicate its regolith may be substantially fluffier. The nucleus exhibits significant variations in macroscopic roughness, with the oldest, darkest terrain being slightly smoother. This result suggests the infilling of low-lying areas with dust and particles that have not been able to leave the comet. The surface of the comet is backscattering, but there are significant variations in the single particle phase function. One region exhibits a flat particle phase function between solar phase angles of 50° and 75° (like cometary dust and unlike planetary surfaces), suggesting that its regolith is controlled by native dust rather than by meteoritic bombardment.     

Coma expansion and photometry of comet Bowell (1980b)

Optical and infrared observations of comet Bowell are presented. The optical observations indicate that the solid grain coma is expanding at only 0.9 ± 0.2 m sec^?1. This is two orders of magnitude slower than the local gas sound speed and may suggest that gas drag is not responsible for stripping the grains from the nucleus. The hypothesis of “electrostatic snap-off” is tentatively advanced to account for the ejection of the grains. Alternatively, the grains may have an unusual size distribution. The extrapolated motion of the grains suggests that the bulk of the coma was formed when the comet was at a heliocentric distance $\text{R ? 10}\text{AU}$. Any water ice in the nucleus would be too cold to give rise to the observed grain coma by equilibrium sublimation at this R. Further evidence against the production of the grain coma by equilibrium sublimation of the nucleus is provided by broadband (J) photometric observations. Almost all of the observed photometric variations of comet Bowell can be ascribed to geometric effects. Simple models indicate that the total grain cross section has been nearly constant since the time of the earliest observations. The present observations, which suggest that water ice sublimation does not control either the optical morphology or the near infrared photometric behavior of comet Bowell, are contrasted with reported high OH production rates. It is concluded that the grain coma may be largely a relic of activity occurring on the nucleus at $\text{R ? 10}\text{AU}$ while the OH may indicate sublimation from the nucleus near perihelion and from coma grains near $\text{R ? 4.6}\text{AU}$.     

Models of P/Wirtanen nucleus: active regions versus non-active regions

From the current understanding we know that comet nuclei have heterogeneous compositions and complex structures. It is believed that cometary activity is the result of a combination of physical processes in the nucleus, like sublimation and recondensation of volatile ices, dust grains release, phase transition of water ice, depletion of the most volatile components in the outer layers and interior differentiation.The evolution of the comet depends on the sublimation of ices and the release of different gases and dust grains: the formation of a dust crust, the surface erosion and the development of the coma are related to the gas fluxes escaping from the nucleus. New observations, laboratory experiments and numerical simulations suggest that the gas and dust emissions are locally generated, in the so-called active regions. This localized activity is probably superimposed to the global nucleus activity. The differences between active and inactive regions can be attributed to differences in texture and refractory material content of the different areas.In this paper we present the results of numerical models of cometary nucleus evolution, developed in order to understand which are the processes leading to the formation of active and non-active regions on the cometary surface. The used numerical code solves the equations of heat transport and gas diffusion within a porous nucleus composed of different ices—such as water (the dominant constituent), CO₂, CO- and of dust grains embedded in the ice matrix.By varying the set of physical parameters describing the initial properties of comet P/Wirtanen, the different behaviour of the icy and dusty areas can be followed.Comet P/Wirtanen is the target of the international ROSETTA mission, the cornerstone ESA mission to a cometary nucleus. The successful design of ROSETTA requires some knowledge of comet status and activity: surface temperatures, amount of active and inactive surface areas, gas production rate and dust flux.     

A comparison of modeled and observed intensity profiles for C2, CN, and the blue continuum for Comet Bradfield (1987s)

Intensity profiles were obtained for the C₂ and CN emission and blue continuum of Comet Bradfield (1987s), from observations obtained over a 10 week period starting shortly before perihelion. Model intensity profiles were produced and then fitted to the observed profiles, and used to put constraints on some of the dust and gas parameters. Most of these parameters, including the gas and dust outflow speeds from the cometary nucleus and the molecular lifetimes, were consistent with expected values. The best fitting models incorporate significant dust particle fragmentation and extended emission of CN from dust, both occurring in the inner coma. In addition, although there may have been enhancement of gas and dust emission on the sunward side of the cometary nucleus, it appears that the tailward side maintained a significant level of activity.     

哈雷彗星主尾流的研究

哈雷彗星在日彗距较大时出现长而直的主彗尾（尾流），这是很有趣的。尾流一般是指等离子体尾流；但是，当地球接近彗星轨道面时，尘埃尾流可能叠加到主彗尾上。在一般感光波段宽的彗星底片上很难区分这两种尾流。本文选取哈雷彗星在不同日彗距的5条主尾流，作了光度测量和比较分析。得出沿各尾轴及其垂直方向几个截面的亮度分布、亮度半极大全宽、尾轴的视风差角和真风差角及彗尾长度。在所分析的蓝敏底片上，过近日点前的2个尾流肯定是等离子体尾流，而5个尾流的相似性以及其他证据说明它们主要都是等离子体尾流，尘埃彗尾的污染是次要的。     

Dynamics of Dust Particles of Different Structure: Application to the Modeling of Dust Motion in the Vicinity of the Nucleus of Comet 67P/Churyumov–Gerasimenko

We consider the estimates of the main forces acting on dust particles near a cometary nucleus. On the basis of these estimates, the motion of dust particles of different structure and mass is analyzed. We consider the following forces: (1) the cometary nucleus gravity, (2) the solar radiation pressure, and (3) the drag on dust particles by a flow of gas produced in the sublimation of cometary ice. These forces are important for modeling the motion of dust particles relative to the cometary nucleus and may substantially influence the dust transfer over its surface. In the simulations, solid silicate spheres and homogeneous ballistic aggregates are used as model particles. Moreover, we propose a technique to build hierarchic aggregates—a new model of quasi-spherical porous particles. A hierarchic type of aggregates makes it possible to model rather large dust particles, up to a millimeter in size and larger, while no important requirements for computer resources are imposed. We have shown that the properties of such particles differ from those of classical porous ballistic aggregates, which are usually considered in the cometary physics problems, and considering the microscopic structure of particles is of crucial significance for the analysis of the observational data. With the described models, we study the dust dynamics near the nucleus of comet 67P/Churyumov–Gerasimenko at an early stage of the Rosetta probe observations when the comet was approximately at 3.2 AU from the Sun. The interrelations between the main forces acting on dust aggregates at difference distances from the nucleus have been obtained. The dependence of the velocity of dust aggregates on their mass has been found. The numerical modeling results and the data of spaceborne observations with the Grain Impact Analyzer and Dust Accumulator (GIADA) and the Cometary Secondary Ion Mass Analyzer (COSIMA) onboard the Rosetta probe are compared at a quantitative level.     

Possibilities of using extreme observations to estimate cometary nucleus sizes

Not considering very rare in situ measurements of cometary nuclei, observations of comets at large heliocentric distances are the only direct source of our knowledge on their sizes. Observations of a cometary nucleus in pure reflected sunlight, at the time when coma is absent, are the way in which the nucleus size can be estimated. Probabilities that extreme observations represent non—active stages of cometary nuclei and also reliability of derived cometary nucleus sizes are investigated. Statistical analysis is based on a sample of 2842 photometric observations of 67 long-period comets observed at large heliocentric distances. For any long-period comet, there is a probability of 2:3 that the sizes derived on the basis of observations at extreme distances are in good agreement with the real nucleus sizes. For new comets in Oort's sense the probability is 3:4 independent of investigated arcs of orbits. For old comets a chance to estimate correct sizes is 1:2 but on the pre-perihelion arc only 1:3. It is also demonstrated that a premature start of activity prior to perihelion or a longer fading after perihelion is more frequent than a short-time isolated activity at large heliocentric distances.     

On the Relationship of Chemical Abundances in the Nucleus to Those in the Coma

One of the goals of comet research is the determination of the chemical composition of the nucleus because it provides us with the clues about the composition of the nebula in which comet nuclei formed. It is well accepted that photo-chemical reactions must be considered to establish the abundances of mother molecules in the coma as they are released from the comet nucleus or from distributed dust sources in the coma. However, the mixing ratios of mother molecules in the coma changes with heliocentric distance. To obtain the abundances in the nucleus relative to those in the coma, we must turn our attention to the release rates of mother molecules from the nucleus as a function of heliocentric distance. For this purpose, we assume three sources for the coma gas: the surface of the nucleus (releasing mostly water vapor), the dust in the coma (the distributed source of several species released from dust particles), and the interior of the porous nucleus (the source of many species more volatile than water). The species diffusing from the interior of the nucleus are released by heat transported into the interior. Thus, the ratio of volatiles relative to water in the coma is a function of the heliocentric distance and provides important information about the chemical composition and structure of the nucleus. Our goal is to determine the abundance ratios of various mother molecules relative to water from many remote-sensing observations of the coma as a function of heliocentric distance. Comet Hale-Bopp is ideal for this purpose since it has been observed using instruments in many different wavelength regions over large ranges of heliocentric distances. The ratios of release rates of species into the coma are than modeled assuming various chemical compositions of the spinning nucleus as it moves from large heliocentric distance through perihelion. Since the heat flow into the nucleus will be different after perihelion from that before perihelion, we can also expect different gas release rates after perihelion compared to those observed before perihelion. Since not all the data are available yet, we report on progress of these calculations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Shape and obliquity effects on the thermal evolution of the Rosetta target 67P/Churyumov-Gerasimenko cometary nucleus

This work is dedicated to the application to 67P/Churyumov-Gerasimenko of a new quasi-3D approach for non-spherically shaped comet nuclei with the aim to interpret the current activity of the comet in terms of initial characteristics and to predict shape and internal stratification evolution of the nucleus. The model is applied to differently shaped nuclei taking into account the characteristics of Comet 67P/Churyumov-Gerasimenko deduced from observations. We focus our attention on the combined effects that shapes and obliquity have on the comet surface and sub-surface evolution. We discuss the results in terms of activity, local dust mantle formation and disruption, erosion of the surface and internal stratigraphy.The results show that differently shaped nuclei can have different internal structures leading to different activity patterns and behaviors. Our calculations have shown that local variations in the dust and gas fluxes can be induced by the nucleus shape. The distribution of “active” areas on Comet 67P/Churyumov-Gerasimenko is different because of different shapes, reflecting the illumination conditions on the surface. These shapes can influence the structure of the inner coma, but the coma far away from the nucleus is only marginally affected by the nucleus shape. However, different comet behaviors can arise from differently shaped comet nuclei, especially in terms of local activity, surface and sub-surface characteristics and properties. The water flux local distribution is the most influenced by the shape as it is directly linked to the illumination. Irregular shapes have large shadowing effects that can result in activity patterns on the comet surface.The effects of different pole directions are discussed to see the relations with the nucleus activity and internal structure. It is shown that the orientation of the rotation axis plays a strong role on the surface evolution of 67P/Churyumov-Gerasimenko, determining seasonal effects on the fluxes. The activity of the comet changes greatly with the nucleus obliquity leading to pre-post-perihelion differences in the activity and seasonal effects. The effects of the dust deposition and crust formation on the cometary activity have also been simulated and are discussed with respect to 67P/Churyumov-Gerasimenko observations. The dust mantling is also strongly obliquity dependent, with different surface distributions of the dust-covered regions according to the different comet pole orientations. Finally, we show that our model can reproduce the fluxes behavior near perihelion in terms of amplitude and asymmetry, and we estimate 20% of the illuminated surface to be active.     

Physical conditions in the plasma tail of comet C/1987 P1 bradfield

Photometric measurements of photographic images of comet C/1987 P1 Bradfield have been carried out with a flat-bed scanner equipped with a slide module. Lengthwise and transverse photometric profiles of the cometary plasma tail have been obtained. Magnetic field induction and some other physical characteristics of the cometary plasma tail observed in November 1987 have been estimated with the use of the diffusion model for a cometary tail by Shul’man and Nazarchuk (1968). It has been shown that the scanned images of comets can be used for estimating the physical characteristics of cometary tails.     

Local subgiants and time-scales of disc formation

This paper concentrates on the relationship between the rate of gas emission from the nucleus of Comet 9P/Tempel 1, the fraction f of the nucleus that is active, and the crater damage inflicted by the recent 2005 July 4 Deep Impact space mission. The cometary nucleus has a surface area of about  1.7 × 10⁸ m²  and a mean radius of about 3700 m. Before the impact it is estimated that only a fraction f = 0.0056 of the nucleus surface was actively producing gas and dust. The active area was about  9.4 × 10⁵ m²  . Absolute magnitudes obtained at recent perihelion passages of this comet indicate that variations in the 0.0074 > f > 0.0039 range can occur from apparition to apparition. Because of the low size of the original active area, the production of a new impact crater in the diameter range 40 to 300 m would lead to a long-term change in the cometary visual magnitude in the range 0.0018 to 0.098 respectively. This is below the limit of detectability. It has been suggested that the cometary dust is in the form of 'talcum powder' not 'beach sand'. We suggest that the dust ejected from the impact site has been broken up by the energetic impact process and thus has a different size distribution from dust locked in the snowy matrix of the nucleus and normally lifted off the nucleus by gentle sublimation processes.     

Cometary dust trail associated with Rosetta mission target: 67P/Churyumov-Gerasimenko

A thin, bright dust cloud, which is associated with the Rosetta mission target object (67P/Churyumov-Gerasimenko), was observed after the 2002 perihelion passage. The neckline structure or dust trail nature of this cloud is controversial. In this paper, we definitively identify the dust trail and the neckline structure using a wide-field CCD camera attached to the Kiso 1.05-m Schmidt telescope. The dust trail of 67P/Churyumov-Gerasimenko was evident as scattered sunlight in all images taken between September 9, 2002 and February 1, 2003, whereas the neckline structure became obvious only after late 2002. We compared our images with a semi-analytical dynamic model of dust grains emitted from the nucleus. A fading of the surface brightness of the dust trail near the nucleus enabled us to determine the typical maximum size of the grains. Assuming spherical compact particles with a mass density of 10³ kg m⁻³ and an albedo of 0.04, we deduced that the maximum diameter of the dust particles was approximately 1 cm. We found that the mass-loss rate of the comet at the perihelion was on or before the 1996 apparition, while the mass-loss rate averaged over the orbit reached . The result is consistent with the studies of the dust cloud emitted in the 2002/2003 return. Therefore, we can infer that the activity of 67P/Churyumov-Gerasimenko has showed no major change over the past dozen years or so, and the largest grains are cyclically injected into the dust tube lying along the cometary orbit.