Cometary Nuclei Research Articles

Development and testing of a pyro-driven launcher for harpoon-based comet sample acquisition

The CORSAIR (COmet Rendezvous, Sample Acquisition, Investigation, and Return) mission was a proposal for the NASA New Frontiers program. It belongs to the Comet Surface Sample Return mission theme which focuses on acquiring and returning to Earth a macroscopic sample from the surface of a comet nucleus. CORSAIR uses a harpoon-based Sample Acquisition System (SAS) with the spacecraft hovering several meters above the comet surface. This stand-off strategy overcomes disadvantages of other systems such as drills. Since comets are low gravity objects, those techniques would require anchoring before sampling, which is not necessary here. Moreover, the harpoon-based system allows for acquiring several samples from different locations on the comet maximizing the scientific output of the mission.Each SAS assembly consists of a pyro-driven launcher, a Sample Acquisition and Retrieval Projectile (SARP) and a retraction system using a deployable composite boom structure. In order to collect enough cometary material, the launcher has to provide the required kinetic energy to the SARP. Due to high energy densities, pyrotechnically actuated devices ultimately reduce the overall system mass and dimensions. First, the scientific and technological background of the CORSAIR mission is explained. Then, an overview of the development, design and testing of the launcher is given. Finally, the launcher theory is introduced explaining the entire reaction chain: initiation → gas dynamics → SARP motion.

Secular light curves of NEAs 2201 Oljato, 3200 Phaethon, 99942 Apophis, 162173 Ryugu, 495848 and 6063 Jason

Using the Secular Light Curve (SLC) formalism (Ferr\'in, 2010a, PSS, 58, 365-391), we searched for cometary activity of six NEAS: 2201 Oljato. The SLC exhibits significant cometary activity and evidence of an eclipse. 3200 Phaethon. Is the parent of the Geminid meteor stream and exhibited a faint tail at perihelion in several oppositions. The SLC fails to show cometary activity using a 1995 to 2017 data set, suggesting that we are in the presence of a dormant cometary nucleus. 99942 Apophis. This SLC does not shows evidence of cometary activity, however it shows evidence of an eclipse. The evidence of a double nature is confirmed with Goldstone and Arecibo radar observations. 162173 Ryugu. This NEA is the target of the Hayabusa 2 spacecraft mission. The SLC exhibits an enhancement consistent with cometary activity. It may be possible to confirm or deny this activity by the end of the mission. 495848= 2002 QD7. This NEA shows deviations from a flat distribution near perihelion in 2007 and 2017. However our own observations in 2018 show a bare nucleus, from which we obtain an absolute nuclear magnitude mV(nucleus) = 18.32+-0.03 (using pV=0.04). The equivalent diameter is D=1.43+-0.10 km. 6063 Jason. The SLC of this NEA exhibits low level cometary activity centred at perihelion. The fact that a cursory search for low level active comets among NEAs has uncovered four positive objects out of 6, hints that many more faint comets should exist as yet undetected.

Photometric observations of ecliptic comet 47P/Ashbrook–Jackson and selected quasi-Hilda and main-belt comets at Kyiv Comet Station (MPC code–585) in 2017

We present an analysis of the photometric data of comets 47P/Ashbrook–Jackson, 65P/Gunn, 362P/2008 GO98, and P/2017 S5 (ATLAS) observed at Kyiv Comet Station (MPC code–585) in 2017. The upper limits of radii for cometary nuclei, Afρ parameters, and color-indexes are measured.

Annihilation of relative equilibria in the gravitational field of irregular-shaped minor celestial bodies

The rotational speeds of irregular-shaped minor celestial bodies can be changed by the YORP effect. This variation in speed can make the numbers, positions, stabilities, and topological cases of the minor body's relative equilibrium points vary. The numbers of relative equilibrium points can be reduced through the collision and annihilation of relative equilibrium points, or increase through the creation and separation of relative equilibrium points. Here we develop a classification system of multiple annihilation behaviors of the equilibrium points for irregular-shaped minor celestial bodies. Most minor bodies have five equilibrium points; there are twice the number of annihilations per equilibrium point when the number of equilibrium points is between one and five. We present the detailed annihilation classifications for equilibria of objects which have five equilibrium points. Additionally, the annihilation classification for the seven equilibria of Kleopatra-shaped objects and the nine equilibria of Bennu-shaped objects are also discussed. Equilibria of different objects fall into different annihilation classifications. By letting the rotational speeds vary, we studied the annihilations and creations of relative equilibria in the gravitational field of ten minor bodies, including eight asteroids, one satellite of a planet, and one cometary nucleus: the asteroids were 216 Kleopatra, 243 Ida, 951 Gaspra, 1620 Geographos, 2063 Bacchus, 2867 Steins, 6489 Golevka, and 101955 Bennu; the satellite of the planet was S16 Prometheus; and the comet was 1682 Q1/Halley. For the asteroid 101955 Bennu, which has the largest number of equilibria among the known asteroids, we find that the equilibrium points with different indices approach each other as the rotational speed varies and thus annihilate each other successively. The equilibrium in the gravitational field and the smooth surface equilibrium collides when the equilibrium touches the surface of the body.

Regional unit definition for the nucleus of comet 67P/Churyumov-Gerasimenko on the SHAP7 model

The previously defined regions on the nucleus of comet 67P/Churyumov-Gerasimenko have been mapped back onto the 3D SHAP7 model of the nucleus (Preusker et al., 2017). The resulting regional definition is therefore self-consistent with boundaries that are well defined in 3 dimensions. The facets belonging to each region are provided as supplementary material. The shape model has then been used to assess inhomogeneity of nucleus surface morphology within individual regions. Several regions show diverse morphology. We propose sub-division of these regions into clearly identifiable units (sub-regions) and a comprehensive table is provided. The surface areas of each sub-region have been computed and statistics based on grouping of unit types are provided. The roughness of each region is also provided in a quantitative manner using a technique derived from computer graphics applications. The quantitative method supports the sub-region definition by showing that differences between sub-regions can be numerically justified.

The big lobe of 67P/Churyumov–Gerasimenko comet: morphological and spectrophotometric evidences of layering as from OSIRIS data

Between 2014 and 2016, ESA’s Rosetta OSIRIS cameras acquired multiple-filters images of the layered nucleus of comet 67P/Churyumov–Gerasimenko, ranging from ultraviolet to near-infrared wavelengths. No correlation between layers disposition and surface spectral variegation has been observed so far. This paper investigates possible spectral differences among decametre-thickness outcropping layers of the biggest lobe of the comet by means of OSIRIS image dataset. A two-classes maximum likelihood classification was applied on consolidated outcrops and relative deposits identified on post-perihelion multispectral images of the big lobe. We distinguished multispectral data on the basis of the structural elevation of the onion-shell Ellipsoidal Model of 67P. The spatial distribution of the two classes displays a clear dependence on the structural elevation, with the innermost class resulting over 50 per cent brighter than the outermost one. Consolidated cometarymaterials located at different structural levels are characterized by different brightness and revealed due to the selective removal of large volumes. This variegation can be attributed to a different texture of the outcrop surface and/or to a different content of refractory materials.

3D radar wavefield migration of comet interiors

Imaging the interior structure of small planetary bodies facilitates a deep understanding of their origin and evolution, thus addressing fundamental questions about the formation of the Solar System. We show that high resolution 3D imaging of their interior structure is possible using radar waves that reflect from internal discontinuities of dielectric properties. A radar imaging mission at a comet nucleus would have the benefit of orbiting all around a finite and transparent body, collecting echoes that derive only from the target, and processing them collectively in phase.As is the case in the medical field, imaging a comet nucleus requires its illumination from multiple directions, which can be accomplished with a spacecraft in slow polar orbit around the studied object. Long acquisition time leads to a dense acquisition array resembling that of conventional synthetic aperture radar systems, but completely surrounding the nucleus (4π steradians). Acquisition with an orbiter at large distance from the comet nucleus (5× the mean diameter) results in relatively coarse data sampling relative to the radar wavelength, thus enabling 3D imaging with a short (<90 days) mission.Radar migration is performed using techniques adapted from terrestrial exploration seismology. Wavefield migration identifies interior reflectors by time reversal of monostatic radar data redatumed to the known comet surface for computational speed-up. This technique is applicable to nuclei of arbitrary shape and interior complexity, and it is complementary to wavefield tomography tasked with constraining physical properties in-between the interior interfaces. Migration resolution is identical in all directions (range and cross-range) and it is equivalent to the range resolution of the radar system. Least-squares migration enhances resolution further by deconvolution of the radar wavelet from the reflectivity image.

Implications of the small spin changes measured for large Jupiter-family comet nuclei

Rotational spin-up due to outgassing of comet nuclei has been identified as a possible mechanism for considerable mass-loss and splitting. We report a search for spin changes for three large Jupiter-family comets (JFCs): 14P/Wolf, 143P/Kowal-Mrkos, and 162P/Siding Spring. None of the three comets has detectable period changes, and we set conservative upper limits of 4.2 (14P), 6.6 (143P) and 25 (162P) minutes per orbit. Comparing these results with all eight other JFCs with measured rotational changes, we deduce that none of the observed large JFCs experiences significant spin changes. This suggests that large comet nuclei are less likely to undergo rotationally-driven splitting, and therefore more likely to survive more perihelion passages than smaller nuclei. We find supporting evidence for this hypothesis in the cumulative size distributions of JFCs and dormant comets, as well as in recent numerical studies of cometary orbital dynamics. We added 143P to the sample of 13 other JFCs with known albedos and phase-function slopes. This sample shows a possible correlation of increasing phase-function slopes for larger geometric albedos. Partly based on findings from recent space missions to JFCs, we hypothesise that this correlation corresponds to an evolutionary trend for JFCs. We propose that newly activated JFCs have larger albedos and steeper phase functions, which gradually decrease due to sublimation-driven erosion. If confirmed, this could be used to analyse surface erosion from ground and to distinguish between dormant comets and asteroids.

On the origin of internal layers in comet nuclei

We present an analysis of the layered structure on 67P/Churyumov–Gerasimenko’ s Hathor cliff and propose a mechanism (self-sustaining, dual-mode propagation of amorphous to crystalline ice phase-change fronts into the nucleus interior) for its origin. If this is a viable mechanism, the strata must be geologically young (∼106 y). The Hathor cliff exposes strata and orthogonal prominent linear structures deep into the small lobe over a radial distance of ∼650 m. We find an average radial thickness of a stratum (defined as the upper stratum boundary and the associated intra-strata material below) of ∼14 m and estimate its average horizontal dimension at 290 ± 110 m. Structures in intra-strata material are highly varied ranging from thin, often distorted, layers (∼1 - 3 m thick), to block-like and globular-shaped features (typically ∼1.5 m radius). Prominent lineaments are typically separated by ∼80 ± 30 m and are ∼5 m, or less, wide.Guided by the results of calculations by Tancredi, Rickman and Greenberg (1994, Astron. Astrophys. 286, 659 – 682) on the dual-mode propagation of such fronts in a simulated comet nucleus, we propose that the bi-modal rates of propagation of these fronts (an essentially stationary “quiescent” mode lasting ∼10 y (or more), alternating with an “active” rapid spurt propagating at ∼100 my−1 over ∼10 - 20 m (or more) in a few months) leads to the establishment of alternating strata boundaries (in the quiescent mode), and the intra-strata material (during the active mode). To achieve the apparent global coordination of the strata we hypothesize that the direction of propagation of the fronts are controlled by the radial outflow of CO coupled with the likely existence of a coarse layered structure in the primeval material below the front.Prominent lineaments were formed after the strata, and we propose that they are the result of tensional stress due to CO and other super-volatile gases released during the phase-change process. The array of different structural forms found in the intra-strata material of 67P are postulated to be the result of various modes of fluidization of the material during the active propagation mode.The independence of the strata systems in the two lobes of 67P (Massironi and 58 colleagues. 2015. Nature 526, 402 – 405) is explained by an assumed, low speed, break-up of the nucleus from a bi-lobate shape into a low energy close binary, soon after the time of onset of crystallization, as the surface is weakened. Eventually, the comet returns to a physically connected bi-lobate shape well after the crystallization of the interior of both lobes is complete (in about ∼2000 y). This “phase-change mechanism” during the Centaur phase may lead to internal layer geometries that vary from one Jupiter Family Comet (JFC) nucleus to another depending on their original shape and internal structure.

Bi-lobed Shape of Comet 67P from a Collapsed Binary

Abstract The Rosetta spacecraft observations revealed that the nucleus of comet 67P/Churyumov–Gerasimenko consists of two similarly sized lobes connected by a narrow neck. Here, we evaluate the possibility that 67P is a collapsed binary. We assume that the progenitor of 67P was a binary and consider various physical mechanisms that could have brought the binary components together, including small-scale impacts and gravitational encounters with planets. We find that 67P could be a primordial body (i.e., not a collisional fragment) if the outer planetesimal disk lasted ≲10 Myr before it was dispersed by migrating Neptune. The probability of binary collapse by impact is ≃30% for tightly bound binaries. Most km-class binaries become collisionally dissolved. Roughly 10% of the surviving binaries later evolve to become contact binaries during the disk dispersal, when bodies suffer gravitational encounters with Neptune. Overall, the processes described in this work do not seem to be efficient enough to explain the large fraction (∼67%) of bi-lobed cometary nuclei inferred from spacecraft imaging.

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.

Cometary ion dynamics observed in the close vicinity of comet 67P/Churyumov–Gerasimenko during the intermediate activity period

Aims.Cometary ions are constantly produced in the coma, and once produced they are accelerated and eventually escape the coma. We describe and interpret the dynamics of the cometary ion flow, of an intermediate active comet, very close to the nucleus and in the terminator plane.Methods.We analysed in situ ion and magnetic field measurements, and characterise the velocity distribution functions (mostly using plasma moments). We propose a statistical approach over a period of one month.Results.On average, two populations were observed, separated in phase space. The motion of the first is governed by its interaction with the solar wind farther upstream, while the second one is accelerated in the inner coma and displays characteristics compatible with an ambipolar electric field. Both populations display a consistent anti-sunward velocity component.Conclusions.Cometary ions born in different regions of the coma are seen close to the nucleus of comet 67P/Churyumov–Gerasimenko with distinct motions governed in one case by the solar wind electric field and in the other case by the position relative to the nucleus. A consistent anti-sunward component is observed for all cometary ions. An asymmetry is found in the average cometary ion density in a solar wind electric field reference frame, with higher density in the negative (south) electric field hemisphere. There is no corresponding signature in the average magnetic field strength.

The sublimative torques of Jupiter Family Comets and mass wasting events on their nuclei

Sublimative outgassing of comets produces torques that alter the rotation state of their nuclei. Recently, parameterized sublimative torque models have been developed to study rotation state changes of individual comet nuclei and populations of cometary bodies. However, these models simplify the interactions between the escaping gas and cometary surface into only a few parameters that hide the details of these complex interactions. Here we directly compare the X-parameter model (Samarasinha and Mueller, 2013) with the SYORP model (Steckloff and Jacobson, 2016) to tease out insights into the details of the gas-surface interactions driving sublimative torques. We find that, for both of these models to accurately model sublimative torques, the number of sublimating molecules that contribute to the net torque is largely independent of the detailed shape and activity of the nucleus, but rather depends primarily on the size of the nucleus and the effective heliocentric distance of the comet. We suggest that cometary activity must be largely restricted to regions of steep gravitational surface slopes (above the angle of repose), where mass wasting can refresh activity by shedding mantles of refractory materials and exposing fresh volatiles. We propose a new classification scheme for comets based on the frequency of this mass-wasting process (relative to the timescale of activity fading): quasi-equilibrium, episodic, quasi-dormant, and extinct.

Cometary Dust.

This review presents our understanding of cometary dust at the end of 2017. For decades, insight about the dust ejected by nuclei of comets had stemmed from remote observations from Earth or Earth's orbit, and from flybys, including the samples of dust returned to Earth for laboratory studies by the Stardust return capsule. The long-duration Rosetta mission has recently provided a huge and unique amount of data, obtained using numerous instruments, including innovative dust instruments, over a wide range of distances from the Sun and from the nucleus. The diverse approaches available to study dust in comets, together with the related theoretical and experimental studies, provide evidence of the composition and physical properties of dust particles, e.g., the presence of a large fraction of carbon in macromolecules, and of aggregates on a wide range of scales. The results have opened vivid discussions on the variety of dust-release processes and on the diversity of dust properties in comets, as well as on the formation of cometary dust, and on its presence in the near-Earth interplanetary medium. These discussions stress the significance of future explorations as a way to decipher the formation and evolution of our Solar System.

3D radar wavefield tomography of comet interiors

Answering fundamental questions about the origin and evolution of small planetary bodies hinges on our ability to image their surface and interior structure in detail and at high resolution. The interior structure is not easily accessible without systematic imaging using, e.g., radar transmission and reflection data from multiple viewpoints, as in medical tomography. Radar tomography can be performed using methodology adapted from terrestrial exploration seismology. Our feasibility study primarily focuses on full wavefield methods that facilitate high quality imaging of small body interiors. We consider the case of a monostatic system (co-located transmitters and receivers) operated in various frequency bands between 5 and 15 MHz, from a spacecraft in slow polar orbit around a spinning comet nucleus. Using realistic numerical experiments, we demonstrate that wavefield techniques can generate high resolution tomograms of comets nuclei with arbitrary shape and complex interior properties.

Comets in UV

Comets are important "eyewitnesses" of Solar System formation and evolution. Important tests to determine the chemical composition and to study the physical processes in cometary nuclei and coma need data in the UV range of the electromagnetic spectrum. Comprehensive and complete studies require for additional ground-based observations and in-situ experiments. We briefly review observations of comets in the ultraviolet (UV) and discuss the prospects of UV observations of comets and exocomets with space-born instruments. A special refer is made to the World Space Observatory-Ultraviolet (WSO-UV) project.

Physical activity of the selected nearly isotropic comets with perihelia at large heliocentric distance

Context.The systematic investigation of comets in a wide range of heliocentric distances can contribute to a better understanding of the physical mechanisms that trigger activity at large distances from the Sun and reveals possible differences in the composition of outer solar system bodies belonging to various dynamical groups.Aims.We seek to analyze the dust environment of the selected nearly isotropic comets with a perihelion distance between 4.5 and 9.1 au, where sublimation of water ice is considered to be negligible.Methods.We present results of multicolor broadband photometric observations for 14 distant active objects conducted between 2008 and 2015 with various telescopes. Images obtained with broadband filters were used to investigate optical colors of the cometary comae and to quantify physical activity of the comet nuclei.Results.The activity level was estimated withAfρparameters ranging between 95 ± 10 cm and 9600 ± 300 cm. Three returning comets were less active than the dynamically new comets. Dust production rates of the comet nuclei were estimated between 1 and 100 kg s−1based on some assumptions about the physical properties of dust particles populating comae. The measured colors point out reddening of the continuum for all the comets. The mean values of a normalized reflectivity gradient within the group of the comets amount to 14 ± 2% per 1000 Å and 3 ± 2% per 1000 Å in the BV and VR spectral domains, respectively. The comae of the dynamically new comets, which were observed on their inbound legs, may be slightly redder in the blue spectral interval than comae of the comets observed after the perihelion passages. The dynamically new comets observed both pre- and post-perihelion, seem to have higher production rates post-perihelion than pre-perihelion for similar heliocentric distances.

Evolution of H2O production in comet C/2012 S1 (ISON) as inferred from forbidden oxygen and OH emission

We present H2O production rates for comet C/2012 S1 (ISON) derived from observations of [O I] and OH emission during its inbound leg, covering a heliocentric distance range of 1.8–0.44 AU. Our production rates are in agreement with previous measurements using a variety of instruments and techniques and with data from the various observatories greatly differing in their projected fields of view. The consistent results across all data suggest the absence of an extended source of H2O production, for example sublimation of icy grains in the coma, or a source with spatial extent confined to the dimensions of the smallest projected field of view (in this case < 1000 km). We find that ISON had an active area of around 10 km2 for heliocentric distances Rh > 1.2 AU, which then decreased to about half this value from Rh = 1.2–0.9 AU. This was followed by a rapid increase in active area at about Rh = 0.6 AU, corresponding to the first of three major outbursts ISON experienced inside of 1 AU. The combination of a detected outburst in the light curve and rapid increase in active area likely indicates a major nucleus fragmentation event. The 5–10 km2 active area observed outside of Rh = 0.6 AU is consistent with a 50–100% active fraction for the nucleus, larger than typically observed for cometary nuclei. Although the absolute value of the active area is somewhat dependent on the thermal model employed, the changes in active area observed are consistent among models. The conclusion of a 50–100+% active fraction is robust for realistic thermal models of the nucleus. However the possibility of a contribution of a spatially unresolved distribution of icy grains cannot be discounted. As our [OI]-derived H2O production rates are consistent with values derived using other methods, we conclude that the contribution of O2 photodissociation to the observed [O I] emission is at most 5–10% that of the contribution of H2O for ISON. This is consistent with the expected contribution of O2 photodissociation if O2/H2O ∼ 4%, meaning [O I] emission can still be utilized as a reliable proxy for H2O production in comets as long as O2/H2O ≲ 4%, similar to the abundance measured by the ROSINA instrument on Rosetta at comet 67P/Churyumov–Gerasimenko.

Картографирование небесных тел со сдвоенной фигурой в проекциях трёхосного эллипсоида на примере бинарного ядра кометы 67P/Чурюмова–Герасименко

Картографирование небесных тел со сдвоенной фигурой в проекциях трёхосного эллипсоида на примере бинарного ядра кометы 67P/Чурюмова–Герасименко

Astrophysical studies in Tajikistan: from traditional approach over modeling to empirical properties

New approach developed in the Institute of Astrophysics, Academy of Sciences of the Republic of Tajikistan, aimed to determination of the relationships between Solar System minor bodies is considered. The method consists of modeling of objects motion in the past or in the future, calculation of theoretical parameters, identification of the theoretically predicted properties with the observed ones. Usage of the method allowed to recognize extinct cometary nuclei among the near-Earth asteroids and to establish the parent bodies of some known asteroid-meteoroid complexes.