Cometary Activity Research Articles

New or Increased Cometary Activity in (2060) 95P/Chiron

Abstract Asteroid Terrestrial-impact Last Alert System (ATLAS) observations of Centaur (2060) 95P/Chiron show a brightening in the apparent magnitude not due rotational light curves or phase effects. This onset or enhancement of cometary activity started after 2021 February 8 UT and continued in later observations from 2021 June 18 UT onward. Recent ATLAS observations and deeper follow-up imaging obtained using the Las Cumbres Observatory 1.0 m robotic telescope network find no confirmed signature of coma or tail-like features.

New constraints on the physical properties and dynamical history of Centaur 174P/Echeclus

ABSTRACT Since 2005 December, recurrent outbursts have been observed for Centaur 174/P Echeclus, confirming it is an active object. Thanks to a large number of photometric data obtained between 2001 April and 2019 December, we were able to compute a shape model of this object. We obtain a sidereal rotation period P = 26.785178 ± 10−6 h and six equally probable pole solutions, each with a large obliquity of the rotational axis (50° or more). We also find the object significantly elongated, with a semi-major axial ratio a/b = 1.32 (and b/c ∼ 1.1 but this second ratio is poorly constrained by the photometric data). Additionally, we present a detailed analysis of the dust emission from the 2016 outburst. Different colour maps are presented that reveal a change in dust colour, which becomes bluer with increasing cometocentric distance. A blue ring-like structure around the nucleus clearly visible in the images obtained on October 4 in the V-R spectral interval points out that the innermost near nucleus region is considerably redder than the surrounding coma. Different jets are also apparent, the main one being oriented southward. A detailed dynamical study is done to investigate past and future orbital elements. These elements appear stable in the period ≈1200 CE to ≈2900 CE. For a period of 12 000 yr the main conclusion is that Echeclus’ perihelion distance was greater than about 4 au, preventing it from following a typical cometary activity like a short-period comet. Close encounters with giant planets nevertheless prevent any study of orbital elements on longer timescale.

Ammonia snow lines and ammonium salts desorption

Context. The nitrogen reservoir in planetary systems is a long-standing problem. Some of the N-bearing molecules are probably incorporated into the ice bulk during the cold phases of the stellar evolution, and may be gradually released into the gas phase when the ice is heated, for example in active comets. The chemical nature of the N-reservoir should greatly influence how, when, and in what form N returns to the gas phase, or is incorporated into the refractory material forming planetary bodies. Aims. We present the study of the thermal desorption of two ammonium salts, ammonium formate and ammonium acetate, from a gold surface and from a water ice substrate. Methods. Temperature-programmed desorption experiments and Fourier transform infrared reflection spectroscopy were conducted to investigate the desorption behavior of ammonium salts. Results. Ammonium salts are semi-volatile species releasing neutral species as major components upon desorption, namely ammonia and the corresponding organic acid (HCOOH and CH3COOH), at temperatures higher than the temperature of thermal desorption of water ice. Their desorption follows a first-order Wigner-Polanyi law. We find the first-order kinetic parameters A = 7.7 ± 0.6 × 1015 s−1 and Ebind = 68.9 ± 0.1 kJ mol−1 for ammonium formate and A = 3.0 ± 0.4 × 1020 s−1 and Ebind = 83.0 ± 0.2 kJ mol−1 for ammonium acetate. The presence of a water ice substrate does not influence the desorption kinetics. Ammonia molecules locked in salts desorb as neutral molecules at temperatures much higher than previously expected, and that are usually attributed to refractory materials. Conclusions. The ammonia snow line has a smaller radius than the water snow line. As a result, the NH3/H2O ratio content in Solar System bodies can be a hint to where they formed and subsequently migrated.

Taurid complex smoking gun: Detection of cometary activity

Using the Secular Light Curve (SLC) formalism (Ferrín, 2010), we have catalogued 88 probable members of the Taurid Complex (TC). 51 of them have useful SLCs and 34 of these (67%) exhibit cometary activity. This high percentage of active asteroids gives support to the hypothesis of a catastrophe that took place during the Upper Paleolithic (Clube and Napier, 1984), when a large short-period comet, arriving in the inner Solar System from the Kuiper Belt, experienced, starting from 20 thousand years ago, a series of fragmentations that produced the present 2P/Encke comet, together with a large number of other members of the TC. The fragmentation of the progenitor body was facilitated by its heterogeneous structure (very similar to a rubble pile) and this also explains the current coexistence in the complex of fragments of different composition and origin. We have found that (2212) Hephaistos and 169P/NEAT are active and members of the TC with their own sub-group. Other components of the complex are groups of meteoroids, that often give rise to meteor showers when they enter the terrestrial atmosphere, and very probably also the small asteroid that in 1908 exploded in the terrestrial atmosphere over Tunguska. What we see today of the TC are the remnants of a very varied and numerous complex of objects, characterized by an intense past of collisions with the Earth which may continue to represent a danger for our planet.

Detection of volatiles undergoing sublimation from 67P/Churyumov-Gerasimenko coma particles using ROSINA/COPS

Context. In an earlier study, we reported that the ram gauge of the COmet Pressure Sensor (COPS), one of the three instruments of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), could be used to obtain information about the sublimating content of icy particles, made up of volatiles and conceivably refractories coming from comet 67P/Churyumov-Gerasimenko. Aims. In this work, we extend the investigation to the second COPS gauge, the nude gauge. In particular, we analyse the volume of the volatile content of coma particles, along with a search for possible dependencies between the nude gauge detection rate (i.e. the rate at which icy particles are detected by the nude gauge) and the position of the Rosetta spacecraft. We also investigate the correlations of the nude gauge detection rate with the quantities associated with cometary activity. Methods. We inspected the density measurements made by the nude gauge for features attributable to the presence of icy particles inside the instrument. These data were then analysed statistically based on the amplitude of the feature and on the position of the spacecraft at the time of detection. Results. Although it was not originally designed for such a purpose, the COPS nude gauge has been able to detect ~67 000 features generated by the sublimation of the volatile content of icy particles. The nude gauge detection rate follows a trend that is inversely proportional to the heliocentric distance. This result is interpreted as a confirmation of a possible relation between the nude gauge detection rate and cometary activity. Thus, we compared the former with parameters related to cometary activity and obtained significant correlations, indicating that the frequency of icy particle detection is driven by cometary activity. Furthermore, by representing the volatile part of the icy particles as equivalent spheres with a density of 1 g cm−3, we obtained a range of diameters between 60 and 793 nm, with the smaller ones (<390 nm in diameter) having a size distribution power index of −4.79 ± 0.26.

Observations of distant comet C/2011 KP36 (Spacewatch): photometry, spectroscopy, and polarimetry

Aims. The main objective of our study is to obtain new observational results for the active long-period comet C/2011 KP36 (Spacewatch). This comet has unusual orbital properties and moves at distances larger than 5 au from the Sun. Methods. We carried out extensive observations of comet C/2011 KP36 (Spacewatch) at the 6-m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences with the focal reducer SCORPIO-2. We obtained quasi-simultaneous long-slit spectra in the visible, as well as photometric and linear polarimetric images with the g-sdss and r-sdss filters on November 25, 2016 when the heliocentric and geocentric distances of the comet were 5.06 au and 4.47 au, respectively. We modeled the behavior of the color and polarization in the coma, considering the dust as an ensemble of polydisperse nonspherical particles. Results. Two strong jet-like structures in solar and antisolar directions and two short and narrow jet features in the perpendicular direction were revealed in the coma. Our simulations showed that the latter two jets originated from the same active area. We determined the orientation of the rotation axis of the nucleus and the position of three active areas. High activity of the comet was characterized by Afρ values of 1065 ± 11 cm in the g-sdss filter and 1264 ± 17 cm in the r-sdss filter. The comet was found to be rich in CO+, while there was no clear detection of CN, C3, C2, and N2+. The dust color g–r varies over the coma from about 0.2m to 0.7m, and the linear polarization degree from about −1% to −6% at the phase angle 9.6°. The color of the nucleus of comet C/2011 KP36 (Spacewatch) is ultrared, B – R = 1.9m ± 0.3m. Conclusions. The high variability of the observed characteristics over the coma of comet C/2011 KP36 (Spacewatch) indicates significant and variable activity of the nucleus with, probably, numerous small active areas. Together with the three identified large active areas, they are characterized by different combinations of water ice, CO2 ice, and refractory dust and sizes of their particles, which are in the micron-size range.

Spatial Distribution of Ultraviolet Emission from Cometary Activity at 67P/Churyumov-Gerasimenko

Abstract The Alice ultraviolet spectrograph on board the Rosetta orbiter provided the first near-nucleus ultraviolet observations of a cometary coma from arrival at comet 67P/Churyumov-Gerasimenko in 2014 August through 2016 September. The characterization of atomic and molecular emissions in the coma revealed the unexpected contribution of dissociative electron impact emission at large heliocentric distances and during some outbursts. This mechanism also proved useful for compositional analysis, and Alice observed many cases that suggested elevated levels of the supervolatile O2, identifiable in part to their emissions resulting from dissociative electron impact. In this paper, we present the first two-dimensional UV maps constructed from Alice observations of atomic emission from 67P during an increase in cometary activity on 2015 November 7–8. Comparisons to observations of the background coma and an earlier collimated jet are used to describe possible changes to the near-nucleus coma and plasma. To verify the mapping method and place the Alice observations in context, comparisons to images derived from the MIRO and VIRTIS-H instruments are made. The spectra and maps we present show an increase in dissociative electron impact emission and an O2/H2O ratio of ∼0.3 for the activity; these characteristics have been previously identified with cometary outbursts seen in Alice data. Further, UV maps following the increases in activity show the spatial extent and emission variation experienced by the near-nucleus coma, informing future UV observations of comets that lack the same spatial resolution.

The similarity of the interstellar comet 2I/Borisov to Solar System comets from high-resolution optical spectroscopy

Aims. 2I/Borisov (hereafter 2I) is the first visibly active interstellar comet observed in the Solar System, allowing us for the first time to sample the composition of a building block from another system. We report on the monitoring of 2I with the Ultraviolet-Visual Echelle Spectrograph, the high-resolution optical spectrograph of the ESO Very Large Telescope at Paranal, for four months from November 15, 2019 to March 16, 2020. Our goal is to characterise the activity and composition of 2I with respect to Solar System comets. Methods. We collected high-resolution spectra at 12 different epochs from 2.1 au pre-perihelion to 2.6 au post-perihelion. Results. On December 24 and 26, 2019, close to perihelion, we detected several OH lines of the 309 nm (0–0) band and derived a water production rate of 2.2 ± 0.2 × 1026 molecules s−1. The three [OI] forbidden oxygen lines were detected at different epochs and we derived a green-to-red doublet intensity ratio (G/R) of 0.31 ± 0.05 close to perihelion. The NH2 ortho and para lines from various bands were measured and allowed us to derive an ortho-to-para abundance ratio (OPR) of 3.21 ± 0.15, corresponding to an OPR and spin temperature of ammonia of 1.11 ± 0.08 and 31−5+10 K, respectively. These values are consistent with the values usually measured for Solar System comets. Emission lines of the radicals NH (336 nm), CN (388 nm), CH (431 nm), and C2 (517 nm) were also detected. Several FeI and NiI lines were identified and their intensities were measured to provide a ratio of log (NiI/FeI) = 0.21 ± 0.18, which is in agreement with the value recently found in Solar System comets. Conclusions. Our high spectral resolution observations of 2I/Borisov and the associated measurements of the NH2 OPR and the Ni/Fe abundance ratio are remarkably similar to Solar System comets. Only the G/R ratio is unusually high, but it is consistent with the high abundance ratio of CO/H2O found by other investigators.

Activity of the Jupiter co-orbital comet P/2019 LD2(ATLAS) observed with OSIRIS at the 10.4 m GTC

Context.The existence of comets with heliocentric orbital periods close to that of Jupiter (i.e., co-orbitals) has been known for some time. Comet 295P/LINEAR (2002 AR2) is a well-known quasi-satellite of Jupiter. However, their orbits are not long-term stable, and they may eventually experience flybys with Jupiter at very close range, close enough to trigger tidal disruptions like the one suffered by comet Shoemaker-Levy 9 in 1992.Aims.Our aim was to study the observed activity and the dynamical evolution of the Jupiter transient co-orbital comet P/2019 LD2(ATLAS) and its dynamical evolution.Methods.We present results of an observational study of P/2019 LD2carried out with the 10.4 m Gran Telescopio Canarias (GTC) that includes image analyses using a Monte Carlo dust tail fitting code to characterize its level of cometary activity, and spectroscopic studies to search for gas emission. We also presentN-body simulations to explore its past, present, and future orbital evolution.Results.Images of P/2019 LD2obtained on May 16, 2020, show a conspicuous coma and tail, but the spectrum obtained on May 17, 2020, does not exhibit any evidence of CN, C2, or C3emission. The comet brightness in a 2.6′′ aperture diameter isr′ = 19.34 ± 0.02 mag, with colors (g′−r′) = 0.78 ± 0.03, (r′−i′) = 0.31 ± 0.03, and (i′−z′) = 0.26 ± 0.03. The temporal dependence of the dust loss rate of P/2019 LD2can be parameterized by a Gaussian function having a full width at half maximum of 350 days, with a maximum dust mass loss rate of 60 kg s−1reached on August 15, 2019. The total dust loss rate from the beginning of activity until the GTC observation date (May 16, 2020) is estimated at 1.9 × 109kg. Comet P/2019 LD2is now an ephemeral co-orbital of Jupiter, following what looks like a short arc of a quasi-satellite cycle that started in 2017 and will end in 2028. On January 23, 2063, it will experience a very close encounter with Jupiter at perhaps 0.016 au; its probability of escaping the solar system during the next 0.5 Myr is estimated to be 0.53 ± 0.03.Conclusions.Photometry and tail model results show that P/2019 LD2is a kilometer-sized object, in the size range of the Jupiter-family comets, with a typical comet-like activity most likely linked to sublimation of crystalline water ice and clathrates. Its origin is still an open question. Our numerical studies give a probability of this comet having been captured from interstellar space during the last 0.5 Myr of 0.49 ± 0.02 (average and standard deviation), 0.67 ± 0.06 during the last 1 Myr, 0.83 ± 0.06 over 3 Myr, and 0.91 ± 0.09 during the last 5 Myr.

Compact pebbles and the evolution of volatiles in the interstellar comet 2I/Borisov

The interstellar traveler, 2I/Borisov, is the first clearly active extrasolar comet, ever detected in our Solar system. We obtained high-resolution interferometric observations of 2I/Borisov with the Atacama Large Millimeter/submillimeter Array (ALMA), and multi-color optical observations with the Very Large Telescope (VLT) to gain a comprehensive understanding of the dust properties of this comet. We found that the dust coma of 2I/Borisov consists of compact "pebbles" of radii exceeding ~1 mm, suggesting that the dust particles have experienced compaction through mutual impacts during the bouncing collision phase in the protoplanetary disk. We derived a dust mass loss rate of >= 200 kg/s and a dust-to-gas ratio >=3. Our long term monitoring of 2I/Borisov with VLT indicates a steady dust mass loss with no significant dust fragmentation and/or sublimation occurring in the coma. We also detected emissions from carbon monoxide gas (CO) with ALMA and derived the gas production rate of Q(CO) (3.3+/-0.8)x10^{26} mole/s. We found that the CO/H$_2$O mixing ratio of 2I/Borisov changed drastically before and after perihelion, indicating the heterogeneity of the cometary nucleus, with components formed at different locations beyond the volatile snow-line with different chemical abundances. Our observations suggest that 2I/Borisov's home system, much like our own system, experienced efficient radial mixing from the innermost parts of its protoplanetary disk to beyond the frost line of CO.

Unusual polarimetric properties for interstellar comet 2I/Borisov

So far, only two interstellar objects have been observed within our Solar System. While the first one, 1I/‘Oumuamua, had asteroidal characteristics, the second one, 2I/Borisov, showed clear evidence of cometary activity. We performed polarimetric observations of comet 2I/Borisov using the European Southern Observatory Very Large Telescope to derive the physical characteristics of its coma dust particles. Here we show that the polarization of 2I/Borisov is higher than what is typically measured for Solar System comets. This feature distinguishes 2I/Borisov from dynamically evolved objects such as Jupiter-family and all short- and long-period comets in our Solar System. The only object with similar polarimetric properties as 2I/Borisov is comet C/1995 O1 (Hale-Bopp), an object that is believed to have approached the Sun only once before its apparition in 1997. Unlike Hale-Bopp and many other comets, though, comet 2I/Borisov shows a polarimetrically homogeneous coma, suggesting that it is an even more pristine object.

Cometary Activity Begins at Kuiper Belt Distances: Evidence from C/2017 K2

Abstract We study the development of activity in the incoming long-period comet C/2017 K2 over the heliocentric distance range 9 ≲ r H ≲ 16 au. The comet continues to be characterized by a coma of submillimeter-sized and larger particles ejected at low velocity. In a fixed co-moving volume around the nucleus we find that the scattering cross section of the coma, C, is related to the heliocentric distance by a power law, , with heliocentric index s = 1.14 ± 0.05. This dependence is significantly weaker than the r H −2 variation of the insolation as a result of two effects. These are, first, the heliocentric dependence of the dust velocity and, second, a lag effect due to very slow-moving particles ejected long before the observations were taken. A Monte Carlo model of the photometry shows that dust production beginning at r H ∼ 35 au is needed to match the measured heliocentric index, with only a slight dependence on the particle size distribution. Mass-loss rates in dust at 10 au are of order 103 kg s−1, while loss rates in gas may be much smaller, depending on the unknown dust to gas ratio. Consequently, the ratio of the nongravitational acceleration to the local solar gravity, α′, may, depending on the nucleus size, attain values of ∼10−7 ≲ α′ ≲ 10−5, comparable to values found in short-period comets at much smaller distances. Nongravitational acceleration in C/2017 K2 and similarly distant comets, while presently unmeasured, may limit the accuracy with which we can infer the properties of the Oort cloud from the orbits of long-period comets.

Contemporaneous Multiwavelength and Precovery Observations of the Active Centaur P/2019 LD2 (ATLAS)

Abstract The Gateway Centaur and Jupiter co-orbital P/2019 LD2 (ATLAS) provides the first opportunity to observe the migration of a solar system small body from a Centaur orbit to a Jupiter Family Comet (JFC) four decades from now. The Gateway transition region is beyond where water ice can power cometary activity, and coma production there is as poorly understood as in all Centaurs. We present contemporaneous multiwavelength observations of LD2 from 2020 July 2–4: Gemini North visible imaging, NASA IRTF near-infrared spectroscopy, and ARO Submillimeter Telescope millimeter-wavelength spectroscopy. Precovery DECam images limit the nucleus’s effective radius to ≤1.2 km and no large outbursts were seen in archival Catalina Sky Survey observations. LD2's coma has g ′ − r ′ = 0.70 ± 0.07 , r ′ − i ′ = 0.26 ± 0.07 , a dust-production rate of ∼10–20 kg s−1, and an outflow velocity between v ∼ 0.6–3.3 m s−1. We did not detect CO toward LD2 on 2020 July 2–3, with a 3σ upper limit of Q(CO) < 4.4 × 1027 mol s−1 (⪅ 200 kg s−1). Near-infrared spectra show evidence for water ice at the 1%–10% level depending on grain size. Spatial profiles and archival data are consistent with sustained activity. The evidence supports the hypothesis that LD2 is a typical small Centaur that will become a typical JFC, and thus, it is critical to understanding the transition between these two populations. Finally, we discuss potential strategies for a community-wide, long-baseline monitoring effort.

Exocometary Activity Around Stars at Different Evolutionary Stages: Current Issues

Modern theories of planetary system formation predict a large population of planetesimals, which are remnants of the primordial matter of the protoplanetary cloud and, at the same time, embryos of small bodies that we observe in our solar system. The planetesimals play an important role in the dynamic and physical evolution of the planetary system. Gravitational scattering of planetesimals which are enriched with volatile elements might cause the volatile and organic compounds to enter the interior of the planetary system, triggering the formation of planetary atmospheres and further development of life. Small bodies inside planetary systems can evaporate due to the increasing insolation at close distances from the mother star, leading to the development of activity akin to the activity of comets in our solar system. The study of cometary activity in our solar system is aimed primarily at the investigation of the physical processes in the early stages of the development of the protoplanetary cloud. Until recently, it was not possible to study small bodies in other planetary systems because their small size makes it very difficult to detect by direct methods. Over the past 10 years, two space missions, Kepler and TESS (The Transiting Exoplanet Survey Satellite), have been equipped for continuous photometric monitoring to find exoplanets by transit, i. e. monitoring the changes in brightness of a star due to the passage of a smaller object across its disk. The presence of high-precision photometric measurements of the luminosity curves of about 200 000 stars, which are available in the public domain, potentially makes it possible to identify rather small changes in the luminosity curves of stars due to the passage of a body with gas-dust coma (exocomet) across the stellar disk. The article considers a number of issues related to the discovery and study of exocomets. The main detection methods based on the analysis of photometric and spectral series of observational data of space missions as well as ground-based observational complexes are considered. We provide a brief review of the main projects devoted to the results of theoretical modeling and experimental studies of the manifestations of exocometary activity. Known cases of manifestations of exocometary activity in planetary systems of stars of different spectral classes are described and the main characteristics of such stars and their planetary systems are given. We discuss the prospects for further research of these still very exotic objects. The importance of such research for understanding evolutionary processes in our own solar system is emphasized.

Ion bulk speeds and temperatures in the diamagnetic cavity of comet 67P from RPC-ICA measurements

ABSTRACT Comets are constantly interacting with the solar wind. When the comet activity is high enough, this leads to the creation of a magnetic field free region around the nucleus known as the diamagnetic cavity. It has been suggested that the ion-neutral drag force is balancing the magnetic pressure at the cavity boundary, but after the visit of Rosetta to comet 67P/Churyumov–Gerasimenko the coupling between ions and neutrals inside the cavity has been debated, at least for moderately active comets. In this study, we use data from the ion composition analyser to determine the bulk speeds and temperatures of the low-energy ions in the diamagnetic cavity of comet 67P. The low-energy ions are affected by the negative spacecraft potential, and we use the Spacecraft Plasma Interaction Software to model the resulting influence on the detected energy spectra. We find bulk speeds of 5–10 km s−1 with a most probable speed of 7 km s−1, significantly above the velocity of the neutral particles. This indicates that the collisional coupling between ions and neutrals is not strong enough to keep the ions at the same speed as the neutrals inside the cavity. The temperatures are in the range 0.7–1.6 eV, with a peak probability at 1.0 eV. We attribute the major part of the temperature to the fact that ions are born at different locations in the coma, and hence are accelerated over different distances before reaching the spacecraft.

Anomalous Sun Flyby of 1I/2017 U1 (`Oumuamua)

The findings of Micheli et al. (Nature2018, 559, 223–226) that 1I/2017 U1 (`Oumuamua) showed anomalous orbital accelerations have motivated us to apply an impact model of gravity in search for an explanation. A small deviation from the 1/r potential, where r is the heliocentric distance, is expected for the gravitational interaction of extended bodies as a consequence of this model. This modification of the potential results from an offset of the effective gravitational centre from the geometric centre of a spherically symmetric body. Applied to anomalous Earth flybys, the model accounts for energy gains relative to an exact Kepler orbit and an increased speed of several spacecraft. In addition, the flat rotation profiles of eight disk galaxies could be explained, as well as the anomalous perihelion advances of the inner planets and the asteroid Icarus. The solution in the case of `Oumuamua is also based on the proposal that the offset leads to an approach and flyby trajectory different from a Kepler orbit without postulating cometary activity. As a consequence, an adjustment of the potential and centrifugal orbital energies can be envisaged outside the narrow uncertainty ranges of the published post-perihelion data without a need to re-analyse the original data. The observed anomalous acceleration has been modelled with respect to the orbit solutions JPL 16 and “Pseudo-MPEC” for 1I/`Oumuamua.

P/2019 LD2 (ATLAS): An Active Centaur in Imminent Transition to the Jupiter Family

Abstract The recently discovered object P/2019 LD2 (by the Asteroid Terrestrial-impact Last Alert System) was initially thought to be a Jupiter Trojan asteroid, until dynamical studies and the appearance of persistent cometary activity revealed that this object is actually an active Centaur. However, the dynamical history, thermal environment, and impact of such environments on the activity of 2019 LD2 are poorly understood. Here we conduct dynamical simulations to constrain its orbital history and resulting thermal environment over the past 3000 yr. We find that 2019 LD2 is currently in the vicinity of a dynamical “Gateway” that facilitates the majority of transitions from the Centaur population into the Jupiter Family of Comets (JFC population). Our calculations show that it is unlikely to have spent significant amounts of time in the inner solar system, suggesting that its nucleus is relatively pristine in terms of physical, chemical, and thermal processing through its history. This could explain its relatively high level of distant activity as a recently activated primordial body. Finally, we find that the median frequency of transition from the Gateway population into the JFC population varies from once every ∼3 yr to less than once every 70 yr, if 2019 LD2's nucleus is ∼1 km in radius or greater than 3 km in radius. Forward modeling of 2019 LD2 shows that it will transition into the JFC population in 2063, representing the first known opportunity to observe the evolution of an active Centaur nucleus as it experiences this population-defining transition.

Constraining the orientation of the spin axes of extrasolar minor bodies 1I/2017 U1 (‘Oumuamua) and 2I/Borisov

Context.The orientation of the spin axis of a comet is defined by the values of its equatorial obliquity and its cometocentric longitude of the Sun at perihelion. These parameters can be computed from the components of the nongravitational force caused by outgassing if the cometary activity is well characterized. The trajectories of known interstellar bodies passing through the Solar System show nongravitational accelerations.Aims.The spin-axis orientation of 1I/2017 U1 (‘Oumuamua) remains to be determined; for 2I/Borisov, the already released results are mutually exclusive. In both cases, the values of the components of the nongravitational force are relatively well constrained. Here, we investigate – within the framework of the forced precession model of a nonspherical cometary nucleus – the orientation of the spin axes of ‘Oumuamua and 2I/Borisov using public orbit determinations that consider outgassing.Methods.We applied a Monte Carlo simulation using the covariance matrix method together with Monte Carlo random search techniques to compute the distributions of equatorial obliquities and cometocentric longitudes of the Sun at perihelion of ‘Oumuamua and 2I/Borisov from the values of the nongravitational parameters.Results.We find that the equatorial obliquity of ‘Oumuamua could be about 93°, if it has a very prolate (fusiform) shape, or close to 16°, if it is very oblate (disk-like). Different orbit determinations of 2I/Borisov gave obliquity values of 59° and 90°. The distributions of cometocentric longitudes were in general multimodal.Conclusions.Our calculations suggest that the most probable spin-axis direction of ‘Oumuamua in equatorial coordinates is (280°, +46°) if very prolate or (312°, −50°) if very oblate. Our analysis favors a prolate shape. For the orbit determinations of 2I/Borisov used here, we find most probable poles pointing near (275°, +65°) and (231°, +30°), respectively. Although our analysis favors an oblate shape for 2I/Borisov, a prolate one cannot be ruled out.

Results of observations of dual-status object 2008 GO98 in 2017

We present the results of optical observations of the dual-status object 2008 GO98 (362P) performed at the Sanglokh Observatory of the Institute of Astrophysics, National Academy of Sciences of Tajikistan and at the Astronomical Observatory of the Taras Shevchenko National University of Kyiv (Ukraine) in July–September 2017. The asteroid showed cometary activity signs such as a dust coma and a tail, the presence of which was confirmed by the morphology analysis. The apparent and absolute magnitudes of the asteroid in the R band were measured. The absolute magnitude gradual decrease from 13.97 to 14.53 showed the activity decline during the observation period. The dust production rate decrease towards the end of the observation period also confirms the ceasing of the object’s activity. We estimated the upper limit of the diameter of the asteroid as ~7 km which is consistent with the available data. The new data support a cometary nature of the object. The equatorial coordinates of the asteroid were determined. The accuracy of astrometric measurements made it possible to calculate the orbit, the elements of which were consistent with the available orbital data. The asteroid activity did not affect the stability of the orbit.

Discrete element modeling of boulder and cliff morphologies on comet 67P/Churyumov-Gerasimenko

Context.Even after the Rosetta mission, some of the mechanical parameters of comet 67P/Churyumov-Gerasimenko’s surface material are not yet well constrained. These parameters are needed to improve our understanding of cometary activity or for planning sample return missions.Aims.We study some of the physical processes involved in the formation of selected surface features and investigate the mechanical and geometrical parameters involved.Methods.Applying the discrete element method (DEM) in a low-gravity environment, we numerically simulated the surface layer particle dynamics involved in the formation of selected morphological features. The material considered is a mixture of polydisperse ice and dust spheres with inter-particle forces given by the Hertz contact model, translational friction, rolling friction, cohesion from unsintered contacts, and optionally due to bonds from ice sintering. We determined a working set of parameters that enables the simulations to be reasonably realistic and investigated morphological changes due to modifications thereof.Results.The selected morphological features are reasonably well reproduced using model materials with a tensile strength on the order of 1–10 Pa. Increasing the diameters of the spherical particles decreases the material strength, and increasing the friction leads to a more brittle but somewhat stronger material. High friction is required to make the material sufficiently brittle to match observations, which points to the presence of very rough, even angular particles. Reasonable seismic activity does not suffice to trigger the collapses of cliffs without material heterogeneities or structural defects.Conclusions.DEM modeling can be a powerful tool to investigate mechanical parameters of cometary surface material. However, many uncertainties arise from our limited understanding of particle shapes, spatial configurations, and size distributions, all on multiple length scales. Further numerical work, in situ measurements, and sample return missions are needed to better understand the mechanics of cometary material and cometary activity.