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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., 12 pages, 4 figures

The discovery of the first interstellar comet 2I/Borisov confirmed the astronomers’ speculation that the passage through the solar system of relatively large objects (asteroids and comets) formed outside of it is a common occurrence. There may be many such objects, but their discovery is still a rare case. Large professional telescopes and existing observation programs with these telescopes have limitations that can be overcome with more modest instruments. The paper describes for the first time how comet 2I/Borisov was discovered with the 65-cm telescope designed by G.V. Borisov and those features of the observation program that made it possible to make this discovery. Recommendations are given for constructing future programs for detecting such bodies. The spatial density of interstellar objects (such as asteroid 1I/'Oumuamua or comet 2I/Borisov) in the solar neighborhood has been estimated. According to our estimate, there may be about 50 interstellar bodies >50 m in size in the Solar System in a sphere with a radius of 50 AU at any given time. Since the speeds of possible approach to the Earth of such objects can be very high, such probable approaches require special attention. The use of special space projects for their study is discussed.

We present results from multiband photometry of the interstellar comet 2I/Borisov (C/2019 Q4). The observations were carried out in the two months before its perihelion passage. The UBVri photometric data obtained for comet 2I were converted to its reflectance by means of observations of neighbouring solar analogues, which allowed the reflectance of the comet with wavelength to be calculated. The registered prominent changes in the reflectance spectra of 2I while it was approaching the Sun from 2.40 to 2.01 au, and numerical simulations of the light scattering by aggregate particles provide insight into the chemical-mineralogical and structural properties of the cometary dust particles. The close agreement between the trends observed in the spectral characteristics of 2I and some models suggests that, during the observations, conglomerates of magnesium-ferrous (and, probably, organic) submicron particles prevailed in the scattering by its matter, H2O ice particles induced no noticeable spectral signals, and the gas contribution to the scattering by the coma was relatively small.

Interstellar comets offer direct samples of volatiles from distant protoplanetary disks. 2I/Borisov is the first notably active interstellar comet discovered in our solar system[1]. Comets are condensed samples of the gas, ice, and dust that were in a star's protoplanetary disk during the formation of its planets and inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration moves volatiles[2], organic material, and prebiotic chemicals in their host system[3]. In our solar system, hundreds of comets have been observed remotely, and a few have been studied up close by space missions[4]. However, knowledge of extrasolar comets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars, with only one detection of carbon monoxide[5]. Here we report that the coma of 2I/Borisov contains significantly more CO than H2O gas, with abundances of at least 173%, more than three times higher than previously measured for any comet in the inner (

Periodic comets move around the Sun on elliptical orbits. As such comet 67P/Churyumov Gerasimenko (hereafter 67P) spends a portion of time in the inner solar system where it is exposed to increased solar insolation. Therefore given the change in heliocentric distance in case of 67P from aphelion at 5.68 AU to perihelion at similar to 1.24 AU the comet's activity the production of neutral gas and dust undergoes significant variations. As a consequence during the inbound portion the mass loading of the solar wind increases and extends to larger spatial scales. This paper investigates how this interaction changes the character of the plasma environment of the comet by means of multifluid MHD simulations. The multifluid MHD model is capable of separating the dynamics of the solar wind ions and the pick up ions created through photoionization and electron impact ionization in the coma of the comet. We show how two of the major boundaries the bow shock and the diamagnetic cavity form and develop as the comet moves through the inner solar system. Likewise for 67P although most likely shifted back in time with respect to perihelion passage this process is reversed on the outbound portion of the orbit. The presented model herein is able to reproduce some of the key features previously only accessible to particle based models that take full account of the ions' gyration. The results shown herein are in decent agreement to these hybrid type kinetic simulations.

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 UVES, the high resolution optical spectrograph of the ESO Very Large Telescope at Paranal, during four months from November 15, 2019 to March 16, 2020. Our goal is to characterize the activity and composition of 2I with respect to solar system comets. We collected high resolution spectra at 12 different epochs from 2.1 au pre-perihelion to 2.6 au post perihelion. 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 $2.2\pm0.2 \times 10^{26}$ molecules/s. The three [OI] forbidden oxygen lines were detected at different epochs and we derive a green-to-red doublet intensity ratio (G/R) of $0.31\pm0.05$ close to perihelion. NH$_2$ ortho and para lines from various bands were measured and allowed us to derive an ortho-to-para ratio (OPR) of $3.21\pm0.15$, corresponding to an OPR and spin temperature of ammonia of $1.11\pm0.08$ and $31^{+10}_{-5}$ 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 C$_2$ (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) = $0.21\pm0.18$ in agreement with the value recently found in solar system comets. Our high spectral resolution observations of 2I/Borisov and the associated measurements of the NH$_2$ OPR and the Ni/Fe abundance ratio are remarkably similar to solar system comets. Only the G/R ratio is unusually high but consistent with the high abundance ratio of CO/H$_2$O found by other investigators., Comment: Accepted for publication in A&A

Comet 2I/Borisov is the first true interstellar comet discovered. Here we present results from observational programs at two Indian observatories, 2 m Himalayan Chandra Telescope at the Indian Astronomical Observatory, Hanle (HCT) and 1.2 m telescope at the Mount Abu Infrared Observatory (MIRO). Two epochs of imaging and spectroscopy were carried out at the HCT and three epochs of imaging at MIRO. We found CN to be the dominant molecular emission on both epochs, 31/11/2019 and 22/12/2019, at distances of r$_H$ = 2.013 and 2.031 AU respectively. The comet was inferred to be relatively depleted in Carbon bearing molecules on the basis of low $C_2$ and $C_3$ abundances. We find the production rate ratio, Q($C_2$)/Q(CN) = 0.54 $\pm$ 0.18, pre-perihelion and Q($C_2$)/Q(CN) = 0.34 $\pm$ 0.12 post-perihelion. This classifies the comet as being moderately depleted in carbon chain molecules. Using the results from spectroscopic observations, we believe the comet to have a chemically heterogeneous surface having variation in abundance of carbon chain molecules. From imaging observations we infer a dust-to-gas ratio similar to carbon chain depleted comets of the Solar system. We also compute the nucleus size to be in the range $0.18\leq r \leq 3.1$ Km. Our observations show that 2I/Borisov's behaviour is analogous to that of the Solar system comets., Comment: Accepted for publication in MNRAS, 10 pages, 8 figures

Recent observations of the long-period comet C/2016 R2 (PanSTARRS; hereafter R2) indicate an unusually high N2/CO abundance ratio, typically larger than ∼0.05, and at least 2–3 times higher than the one measured in 67P/Churyumov–Gerasimenko. Another striking compositional feature of this comet is its heavy depletion in H2O (H2O/CO ∼ 0.32%), compared to other comets. Here we investigate the formation circumstances of a generic comet whose composition reproduces these two key features. We first envisage the possibility that this comet agglomerated from clathrates, but we find that such a scenario does not explain the observed low water abundance. We then alternatively investigate the possibility that the building blocks of R2 agglomerated from grains and pebbles made of pure condensates via the use of a disk model describing the radial transport of volatiles. We show that N2/CO ratios reproducing the value estimated in this comet can be found in grains condensed in the vicinity of the CO and N2 ice lines. Moreover, high CO/H2O ratios (>100 times the initial gas-phase value) can be found in grains condensed in the vicinity of the CO ice line. If the building blocks of a comet assembled from such grains, they should present N2/CO and CO/H2O ratios consistent with the measurements made in R2’s coma. Our scenario indicates that R2 formed in a colder environment than the other comets that share more usual compositions. Our model also explains the unusual composition of the interstellar comet 2l/Borisov.

On 31 August 2019, an interstellar comet was discovered as it passed through the Solar System (2I/Borisov). Based on initial imaging observations, 2I/Borisov appeared to be completely similar to ordinary Solar System comets - an unexpected characteristic after the multiple peculiarities of the only previous known interstellar visitor 1I/'Oumuamua. Spectroscopic investigations of 2I/Borisov identified the familiar cometary emissions from CN, C2, O I, NH2, OH, HCN, and CO, revealing a composition similar to that of carbon monoxide-rich Solar System comets. At temperatures >700 K, comets additionally show metallic vapors produced by the sublimation of metal-rich dust grains. However, due to the high temperature needed, observation of gaseous metals has been limited to bright sunskirting and sungrazing comets and giant star-plunging exocomets. Here we report spectroscopic detection of atomic nickel vapor in the cold coma of 2I/Borisov observed at a heliocentric distance of 2.322 au - equivalent to an equilibrium temperature of 180 K. Nickel in 2I/Borisov seems to originate from a short-lived nickelbearing molecule with a lifetime of $340^{+260}_{-200}$ s at 1 au and is produced at a rate of $0.9 \pm 0.3 \times 10^{22}$ atoms s$^{-1}$, or 0.002% relative to OH and 0.3% relative to CN. The detection of gas-phase nickel in the coma of 2I/Borisov is in line with the concurrent identification of this atom (as well as iron) in the cold comae of Solar System comets., Accepted manuscript with minor post-acceptance corrections

We will present the results of coordinated observations of 2I/Borisov with the Neil Gehrels-Swift observatory (Swift) and Hubble Space Telescope (HST), which allowed us to provide the first glimpse into the ice content and chemical composition of the protoplanetary disk of another star. Comets are condensed samples of the gas, ice and dust that were in a star’s protoplanetary disk during the formation of its planets, and inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration distributes volatiles [1], organic material and prebiotic chemicals around their host system [2]. In our Solar System, hundreds of comets have been observed remotely, and a few have been studied up close by space missions [3]. Similarly, interstellar comets offer a glimpse into the building blocks, formation, and evolution of other planetary systems. However, knowledge of extrasolar comets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars. 2I/Borisov, discovered in Aug. 2019, is the first notably active interstellar comet discovered in our Solar System [4]. We used the UltraViolet Optical Telescope (UVOT) of Swift to determine 2I/Borisov’s water production rates and dust content surrounding the nucleus at six epochs spaced before and after perihelion on Dec. 8.55, 2019 UTC (-2.56AU to 2.54AU) [5]. Water production rates increased steadily before perihelion at a rate of increase quicker than that of most dynamically new comets but slower than most Jupiter-family comets. After perihelion, the water production rate decreased much more rapidly than that of all previously observed comets. We used a sublimation model to constrain the active area and minimum radius of the nucleus, and found that a significant fraction of the surface of Borisov is active. We also used Cosmic Origins Spectrograph (COS) on the HST during four epochs around the perihelion and clearly detected the emissions of several bands of the CO Fourth Positive system, which we used to derive CO production rates [6]. Comparing these with the water production rates determined by Swift, we found that after perihelion, the coma of 2I/Borisov contains substantially more CO than H2O gas. Our abundances were more than three times higher than previously measured for any comet in the inner ( Fig. 1 Volatile production rates as a function of time relative to perihelion [6]. The production rates of CO measured with HST/COS (this work) and the water production rate measured by Swift (based on OH, open circles; 5) and the Very Large Telescope/UVES (based on OH, 8), and at the Apache Point Observatory (based on [OI], 9). Arrows indicate 3-σ upper limits, and error bars indicate 1-σ stochastic uncertainties. The grey line indicates the temporal trend of water production rates used to derive the elemental composition. References: [1] Cleeves, L. I. et al. The ancient heritage of water ice in the solar system. Science 345, 1590–1593 (2014). [2] Rubin, M., Bekaert, D. V., Broadley, M. W., Drozdovskaya, M. N., and Wampfler, S. F. Volatile Species in Comet 67P/Churyumov-Gerasimenko: Investigating the Link from the ISM to the Terrestrial Planets. ACS Publ. 3, 1792–1811 (2019). [3] Bockelée-Morvan, D. & Biver, N. The composition of cometary ices. Phil. Trans. R. Soc. A 375, 20160252–11 (2017). [4] Guzik, P. et al. Initial characterization of interstellar comet 2I/Borisov. Nature Astron. 4, 53 - 57 (2019). [5] Xing, Z., Bodewits, D., Noonan, J., and Bannister, M. T. Water Production Rates and Activity of Interstellar Comet 2I/Borisov. Astrophys. J. 893, L48 (2020). [6] Bodewits, D., Noonan, J. et al. The carbon monoxide-rich interstellar comet 2I/Borisov. Nature Astron. doi:10.1038/s41550-020-1095-2 (2020). [7] Cordiner, M.A., Milam, S.N., Biver, N. et al. Unusually high CO abundance of the first active interstellar comet. Nature doi:Astron.https://doi.org/10.1038/s41550-020-1087-2 (2020). [8] Lupu, R. E., Feldman, P. D., Weaver, H. A. & Tozzi, G.-P. The Fourth Positive System of Carbon Monoxide in the Hubble Space Telescope Spectra of Comets. Astrophys. J. 670, 1473-1484 (2007). [9] McKay, A. J., Cochran, A. L., Dello Russo, N. & DiSanti, M. A. Detection of a Water Tracer in Interstellar Comet 2I/Borisov. Astrophys. J. 889, L10 (2020)

During the journey through the solar system, interstellar comet 2I/Borisov has been observed spectroscopically by most of the largest telescopes on Earth, enabling comparative studies of its chemical composition versus solar system comets. Already a few weeks after the discovery, the detection of the CN (0-0 band) in the coma has been reported [1]. Subsequent detections of C2 suggested a significant depletion of this molecule [2,3], however, later evolution of C2 placed it close to the typical values [4]. Pre-discovery images of 2I/Borisov showing this object to be active when far away from the sun indicated that its activity is driven by low-temperature volatiles, later confirmed by the detection of a high abundance of CO [5,6]. High abundance was also reported for NH2 [4]. The water production rates derived from the detection of [OI] 6300 A line [7] were consistent with SWIFT/UVOT observations [8]. Here we report our spectroscopic observations of 2I/Borisov from VLT X-Shooter. We collected over 10 hours of data on UT 2020 January 28th, 30th, and 31st obtaining the deep spectrum of this object taken around the time of its maximum brightness. The spectrum covers an unprecedented wavelength range of 300 - 2500 nm that comprises numerous characteristic cometary emissions. The excellent sensitivity of the spectrum, combined with the decent spectral resolution provided by X-Shooter over the entire, enormous wavelength range makes the collected material unique. Acknowledgements Authors are grateful for support from the National Science Centre of Poland through SONATA BIS grant number 2016/22/E/ST9/00109 to M.D. References [1] Fitzsimmons, A., Hainaut, O., Meech, K. et al.: Detection of CN Gas in Interstellar Object 2I/Borisov, ApJL, Vol. 885, L9, (2019)[2] Kareta, T., Andrews, J., Noonan, J. et al.: Carbon Chain Depletion of 2I/Borisov, ApJL, Vol. 889, L38, (2020)[3] Lin, H., Lee, C., Gerdes, D. et al.: Detection of Diatomic Carbon in 2I/Borisov, ApJL, Vol. 889, L30, (2020)[4] Bannister, M., Opitom, C., Fitzsimmons, A. et al.: Interstellar comet 2I/Borisov as seen by MUSE: C2, NH2 and red CN detections, https://arxiv.org/abs/2001.11605, (2020)[5] Bodewits, D., Noonan, J., Feldman, P. et al.: The carbon monoxide-rich interstellar comet 2I/Borisov, NatAst, (in press)[6] Cordiner, M., Milam, S., Biver, N. et al.: Unusually high CO abundance of the first active interstellar comet, NatAst, (in press)[7] McKay, A., Cochran, A., Dello Russo, N. et al.: Detection of a Water Tracer in Interstellar Comet 2I/Borisov, ApJL, Vol. 889, L10, (2020)[8] Xing, Z., Bodewits, D, Noonan, J. et al.: Water Production Rates and Activity of Interstellar Comet 2I/Borisov, ApJL, Vol. 893, L48, (2020)

The interstellar comet 2I/Borisov was discovered on August 20, 2019. It is only the second interstellar object to be observed crossing our Solar System, and the first one for which outgassing was detected directly [1]. Early observations indicated that 2I/Borisov is depleted in C2, similarly to about 30% of Solar System comets [2,3]. Preliminary observations with the MUSE IFU performed in November 2019 confirmed that 2I is depleted in C2 but also showed it is rich in NH2 [4]. We present here results from the full observing campaign performed with the MUSE instrument. MUSE is a multi-unit integral field spectrograph mounted on the UT4 telescope of the VLT [5]. The instrument covers the wavelength range from 480 to 930 nm with a resolving power of about 3000. It has a large field of view of 1’x1’ and a spatial resolution of 0.2”, which makes it an ideal instrument to study extended sources. We observed 2I with MUSE on 16 different dates between November 14, 2019 and March 19, 2020. The observations started about one month before the perihelion passage and continued until the comet reached 3 au post-perihelion. This data sets constitutes a great opportunity to study the activity and coma composition of 2I over several months. Our observations allow us to detect emission bands from C2, NH2, and CN. Using a Haser model [6] we derive production rates for those 3 species and follow their evolution. We also study the evolution of the ratio between those production rates, to monitor how the composition of 2I coma changes as a function of time and distance from the Sun. References: [1] Fitzsimmons et al., 2019, The Astrophysical Journal Letters, Volume 885, Issue 1, article id. L9, 6 pp.[2] Opitom et al., 2019, Astronomy & Astrophysics, Volume 631, id.L8, 5 pp.; [3] Lin et al., 2019, The Astrophysical Journal Letters, Volume 889, Issue 2, id.L30;[4] Bannister et al, 2020, submitted to ApJ Letters; [5] Bacon et al, 2010, Proceedings of the SPIE, Volume 7735, id. 773508; [6] Haser, 1957,Bulletin de la Classe des Sciences de l'Académie Royale de Belgique, vol. 43, p. 740-750

In August 2019, 2I/Borisov, the second interstellar object and first visibly active interstellar comet, was discovered on a trajectory nearly perpendicular to the ecliptic. Observations of planet forming disks and debris disks serve as probes of the ensemble properties of extrasolar planetesimals, but the passage of an active interstellar comet through our Solar System provides a rare opportunity to individually study these small bodies up close in the same ways in which we investigate objects originating from our own Outer Solar System. Ground-based observations of short period comet 67P/Churyumov–Gerasimenko revealed a coma dust composition indistinguishable from what was measured on its nucleus by the orbiting Rosetta spacecraft. Therefore when 2/I Borisov had a dust dominated tail, we attempted to study its composition with near-simultaneous griJ photometry with the Gemini North Telescope. We obtained two epochs of GMOS-N and NIRI observations in November 2019, separated by two weeks. We will report on the inferred optical-near-IR colors of 2I/I Borisov’s dust coma/tail and nucleus. We will compare our measurements to other observations of 2I/Borisov and place the interstellar comet in context with the Col-OSSOS (Colours of the Outer Solar System Survey) sample of small KBOs and interstellar object ʻOumuamua observed in grJ with Gemini North, using the same setup.

VISIR Characterization of the Nucleus, Morphology, Activity, Spin-Pole Orientation & Rotation of Interstellar Comet 2I/Borisov by Earth- and Space-based Facilities Bryce T. Bolin (1,2), Carey M. Lisse (3), Mansi M. Kasliwal (1), Robert Quimby (4,5), Dennis Bodewits (6), Alessandro Morbidelli (7), George Helou (2) (1) Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA (bbolin@caltech.edu) (2) IPAC, Mail Code 100-22, Caltech, 1200 E. California Blvd., Pasadena, CA 91125, USA (3) Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA (4) Department of Astronomy, San Diego State University, 5500 Campanile Dr., San Diego, CA 92182, USA (5) Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan (6) Physics Department, Leach Science Center, Auburn University, Auburn, AL 36832, USA (7) Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Boulevard de l’Observatoire, CS 34229, F-06304 Nice cedex 4, France We present visible and near-infrared photometric and spectroscopic observations of interstellar object 2I/Borisov taken from 2019 September 10 to 2020 January 27 using ground-based facilities FROM the ZTF, the Keck Telescope, the GROWTH network, the APO ARC 3.5m, & the NASA/IRTF 3.0m combined high-resolution observations from HST. The photometry, taken in filters spanning the visible and NIR range shows 2I having a reddish object becoming neutral in the NIR. The lightcurve from recent and pre-discovery data reveals a brightness trend suggesting the recent onset of significant H2O sublimation with the comet being active with super volatiles such as CO at heliocentric distances >6 au consistent with its extended morphology (Fig. 1., left panel). Using the advanced capability to significantly reduce the scattered light from the coma enabled by high-resolution NIR images from Keck adaptive optics taken on 2019 October 04, we estimate a diameter of 2I's nucleus of 1 km (Boe et al. 2019). We combine our deep imaging of interstellar comet 2I/Borisov taken with the Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) on 2019 December 8 UTC and 2020 January 27 UTC (HST GO 16040, PI Bolin) before and after its perihelion passage in combination with HST/WFC3 images taken on 2019 October 12 UTC and 2019 November 16 UTC (HST GO/DD 16009, PI Jewitt) before its outburst and fragmentation of March 2020, thus observing the comet in a relatively undisrupted state. We locate 1-2” long (2,000 - 3,000 km projected length) jet-like structures near the optocenter of 2I that appear to change position angles from epoch to epoch (Fig. 2, left panel). With the assumption that the jet is located near the rotational pole, we determine that 2I's pole points near RA = 322 deg, dec = 37 deg, ecliptic longitude = 341 deg, ecliptic latitude = 48 deg (Fig 2., right panel). We find evidence for periodicity in the time-series lightcurve explained by a nucleus rotation period of ~10.6 h and small amplitude of ~0.05 implying a b/a axial ratio of ~1.5 when combined with our pole solution, unlike the b/a of 4 to 10 found for 1I/`Oumuamua (Bolin et al. 2018). Unless 2I's nucleus was References: Boe et al. 2019. Icarus, Volume 333, p. 252-272., Bolin et al. 2018. ApJL, 852, L2., Bolin et al. 2020a, AJ, 160, 16pp., Bolin et al. 2020b, ATel, 13613., Drahus et al. 2020, ATel, 13549., Fitzsimmons et al. 2019. ApJL, 885, L9., Jewitt et al. 2020, arxiv:2006.01242. Figure 1. Left panel: r magnitude of 2I as a function of the true anomaly using photometry taken between 2019 March 17 and 2019 November 29 UTC. The blue and orange lines are the predicted brightness as a function of true anomaly angle for H2O and CO-dominated activity comet from the out-gassing model of 2I from Fitzsimmons et al. 2019. Right panel: the size distribution of ISOs within 3 au of the Sun estimated from the detection of 1I with D~250 m and 2I with D~1.4 km. The solid grey line is fit to data with the function y = axb and is based on the estimated size of 1I from the literature and the average of the upper limits on the diameter of 2I assuming 0.04 and 0.1 albedo where a = 0.12 +/- 0.05 and b = 3.38 +/- 1.18. Figure 2. Left panel: Mosaic of radial profile normalized composite images of 2I taken with HST on 2019 October 12 UTC and 2019 November 16 UTC using the F350LP filter (HST GO/DD 16009, PI Jewitt) and on 2019 December 8 UTC and 2020 January 27 UTC using the F689M and F845M filters (HST GO 16040, PI Bolin) A south-facing jet is observed in the near-nucleus coma in the images taken on 2019 October 12 and 2019 November 16 and a south-west facing jet is observed in the December 8 UTC and 2020 January 27 images (outlined by the dotted purple wedges). Right panel: Pole/line-of-sight planes from our 4 observing dates. The intersection zone of around RA = 322 deg, dec = 37 deg in the grey square defines the rotation axis with an uncertainty of ~10 degs.

Interstellar comets penetrating through the Solar System had been anticipated for decades. The discovery of asteroidal-looking 'Oumuamua was thus a huge surprise and a puzzle. Furthermore, the physical properties of the 'first scout' turned out to be impossible to reconcile with Solar System objects, challenging our view of interstellar minor bodies. Here, we report the identification and early characterization of a new interstellar object, which has an evidently cometary appearance. The body was discovered by Gennady Borisov on 30 August 2019 UT and subsequently identified as hyperbolic by our data mining code in publicly available astrometric data. The initial orbital solution implies a very high hyperbolic excess speed of ~32 km/s, consistent with 'Oumuamua and theoretical predictions. Images taken on 10 and 13 September 2019 UT with the William Herschel Telescope and Gemini North Telescope show an extended coma and a faint, broad tail. We measure a slightly reddish colour with a g'-r' colour index of 0.66 +/- 0.01 mag, compatible with Solar System comets. The observed morphology is also unremarkable and best explained by dust with a power-law size-distribution index of -3.7 +/- 1.8 and a low ejection speed (44 +/- 14 m/s for $\beta$ = 1 particles, where $\beta$ is the ratio of the solar gravitational attraction to the solar radiation pressure). The nucleus is probably ~1 km in radius, again a common value among Solar System comets, and has a negligible chance of experiencing rotational disruption. Based on these early characteristics, and putting its hyperbolic orbit aside, 2I/Borisov appears indistinguishable from the native Solar System comets., Comment: Published in Nature Astronomy on 14 October 2019

We observed the interstellar comet 2I/Borisov using the Neil Gehrels-Swift Observatory's Ultraviolet/Optical Telescope. We obtained images of the OH gas and dust surrounding the nucleus at six epochs spaced before and after perihelion (-2.56 AU to 2.54 AU). Water production rates increased steadily before perihelion from $(7.0\pm1.5)\times10^{26}$ molecules s$^{-1}$ on Nov. 1, 2019 to $(10.7\pm1.2)\times10^{26}$ molecules s$^{-1}$ on Dec. 1. This rate of increase in water production rate is quicker than that of most dynamically new comets and at the slower end of the wide range of Jupiter-family comets. After perihelion, the water production rate decreased to $(4.9\pm0.9)\times10^{26}$ molecules s$^{-1}$ on Dec. 21, which is much more rapidly than that of all previously observed comets. Our sublimation model constrains the minimum radius of the nucleus to 0.37 km, and indicates an active fraction of at least 55% of the surface. $A(0)f\rho$ calculations show a variation between 57.5 and 105.6 cm with a slight trend peaking before the perihelion, lower than previous and concurrent published values. The observations confirm that 2I/Borisov is carbon-chain depleted and enriched in NH$_2$ relative to water., Comment: 12 pages, 3 figures, 2 tables, submitted to ApJL

On 30 August 2019 the amateur Borisov discovered a new comet; after few days it was clear from the characteristics of its orbit (eccentricity > 3 and high hyperbolic excess velocity) that the second interstellar object had been detected and the object received the name of 2I/Borisov. It appears to be very different from 1I/’Oumuamua and can be considered as the first interstellar comet. According to the first observations the comet had a nucleus with a radius of few km and a dust coma and tail due to the activity started in June 2019 (Jewitt et al., 2019). At the beginning of October we submitted the Discretionary Director Time (DDT) proposal to the TNG in order to monitor the comet. Some images have been acquired, in November and December 2019, with the DOLORES instrument in the R filter. We have applied the dust model described in Fulle et al. (2010), that has been tested on the comet 67P/Churyumov-Gerasimenko and validated with the Rosetta measurements. According to the results of our dust model and the activity model (Fulle et al., 2020) we derived a water flux from the nucleus of 8x10-6 kg m-2 s-1 and a dust loss rate of 35 and 30 kg s-1 in November and December 2019 respectively (Cremonese et al., 2020). This slight decrease has been observed around the perihelion on 8 December, few months later the comet fragmented. In this work we will describe the dust tail observations and the dust model results, even comparing them with the Jupiter family comet 67P. References: G.Cremonese, et al., 2020, ApJL, 893, L12 M.Fulle et al., 2010, A&A, 522, A63. M.Fulle et al., 2020, MNRAS, 493, 4039. Jewitt et al., 2019, ApJ, 886, L29.

Follow-up imaging observations of the first detected interstellar comet 2I/Borisov are presented, which were carried out with the Schmidt-Teleskop-Kamera at the University Observatory Jena in 11 observing epochs in October and November 2019. The orbital solution of the comet, derived from the obtained astrometric measurements, confirms its highly eccentric ($e=3.3570 \pm 0.0006$) and inclined ($i=44.0524 \pm 0.0004\,^\circ$) orbit, that proves the interstellar origin of the comet. According to our best-fitting orbital solution, comet 2I/Borisov reaches its perihelion on 2019 Dec 8 ($q=2.0066$ au) and its closest encounter with Earth on 2019 Dec 28 ($\Delta_{\text{min}}=1.9368$ au). The distance corrected brightness of the nucleus of the comet clearly exhibits a linear correlation with its phase angle. The slope of this correlation indicates the activity of the comet, which is also detected in deep imaging data, taken in 5 observing epochs, showing the coma and the tail of the comet. During our observing campaign the coma of comet 2I/Borisov exhibits on average a diameter of $(4.57\pm0.38) \cdot 10^4$ km, and the length of the tail of the comet measures $(1.52\pm0.12) \cdot 10^5$ km, assuming an anti-solar orientation., Comment: 14 pages, 4 figures, 8 tables, accepted for publication in AN

We present Hubble Space Telescope observations of a photometric outburst and splitting event in interstellar comet 2I/Borisov. The outburst, first reported with the comet outbound at 2.8 AU (Drahus et al.~2020), was caused by the expulsion of solid particles having a combined cross-section about 100 sq. km and a mass in 0.1 mm sized particles about 2e7 kg. The latter corresponds to 1e-4 of the mass of the nucleus, taken as a sphere of radius 500 m. A transient ``double nucleus'' was observed on UT 2020 March 30 (about three weeks after the outburst), having a cross-section about 0.6 sq. km and corresponding dust mass 1e5 kg. The secondary was absent in images taken on and before March 28, and in images taken on and after April 03. The unexpectedly delayed appearance and rapid disappearance of the secondary are consistent with an origin through rotational bursting of one or more large (meter-sized) boulders under the action of outgassing torques, following their ejection from the main nucleus. Overall, our observations reveal that the outburst and splitting of the nucleus are minor events involving a negligible fraction of the total mass: 2I/Borisov will survive its passage through the planetary region largely unscathed., 21 pages, 5 figures, 1 appendix

2I/Borisov is the first interstellar comet discovered on 2019 August 30, and it soon showed a coma and a dust tail. This study reports the results of images obtained at the Telescopio Nazionale Galileo telescope, on La Palma - Canary Islands, in 2019 November and December. The images have been obtained with the R filter in order to apply our dust tail model. The model has been applied to the comet 67P/Churyumov-Gerasimenko and compared to the Rosetta dust measurements showing a very good agreement. It has been applied to the comet 2I/Borisov, using almost the same parameters, obtaining a dust environment similar to that of 67P/Churyumov-Gerasimenko, suggesting that the activity may be very similar. The dust tail analysis provided a dust-loss rate Qd ≍ 35 kg s-1 in 2019 November and Qd ≍ 30 kg s-1 in 2019 December.

We present visible and near-infrared photometric and spectroscopic observations of interstellar object 2I/Borisov taken from 2019 September 10 to 2019 November 29 using the GROWTH, the APO ARC 3.5 m and the NASA/IRTF 3.0 m combined with post and pre-discovery observations of 2I obtained by ZTF from 2019 March 17 to 2019 May 5. Comparison with imaging of distant Solar System comets shows an object very similar to mildly active Solar System comets with an out-gassing rate of $\sim$10$^{27}$ mol/sec. The photometry, taken in filters spanning the visible and NIR range shows a gradual brightening trend of $\sim0.03$ mags/day since 2019 September 10 UTC for a reddish object becoming neutral in the NIR. The lightcurve from recent and pre-discovery data reveals a brightness trend suggesting the recent onset of significant H$_2$O sublimation with the comet being active with super volatiles such as CO at heliocentric distances $>$6 au consistent with its extended morphology. Using the advanced capability to significantly reduce the scattered light from the coma enabled by high-resolution NIR images from Keck adaptive optics taken on 2019 October 04, we estimate a diameter of 2I's nucleus of $\lesssim$1.4 km. We use the size estimates of 1I/'Oumuamua and 2I/Borisov to roughly estimate the slope of the ISO size-distribution resulting in a slope of $\sim$3.4$\pm$1.2, similar to Solar System comets and bodies produced from collisional equilibrium., 22 pages, 7 figures, 1 table. Accepted for publication in AJ on 12 May 2020

We present high resolution, deep imaging of interstellar comet 2I/Borisov taken with the Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) on 2019 December 8 UTC and 2020 January 27 UTC (HST GO 16040, PI Bolin) before and after its perihelion passage in combination with HST/WFC3 images taken on 2019 October 12 UTC and 2019 November 16 UTC (HST GO/DD 16009, PI Jewitt) before its outburst and fragmentation of March 2020, thus observing the comet in a relatively undisrupted state. We locate 1-2\arcsec~long (2,000 - 3,000 km projected length) jet-like structures near the optocenter of 2I that appear to change position angles from epoch to epoch. With the assumption that the jet is located near the rotational pole supported by its stationary appearance on $\sim$10-100 h time frames in HST images, we determine that 2I's pole points near $\alpha$ = 322$\pm$10$^\circ$, $\delta$ = 37$\pm$10$^\circ$ ($\lambda$ = 341$^\circ$, $\beta$ = 48$^\circ$) and may be in a simple rotation state. Additionally, we find evidence for possible periodicity in the HST time-series lightcurve on the time scale of $\sim$5.3 h with a small amplitude of $\sim$0.05 mag implying a lower limit on its $b/a$ ratio of $\sim$1.5 unlike the large $\sim$2 mag lightcurve observed for 1I/`Oumuamua. However, these small lightcurve variations may not be the result of the rotation of 2I's nucleus due to its dust-dominated light-scattering cross-section. Therefore, uniquely constraining the pre-Solar System encounter, pre-outburst rotation state of 2I may not be possible even with the resolution and sensitivity provided by HST observations., Comment: 14 pages, 6 figures and 3 tables, accepted for publication in MNRAS on 23 July 2020

2I/Borisov is the first-ever observed interstellar comet (and the second detected interstellar object). It was discovered on 30 August 2019 and has a heliocentric orbital eccentricity of ~ 3.35, corresponding to a hyperbolic orbit that is unbound to the Sun. Given that it is an interstellar object, it is of interest to compare its properties -- such as composition and activity -- with the comets in our Solar System. This study reports low-resolution optical spectra of 2I/Borisov. The spectra were obtained by the MDM observatory Hiltner 2.4m telescope/Ohio State Multi-Object Spectrograph (on 1 and 5 November 2019). The wavelength coverage spanned from 3700A to 9200A. The dust continuum reflectance spectra of 2I/Borisov show that the spectral slope is stepper in the blue end of the spectrum (compared to the red). The spectra of 2I/Borisov clearly show CN emission at 3880A, as well as C2 emission at both 4750A and 5150A. Using a Haser model to covert the observed fluxes into estimates for the molecular production rates, we find Q(CN) = 2.4 +/- 0.2 x 10^24 s^-1, and Q(C2) = 5.5 +/- 0.4 x 10^23 s^-1 at the heliocentric distance of 2.145 au. Our Q(CN) estimate is consistent with contemporaneous observations, and the Q(C2) estimate is generally below the upper limits of previous studies. We derived the ratio Q(C2)/Q(CN) = 0.2 +/- 0.1, which indicates that 2I/Borisov is depleted in carbon chain species, but is not empty. This feature is not rare for the comets in our Solar System, especially in the class of Jupiter Family Comets., 8 pages, 3 figures, accepted for publication in Astrophysical Journal Letters

The aim of this paper is to report on the results of a multicolour observing campaign performed at the LBT on the interstellar comet 2I/Borisov during three epochs while target was approaching its perihelion: UBVRI images have been observed in October and December 2019, when the comet was at ∼2.3 and 2.0 au from the Sun, respectively. Comet Borisov presented a rather complex morphology, especially in the December observing epoch, when a bi-lobate structure on a global scale was visible, with a jet-like structure characterised by a disconnection event, probably due to the presence of an active area on the nucleus' surface combined with its rotational properties. U – B colour, measured for the first time for this comet, monotonically decreases as Borisov approaches perihelion, from 0.35 ± 0.05 in October 19th to −0.07 ± 0.05 in December 2nd; B – V colour shows a more chaotic behaviour, with values for October epochs quite similar to solar colours, and a higher value of 0.76 ± 0.04 in December 2nd, probably due to a combination of an increase of the B flux (CN emission) and of an increase in V flux (due to C2). V – R and R–I colours derived in both the October and December observing epochs depict a scenario of a coma slightly redder than the Sun, with a RI-reddening value of ∼11%/1000 A and ∼5%/1000 A, respectively. The R- Afρ at the reference aperture of r ∼104 km from the optocentre slightly decreases from ∼100 cm (October) to ∼ 70 cm (December), showing that the Borisov's dust environment is quite similar to those of Short Period Comets of our Solar System. The application of a first-order photometric model, with the plausible scenario of grains with radius a = 100 μm and bulk density ρd = 1000 kg/m3 moving at vd in the range of 3–30 m/s, allows to derive a dust production rate Qd decrease from a range of 3–25 kg/s in October to a range of 2–18 kg/s in December. All these results confirm that the interstellar comet 2I/Borisov has a coma environment quite similar to that of the Short Period Comets (mostly Jupiter Family targets) orbiting in our inner Solar System.

FeI and NiI emission lines have recently been found in the spectra of 17 Solar System comets observed at heliocentric distances between 0.68 and 3.25 au and in the interstellar comet 2I/Borisov. The blackbody equilibrium temperature at the nucleus surface is too low to vaporize the refractory dust grains that contain metals, making the presence of iron and nickel atoms in cometary atmospheres a puzzling observation. Moreover, the measured NiI/FeI abundance ratio is on average one order of magnitude larger than the solar photosphere value. We report new measurements of FeI and NiI production rates and abundance ratios for the Jupiter-family comet (JFC) 46P/Wirtanen in its 2018 apparition and from archival data of the Oort-cloud comet (OCC) C/1996 B2 (Hyakutake). The comets were at geocentric distances of 0.09 au and 0.11 au, respectively. The emission line surface brightness was found to be inversely proportional to the projected distance to the nucleus, confirming that FeI and NiI atoms are ejected from the surface of the nucleus or originate from a short-lived parent. Considering the full sample of 20 comets, we find that the range of NiI/FeI abundance ratios is significantly larger in JFCs than in OCCs. We also unveil significant correlations between NiI/FeI and C$_2$/CN, C$_2$H$_6$/H$_2$O, and NH/CN. Carbon-chain- and NH-depleted comets show the highest NiI/FeI ratios. The existence of such relations suggests that the diversity of NiI/FeI abundance ratios in comets could be related to the cometary formation rather than to subsequent processes~in~the~coma., Comment: Accepted for publication in A&A Letters

This paper presents the results of a statistical analysis of the dynamic parameters of 300 comets that have osculating hyperbolic orbits. It is shown that such comets differ from other comets by their large perihelion distances and by a predominance of retrograde motion. It is shown that the values of i, the inclination of the hyperbolic comets, are in comparative excess over the interval 90–120°. The dominance by q, the perihelion distance, renders it difficult to suggest that the excess hyperbolic velocity of these comets can be the result of physical processes that take place in their nuclei. Aspects of the following working hypothesis, that the hyperbolic excess of parameter e might be formed after comets pass through meteoroid streams, are also studied. To evaluate this hypothesis, the distribution of the orbits of hyperbolic comets relative to the plane of motion of 112 established meteoroid streams are analyzed. The number (N) of orbit nodes for hyperbolic comets with respect to the plane of each stream at various distances is calculated. To determine the degree of redundancy of N, a special computing algorithm was applied that provided the expected value nav as well as the standard deviation σ for the number of cometary nodes at the plane of each stream. A comparative analysis of the N and nav values that take σ into account suggests an excess in 40 stream cases. This implies that the passage of comets through meteoroid streams can lead to an acceleration of the comets’ heliocentric velocity.

International audience; We suggest that the large organic molecules found in the dust of comet 67P/CG originated from the interstellar medium (ISM), and that this material is the source of absorption features in stellar spectra known as diffuse interstellar bands (DIBs). These large organic molecules were present in the ISM prior to the emergence of the pre-solar nebula and were conserved during comet formation in the solar nebula due to gentle, hierarchical accretion, a scenario based on many lines of evidences collected with Rosetta rendezvous mission (Davidsson et al. 2016). This is in contrast with the sublimation of H2O ice, according to diverse comet D/H values. While the organic to mineral mass ratios RC for comet Halley and 67P/CG were measured in the range ∼0.32–1.0, we estimate that the DIB carriers alone can provide RISM = 0.32, but that this ratio could be increased by other organic molecules in the ISM that do not show up in absorption. The decrease of DIB during line-of-sight crossings of the dense cores of interstellar clouds and the simultaneous steepening of the far-UV part of the reddening curve suggest that DIB carriers coagulate and are constituent of the very small grains eventually preserved in comets. This conclusion implies that a future sample-return mission of a comet nucleus would not only provide unique information on comets, but also on the exact nature of the interstellar species producing the hundreds of DIBs, an unanswered question since their discovery several decades ago.

In the past, the formation process of the distant comet Oort Cloud was simulated assuming the ejecting giant planets in their current orbits and with their current masses. Today, it is however known that the planets formed in a tighter configuration and the comet cloud could be replenished, mostly, in the era of their radial migration. We simulate the Oort-Cloud formation during the planetary migration in course to see if the formation efficiency is larger than that for the stable planetary orbits. As well, we try to answer the question if the ratio of the Oort-Cloud and scattered-disk populations is larger in the case of migration in course to alleviate the problem of too a low ratio in the past simulations. The formation efficiency in the simulations with migrating planets appears to be larger than in that with the planets in stable orbits, but only for the first 100 Myr. Later, it practically disappears. The simulations with the migrating planets result in a higher ratio of the Oort-Cloud and scattered-disk populations. However, this enhancement is only moderate and cannot explain the problem of too a low ratio of both populations.

Comet 2I/Borisov, the first unambiguous interstellar comet ever found, was discovered in August 2019 at $\sim3$ au from the Sun on its inbound leg. No pre-discovery detection beyond 3 au has yet been reported, mostly due to the comet's proximity to the Sun as seen from the Earth. Here we present a search for pre-discovery detections of comet Borisov using images taken by the Catalina Sky Survey (CSS), Pan-STARRS and Zwicky Transient Facility (ZTF), with a further comprehensive follow-up campaign being presented in \citet{Bolin2019}. We identified comet Borisov in ZTF images taken in May 2019 and use these data to update its orbit. This allowed us to identify the comet in images acquired as far back as December 2018, when it was 7.8 au from the Sun. The comet was not detected in November 2018 when it was 8.6 au from the Sun, possibly implying an onset of activity around this time. This suggests that the activity of the comet is either driven by a more volatile species other than H$_2$O, such as CO or CO$_2$, or by exothermic crystallization of amorphous ice. We derive the radius of the nucleus to be $, AJ in press

Scientists have spotted signs of water as the object, 2I/Borisov, streaks towards the Sun. Scientists have spotted signs of water as the object, 2I/Borisov, streaks towards the Sun.

In August 2019, a second interstellar object 2I/Borisov was discovered 2 years after the discovery of the first known interstellar object, 1I/'Oumuamua. Can we send a spacecraft to this object, using existing technologies? In this paper we assess the technical feasibility of a near-term mission to 2I/Borisov. We apply the Optimum Interplanetary Trajectory Software (OITS) tool to generate trajectories to 2I/Borisov. As results, we get the minimal $\Delta V$ trajectory with a launch date in July 2018. For this trajectory, a Falcon Heavy launcher could have hauled an 8 ton spacecraft to 2I/Borisov. For a later launch date, results for a combined powered Jupiter flyby with a Solar Oberth maneuver are presented. For a launch in 2027, we could reach 2I/Borisov in 2052, using the Space Launch System (SLS), up-scaled Parker probe heat shield technology, and solid propulsion engines. Using a SLS a spacecraft with a mass of 765 kg could be sent to 2I/Borisov. A Falcon Heavy could deliver 202 kg to 2I/Borisov. Arrival times sooner than 2052 can potentially be achieved but with higher $\Delta V$ requirements and lower spacecraft payload masses. 2I/Borisov's discovery shortly after the discovery of 1I/'Oumuamua implies that the next interstellar object might be discovered in the near future. The feasibility of a mission to both, 1I/'Oumuamua and 2I/Borisov using existing technologies indicates that missions to at least some future interstellar objects are feasible as well., Comment: 25 pages 5 figures

The detection of Interstellar Objects passing through the Solar System offers the promise of constraining the physical and chemical processes involved in planetary formation in other extrasolar systems. While the effect of outgassing by 1I/2017 U1 ('Oumuamua) was dynamically observed, no direct detection of the ejected material was made. The discovery of the active interstellar comet 2I/Borisov means spectroscopic investigations of the sublimated ices is possible for this object. We report the first detection of gas emitted by an interstellar comet via the near-UV emission of CN from 2I/Borisov at a heliocentric distance of $r$ = 2.7 au on 2019 September 20. The production rate was found to be Q(CN) = $(3.7\pm0.4)\times10^{24}$ s$^{-1}$, using a simple Haser model with an outflow velocity of 0.5 km s$^{-1}$. No other emission was detected, with an upper limit to the production rate of C$_2$ of $4\times10^{24}$ s$^{-1}$. The spectral reflectance slope of the dust coma over $3900$ \AA\ $< \lambda< 6000$ \AA \ is steeper than at longer wavelengths, as found for other comets. Broad band $R_c$ photometry on 2019 September 19 gave a dust production rate of $Af\rho=143\pm10$ cm. Modelling of the observed gas and dust production rates constrains the nuclear radius to $0.7-3.3$ km assuming reasonable nuclear properties. Overall, we find the gas, dust and nuclear properties for the first active Interstellar Object are similar to normal Solar System comets., Comment: 9 pages, 4 figures, accepted for publication in ApJL

The discovery of the first active interstellar object 2I/Borisov provides an unprecedented opportunity to study planetary formation processes in another planetary system. In particular, spectroscopic observations of 2I allow us to constrain the composition of its nuclear ices. We obtained optical spectra of 2I with the 4.2 m William Herschel and 2.5 m Isaac Newton telescopes between 2019 September 30 and October 13, when the comet was between 2.5 au and 2.4 au from the Sun. We also imaged the comet with broadband filters on 15 nights from September 11 to October 17, as well as with a CN narrow-band filter on October 18 and 20, with the TRAPPIST-North telescope. Broadband imaging confirms that the dust coma colours (B-V=0.82$\pm$0.02, V-R=0.46$\pm$0.03, R-I=0.44$\pm$0.03, B-R=1.28$\pm$0.03) are the same as for Solar System comets. We detect CN emission in all spectra and in the TRAPPIST narrow-band images with production rates between 1.6$\times10^{24}$ and 2.1$\times10^{24}$ molec/s. No other species are detected. We determine three-sigma upper limits for C$_2$, C$_3$, and OH production rates of 6$\times10^{23}$ molec/s, 3$\times10^{23}$ molec/s and 2$\times10^{27}$ molec/s, respectively, on October 02. There is no significant increase of the CN production rate or A(0)f$\rho$ during our observing period. Finally, we place a three-sigma upper limit on the Q(C$_2$)/Q(CN) ratio of 0.3 (on October 13). From this, we conclude that 2I is highly depleted in C$_2$, and may have a composition similar to Solar System carbon-chain depleted comets., Comment: Revised version, Accepted for publication in A&A Letters

Spectra and images of comet C/2006 OF2 (Broughton) were obtained with the 4.2-m William Herschel Telescope (La Palma, Spain) and with the 6-m Big Telescope Alt-azimuth (SAO RAN, Russia) at two observing epochs on October 15, 2007 and December 05, 2008.The comet was at heliocentric distances of 4.24 au and 2.59 au before and after the perihelion, respectively. The emissions CN 0–0, 0–1 (B2Σ+ − X2Σ+), C3 (A1Πu − X1Σg+) bands and C2 0–0 (A3Πg – X3Πu) of the Swan band system were identified in the post-perihelion cometary spectrum. Using the simple Haser model, the gas production rates of CN, C2 and C3 molecules were estimated at level of 1.35 × 1024 mol/s, 0.45 × 1024 mol/s, and 0.12 × 1024 mol/s, respectively. The gas production rate ratios, log(C2/CN) = −0.48 and log(C3/CN) = −1.05, point out that comet C/2006 OF2 probably belongs to the group of “carbon-chain depleted” comets. The ratios between C2, C3 and CN abundances of comet C/2006 OF2 are similar to those recently measured for interstellar comet 2I/Borisov at comparable heliocentric distances. The normalized reflectivity gradients of 12.7 ± 0.2%/1000 Å and − 2.3 ± 0.2%/1000 Å were calculated from the post perihelion spectrum in the intervals 4450 Å – 5500 Å and 5500–6935 Å, respectively, using the spectral regions free from comet emissions. The continuum slope, measured in the interval 4200 Å – 4800 Å, is even steeper, reaching 32.7 ± 0.2%/1000 Å. The normalized reflectivity gradient measured from a pair of the pre-perihelion V and R-images varies between 8%/1000 Å and 11%/1000 Å in the inner part of the coma, which exceeds the value obtained from the similar spectral interval of the post-perihelion spectrum, indicating that the coma became bluer after the perihelion passage. The morphology of the pre-perihelion images of the comet is governed by faint morphological features hidden in the bright coma. The color of the jets isolated from the pre-perihelion images is redder than the color of the ambient coma. The pre- and post-perihelion V-band Afρ parameters were found to be 1990 ± 90 cm and 2500 ± 100 cm for heliocentric distances of 4.24 au and 2.59 au, respectively. © 2020 Elsevier Inc., This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. The part of the data, which is paper based on, was obtained from the Isaac Newton Group Archive maintained as part of the CASU Astronomical Data Centre at the Institute of Astronomy, Cambridge. We also acknowledge the generous allocation of telescope time, which have been granted to us by Special Astrophysical Observatory RAN (Russia). L. M. Lara acknowledges the financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award SEV-2017-0709, and from the research projects ESP2016-76076-R and PGC2018-099425-B-I00. Finally, we thank the anonymous reviewers for their valuable advices resulting in improving of this paper.

The paper presents an analysis of the actual change in brightness of comet 2I/Borisov. The calculations take into account total scattering cross-sections coming from ice, dust particles, and their combinations. It was assumed that dust particles are so-called silicate agglomerates which consist of single monomers. We analyze the actual outburst of the interstellar comet 2I/Borisov in terms of mass loss. This parameter (mass ejected) is one of the main factors determining the amplitude of the comet’s brightness jump. A numerical model was developed and used to analyze the loss of mass ejected during the cometary outburst 2I/Borisov. The calculations presented in this paper are related to the actual outburst of comet 2I/Borisov which was observed in March 2020 with a change in brightness Δ m ​= ​−0.7 magnitude. The ratio of the mass of thrown dust to the mass of ejected ice, which determines the observed change in brightness, was also determined. In the case of water ice controlled sublimation, the ratio is between 9.51 and 8.16. However, in the case of the sublimation of carbon monoxide, this parameter takes values from 7.47 to 3.07. The values of these mass ratios are significantly related to the η(t1) parameter.

Comets have been invoked in numerous studies as a potentially important source of dust and gas around stars, but none has studied the thermo-physical evolution, out-gassing rate, and dust ejection of these objects in such stellar systems. We investigate the thermo-physical evolution of comets in exo-planetary systems in order to provide valuable theoretical data required to interpret observations of gas and dust. We use a quasi 3D model of cometary nucleus to study the thermo-physical evolution of comets evolving around a single star from 0.1 to 50 AU, whose homogeneous luminosity varies from 0.1 to 70 solar luminosities. This paper provides mass ejection, lifetimes, and the rate of dust and water gas mass productions for comets as a function of the distance to the star and stellar luminosity. Results show significant physical changes to comets at high stellar luminosities. The models are presented in such a manner that they can be readily applied to any planetary system. By considering the examples of the Solar System, Vega and HD 69830, we show that dust grains released from sublimating comets have the potential to create the observed (exo)zodiacal emission. We show that observations can be reproduced by 1 to 2 massive comets or by a large number of comets whose orbits approach close to the star. Our conclusions depend on the stellar luminosity and the uncertain lifetime of the dust grains. We find, as in previous studies, that exozodiacal dust disks can only survive if replenished by a population of typically sized comets renewed from a large and cold reservoir of cometary bodies beyond the water ice line. These comets could reach the inner regions of the planetary system following scattering by a (giant) planet., Comment: 21 pages, 10 figures

The outbursts of comets, sudden large increases in their luminosity, are still very interesting and mysterious signs of activity of this celestial bodies. Most of the cometary outbursts are taking place at heliocentric distances where sublimation of water ice plays an important role in the activity of comets. However, the phenomenon is also observed far away from the Sun (i.e. ≃5–20 au) where the sublimation of water ice is negligible and the activity of comets is dominated by more volatile chemicals. Not only typical comets show ‘cometary-like’ activity but also Centaurs. In addition to the long-term changes in brightness related to heliocentric distances and short-periodic variations connected with the rotation of the nucleus, they also exhibit a random variations in luminosity which are similar to the cometary outbursts. Paper presents an overview of the most likely hypotheses and models which try to explain this phenomenon.

Differential evolution of the orbits of near-Earth asteroid (NEA) 2003 EH1 and comet 96P/Machholz 1 under perturbing action of planets was investigated for the time interval of 28000 years. The similarity of the orbits was analyzed with the Southworth–Hawkins criterion DSH. It has been shown that both the comet and the asteroid can be fragments of a nucleus of the same larger comet being a progenitor of the Quadrantid complex. A break-up of the parent comet apparently occurred approximately 9500 years ago. NEA 2003 EH1 is actually a dormant fragment of a nucleus of the parent comet. It was concluded that comet 96P/Machholz 1, NEA (186256) 2003 EH1 of the Amore group, and the Quadrantid meteorite swarm form a family of related objects.

Recent advances in space exploration of comets which are the most primitive objects in the outer solar system and numerical simulations have brought new insights to the history of solar system formation. The new idea of the pebble accretion mechansim might lead to possible solutions to the rapid formation of the planetary system. The detection of a large amount of oxygen molecules in the coma of comet 67P/Churyumov-Gerasimenko has also important implication on the initial environment of the molecular cloud and the solar nebula. Finally, the presence of meteoritic material in the dust grains of comet 81P/Wild 2 has raised the possibility of large-scale mass transport in the early solar nebula mi=hich might be associated with magnetohydrodynamic process.

An overview is given of close encounters of nearly parabolic comets (NPCs; with periods of P > 200 years and perihelion distances of q > 0.1 AU; the number of the comets is N = 1041) with planets. The minimum distances Δmin between the cometary and planetary orbits are calculated to select comets whose Δmin are less than the radius of the planet’s sphere of influence. Close encounters of these comets with planets are identified by numerical integration of the comets’ equations of motion over an interval of ±50 years from the time of passing the perihelion. Close encounters of NPCs with Jupiter in 1663–2011 are reported for seven comets. An encounter with Saturn is reported for comet 2004 F2 (in 2001).

Observations of Comet C/2010 X1 (Elenin) in August 2011 showed a pattern of evolution of coma morphology into an expanding, amorphous cloud of rapidly decreasing surface brightness shortly before perihelion passage, similar to C/1999 S4 (LINEAR) in 2000. However, the comet’s difficult viewing geometry at the time of disruption made it impossible to study the event using high-resolution imaging techniques and, in particular, to measure the ratio of dust to cometessimals which gives information on the internal structure of the nucleus before disruption. We report Herschel/PACS observations of the defunct Comet C/2010 X1 (Elenin), aimed at detecting its infrared ghost: the debris cloud and any remaining inert, or low-activity cometessimals that may have been released in the break-up of the nucleus, taking advantage of the comet’s close pass to Earth in mid-October 2011. These observations would have allowed us to detect, potentially, both individual surviving inert fragments of the nucleus of ≈260 m diameter (and significantly smaller, active cometessimals) and the expanding dust cloud, being particularly sensitive to coarse dust and boulders in the millimetre to metre size range. No discrete bodies were detected by Herschel to this limiting size.

The principle of habitable planets creation of near stars in our galaxy was developed. It was shown that the Earth and the Moon formed as a result of thermal nuclear explosion (TNE) after a collision with a small comet. The comet has broken through the proto-Earth crust, and created conditions for liquid-drop division into the Earth and the Moon. It is shown that the comet impact on the proto-Earth led to formation of continents as well as to formation of the Moon, seas, oceans and atmosphere and to creation of conditions for life origin. The analysis of kimberlitic pipes distribution, as markers of comet fragments motion under a crust, shows that the comet has broken up under crust with formation of many gas bubbles. An analysis of the Martian topography revealed that the comet hit Mars not under a glancing angle. As a result, the products of TNE remained under the Marian crust. The track of the comet movement under the Venus crust was also established. It is shown that the planet surface near comet track is spotted by numerous cracks.

The discovery of the second interstellar object 2I/Borisov on 2019 August 30 raises the question of whether it was ejected recently from a nearby stellar system. Here we compute the asymptotic incoming trajectory of 2I/Borisov, based on both recent and pre-discovery data extending back to December 2018, using a range of force models that account for cometary outgassing. From Gaia DR2 astrometry and radial velocities, we trace back in time the Galactic orbits of 7.4 million stars to look for close encounters with 2I/Borisov. The closest encounter we find took place 910kyr ago with the M0V star Ross 573, at a separation of 0.068pc (90% confidence interval of 0.053-0.09pc) with a relative velocity of 23km/s. This encounter is nine times closer than the closest past encounter identified for the first interstellar object 1I/'Oumuamua. Ejection of 2I/Borisov via a three-body encounter in a binary or planetary system is possible, although such a large ejection velocity is unlikely to be obtained and Ross 573 shows no signs of binarity. We also identify and discuss some other recent close encounters, recognizing that if 2I/Borisov is more than about 10Myr old, our search would be unlikely to find its parent system., Comment: 7 pages; accepted to A&A

We present high spectral resolution optical spectra obtained with the ARCES instrument at Apache Point Observatory showing detection of the [OI]6300 A line in interstellar comet 2I/Borisov. We employ the observed flux in this line to derive an H$_2$O production rate of (6.3$\pm$1.5)$\times$10$^{26}$ mol s$^{-1}$. Comparing to previously reported observations of CN, this implies a CN/H$_2$O ratio of $\sim$0.3-0.6%. The lower end of this range is consistent with the average value in comets, while the upper end is higher than the average value for Solar System comets, but still within the range of observed values. C$_2$/H$_2$O is depleted, with a value likely less than 0.1%. The dust-to-gas ratio is consistent with the normal value for Solar System comets. Using a simple sublimation model we estimate an H$_2$O active area of 1.7 km$^2$, which for current estimates for the size of Borisov suggests active fractions between 1-150%, consistent with values measured in Solar System comets. More detailed characterization of 2I/Borisov, including compositional information and properties of the nucleus, is needed to fully interpret the observed H$_2$O production rate., 8 pages, 2 figures, 1 table, Accepted in ApJ Letters

We present high resolution imaging observations of interstellar comet 2I/Borisov (formerly C/2019 Q4) obtained using the Hubble Space Telescope. Scattering from the comet is dominated by a coma of large particles (characteristic size 0.1 mm) ejected anisotropically. Convolution modeling of the coma surface brightness profile sets a robust limit to the spherical-equivalent nucleus radius r_n < 0.5 km (geometric albedo 0.04 assumed). We obtain an independent constraint based on the non-gravitational acceleration of the nucleus, finding r_n > 0.2 km (nucleus density 500 kg/m3 assumed). The profile and the non-gravitational constraints cannot be simultaneously satisfied if density < 25 kg/m3; the nucleus of comet Borisov cannot be a low density fractal assemblage of the type proposed elsewhere for the nucleus of 1I/'Oumuamua. We show that the spin-up timescale to outgassing torques, even at the measured low production rates, is comparable to or shorter than the residence time in the Sun's water sublimation zone. The spin angular momentum of the nucleus should be changed significantly during the current solar fly-by. Lastly, we find that the differential interstellar size distribution in the 0.5 mm to 100 m size range can be represented by power laws with indices < 4 and that interstellar bodies of 100 m size scale strike Earth every one to two hundred million years., 24 Pages, 5 Figures