The low velocity of interstellar asteroid 1I/'Oumuamua with respect to our galaxy's Local Standard of Rest implies it is young. Adopting the young age hypothesis, we assess possible origin systems for this interstellar asteroid and for 2I/Borisov, though the latter's higher speed means it is unlikely to be young. First, their past trajectories are modelled under gravitational scattering by galactic components ('disk heating') to assess how far back one can trace them. The stochastic nature of disk heating means that a back-integration can only expect to be accurate to within 15 pc and 2 kms$^{-1}$ at -10 Myr, dropping steeply to 400 pc and 10 kms$^{-1}$ at -100 Myr, sharply limiting our ability to determine a precise origin. Nevertheless, we show `Oumuamua's origin system is likely currently within 1 kpc of Earth, in the local Orion Arm. Second, we back-integrate 'Oumuamua's trajectory to assess source regions, emphasizing young systems and moving groups. Though disk heating allows for only a statistical link to source regions, `Oumuamua passed through a considerable subset of the Carina and Columba moving groups when those groups were forming. This makes them perhaps the most plausible source region, if 'Oumuamua was ejected during planet formation or via intra-cluster interactions. We find three stars in the Ursa Major group, one brown dwarf, and seven other stars to have plausible encounters with 2I/Borisov, within 2 pc and 30 kms$^{-1}$. These encounters' high relative speeds mean none are likely to be the home of 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. 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. Here, we investigate the ori…
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 $ΔV$ 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…
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 addition…
The first known interstellar object, 1I/2017 U1 'Oumuamua, displayed such unusual properties that its origin remains a subject of much debate. We propose that 'Oumuamua's properties could be explained as those of a fractal dust aggregate (a "dust bunny") formed in the inner coma of a fragmenting exo-Oort cloud comet. Such fragments could serve as accretion sites by accumulating dust particles, resulting in the formation of a fractal aggregate. The fractal aggregate eventually breaks off from the fragment due to hydrodynamic stress. With their low density and tenuously bound orbits, most of the…
Observations of the first interstellar minor object 1I/2017 'Oumuamua did not reveal direct signs of outgassing that would have been natural if it had volatile-rich composition. However, a recent measurement by Micheli et al (2018) of a substantial non-gravitational acceleration affecting the orbit of this object has been interpreted as resulting from its cometary activity, which must be rather vigorous. Here we critically re-assess this interpretation by exploring the implications of measured non-gravitational acceleration for the 'Oumuamua's rotational state. We show that outgassing torques …
A 2000-2017 set of long-period comets with high-quality orbits of perihelion distance <1 AU is used to show that the objects that perish shortly before perihelion are nearly exclusively the Oort Cloud comets, especially those with perihelia within 0.6 AU of the Sun, intrinsically fainter, and dust poor. Their propensity for disintegration is much higher than predicted by Bortle's perihelion survival rule, prompting the author to propose a new synoptic index to be tested in future prognostication efforts. By their susceptibility to demise near the Sun, the nuclei of Oort Cloud comets differ dra…
2I/Borisov is the second interstellar object (ISO) after 'Oumuamua (Meech et al. 2017), but differs from 'Oumuamua drastically with its extensive cometary activity. A key ingredient to understand the nature of this comet is its size. However, due to its cometary activity and extended coma in the optical, only rough estimates and upper limits can be made for 2I/Borisov, ranging in a wide spread from 0.7 to 3.8 km (Guzik et al. 2019; Fitzsimmons et al. 2019; Jewitt, & Luu 2019; Bolin et al. 2019). It has been shown that observations at longer wavelengths (i.e. infrared) are less susceptible to t…
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 ($q=2.0066$ …
Interstellar interlopers are bodies formed outside of the solar system but observed passing through it. The first two identified interlopers, 1I/`Oumuamua and 2I/Borisov, exhibited unexpectedly different physical properties. 1I/`Oumuamua appeared unresolved and asteroid-like whereas 2I/Borisov was a more comet-like source of both gas and dust. Both objects moved under the action of non-gravitational acceleration. These interlopers and their divergent properties provide our only window so far onto an enormous and previously unknown galactic population. The number density of such objects is $\si…
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 …
`Oumuamua (1I/2017 U1) is the first object of interstellar origin observed in the Solar System. Recently, \citet{Micheli2018} reported that `Oumuamua showed deviations from a Keplerian orbit at a high statistical significance. The observed trajectory is best explained by an excess radial acceleration $Δa \propto r^{-2}$, where $r$ is the distance of `Oumuamua from the Sun. Such an acceleration is naturally expected for comets, driven by the evaporating material. However, recent observational and theoretical studies imply that `Oumuamua is not an active comet. We explore the possibility that th…
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 …
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 o…
The image of comet 2I/Borisov taken by the Hubble Space Telescope on October 12 shows a fan-like feature in the inner coma, which extends over a few arcseconds from the nuclear condensation, its axis pointing a little west of the north. It is proposed that this protrusion is a halo of centimeter-sized and larger dust grains or pebbles released at low velocities from the nucleus at large heliocentric distances (on the order of 10 AU or more) before perihelion and subjected to very low radiation pressure accelerations. Unlike potential microscopic dust from ongoing activity, this population of l…
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 a…
For the second discovered interstellar comet 2I/Borisov we searched for a candidate for its home system. We will never be sure which star or stellar system does this comet come from but obtaining a very small relative velocity and a promisingly small miss-distance, when tracing the motion of 2I/Borisov back in time in its movement through the space, makes an encountered body a good candidate for a source of this comet. In our long-standing project on studying Oort spike comets dynamics, we recently updated a list of potential stellar perturbers of cometary motion. This list was checked against…
The interstellar comet 2I/Borisov bears a strong resemblance to Oort Cloud comets, judging from its appearance in images taken over the first six weeks of observation. To test the proposed affinity in more diagnostic terms, 2I is compared to Oort Cloud comets of similar perihelion distance, near 2 AU. Eight such objects are identified among the cataloged comets whose orbits have been determined with high accuracy. This work focuses on three particular characteristics: the light curve, the geometry of the dust tail, and the dust parameter Afrho. Unlike Oort Cloud comets with perihelia beyond th…
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 bou…
Hubble Space Telescope observations of interstellar comet 2I/Borisov near perihelion show the ejection of large (>~100 um) particles at <~9 m/s speeds, with estimated mass-loss rates of ~35 kg/s. The total mass loss from comet Borisov corresponds to loss of a surface shell on the nucleus only ~0.4 m thick. This shell is thin enough to be susceptible to past chemical processing in the interstellar medium by cosmic rays, meaning that the ejected materials cannot necessarily be considered as pristine. Our high-resolution images reveal persistent asymmetry in the dust coma, best explained by a the…
'Oumuamua was discovered passing through our Solar System on a hyperbolic orbit. It presents an apparent contradiction, with colors similar to those of volatile-rich Solar System bodies but with no visible outgassing or activity during its close approach to the Sun. Here we show that this contradiction can be explained by the dynamics of planetesimal ejection by giant planets. We propose that 'Oumuamua is an extinct fragment of a comet-like planetesimal born in a planet-forming disk that also formed Neptune- to Jupiter-mass giant planets. On its pathway to ejection 'Oumuamua's parent body unde…
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-fami…
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 …
We processed images taken with the Hubble Space Telescope (HST) to investigate any morphological features in the inner coma suggestive of a peculiar activity on the nucleus of the interstellar comet 2I/Borisov. The coma shows an evident elongation, in the position angle (PA) ~0-180d direction, which appears related to the presence of a jet originating from a single active source on the nucleus. A counterpart of this jet directed towards PA ~10d was detected through analysis of the changes of the inner coma morphology on HST images taken in different dates and processed with different filters. …
Intrinsically faint comets in nearly-parabolic orbits with perihelion distances much smaller than 1 AU exhibit strong propensity for suddenly disintegrating at a time not long before perihelion, as shown by Bortle (1991). Evidence from available observations of such comets suggests that the disintegration event usually begins with an outburst and that the debris is typically a massive cloud of dust grains that survives over a limited period of time. Recent CCD observations revealed, however, that also surviving could be a sizable fragment, resembling a devolatilized aggregate of loosely-bound …
Micheli, et al. (2018) reported that a seven-parameter fit to the orbit of 1I/2017 U1 `Oumuamua indicated a non-gravitational acceleration in the anti-Solar direction, and attributed it to recoil from comet-like outgassing. The implied gas to dust ratio is at least 100 times greater than that of known Solar System comets. The reported collapse of the scatter of nearly contemporaneous coordinate residuals upon inclusion of the non-gravitational term in the orbital fits is difficult to understand. There are grounds for skepticism.
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 profil…
The composition of comets in the Solar System come in multiple groups thought to encode information about their formation in different regions of the outer protosolar disk. The recent discovery of the second interstellar object, 2I/Borisov, allows for spectroscopic investigations into its gas content and a preliminary classification of it within the Solar System comet taxonomies to test the applicability of planetesimal formation models to other stellar systems. We present spectroscopic and imaging observations from 2019 September 20th to October 26th at the Bok, MMT, and LBT telescopes. We id…
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…
The upcoming commencement of the Vera C. Rubin Observatory's Legacy Survey of Space of Time (LSST) will greatly enhance the discovery rate of interstellar objects (ISOs). `Oumuamua and Borisov were the first two ISOs confirmed in the Solar system, although the first interstellar meteor may have been discovered earlier. We discuss the properties of `Oumuamua and Borisov and explore the expected abundance of ISOs as a function of size in the solar neighborhood. We compare the expected abundance of ISOs to that of objects in the Oort cloud, and draw conclusions about the mass budget per star that…
C/2019 Q4 (Borisov), discovered by Gennady Borisov on August 30, 2019, is tentatively an interstellar comet. We show that a kilometer-scale nucleus would provide consistency with both `Oumuamua and CNEOS-2014-01-08, resulting in a single power-law distribution with an equal amount of mass per logarithmic bin of interstellar objects.
We present initial observations of the interstellar body 2I/(2019 Q4) Borisov taken to determine its nature prior to the perihelion in 2019 December. Images from the Nordic Optical Telescope show a prominent, morphologically stable dust coma and tail. The dust cross-section within 15,000 km of the nucleus averages 130 sq. km (assuming geometric albedo 0.1) and increases by about 1 percent per day. If sustained, this rate indicates that the comet has been active for 100 days prior to the observations. Cometary activity thus started in 2019 June, at which time C/Borisov was at 4.5 AU from the Su…
We present a study of interstellar comet 2I/2019 Q4 (Borisov) using both preperihelion and postperihelion observations spanning late September 2019 through late January 2020. The intrinsic brightness of the comet was observed to continuously decline throughout the timespan, likely due to the decreasing effective scattering cross-section as a result of volatile sublimation with a slope of $-0.43 \pm 0.02$ km$^{2}$ d$^{-1}$. We witnessed no significant change in the slightly reddish colour of the comet, with mean values of $\left \langle g - r \right \rangle = 0.68 \pm 0.04$, $\left \langle r - …
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 peri…
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 ou…
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 …
We simulate the passage through the Sun-Jupiter system of interstellar objects (ISOs) similar to 1I/`Oumuamua or 2I/Borisov. Capture of such objects is rare and overwhelmingly from low incoming speeds onto orbits akin to those of known long-period comets. This suggests that some of these comets could be of extra-solar origin, in particular inactive ones. Assuming ISOs follow the local stellar velocity distribution, we infer a volume capture rate of $0.051\,\mathrm{au}^3 \mathrm{yr}^{-1}$. Current estimates for orbital lifetimes and space densities then imply steady-state captured populations o…
In this work, we present the results of an observational study of 2I/Borisov carried out with the 10.4-m Gran Telescopio Canarias (GTC) and the 3.6-m Telescopio Nazionale Galileo (TNG), both telescopes located at the Roque de Los Muchachos Observatory, in the island of La Palma (Spain). The study includes images in the visible and near-infrared, as well as visible spectra in the 3600 - 9200 A wavelength range. N-body simulations were also performed to explore its orbital evolution and Galactic kinematic context. The comet's dust continuum and near-infrared colours are compatible with those obs…
The detection of interstellar bodies passing near the Sun offers the opportunity to observe not just objects similar to those in the solar system, but also unfamiliar objects without solar system analogues. Here I show that Asymptotic Giant Branch (AGB) stellar evolution may lead to the creation, out of stardust, of substantial numbers of nomadic Post-Main-Sequence Objects (PMSOs). ABG nucleosynthesis will produce three broad classes of PMSO chemistry, oxygen, carbon and nitrogen rich (O-rich, C-rich, N-rich, respectively), depending largely on the original stellar mass. I further show that th…
During the formation and evolution of the Solar System, significant numbers of cometary and asteroidal bodies were ejected into interstellar space$^{1,2}$. It can be reasonably expected that the same happened for planetary systems other than our own. Detection of such Inter- stellar Objects (ISOs) would allow us to probe the planetesimal formation processes around other stars, possibly together with the effects of long-term exposure to the interstellar medium. 1I/2017 U1 'Oumuamua is the first known ISO, discovered by the Pan-STARRS1 telescope in October 2017$^3$.The discovery epoch photometry…
Potential effects of sublimation of water ice from very slowly moving millimeter-sized and larger grains, a product of activity at ~10 AU or farther from the Sun driven presumably by annealing of amorphous water ice, are investigated by comparing 2I/Borisov with a nominal Oort Cloud comet of equal perihelion distance of 2 AU. This comparison suggests that the strongly hyperbolic motion of 2I mitigates the integrated sublimation effect. The population of these grains near the nucleus of 2I is likely to have been responsible for the comet appearing excessively bright in pre-discovery images at 5…
At present, there exists no consensus in the astronomical community regarding either the bulk composition or the formation mechanism for the interstellar object 1I/2017 U1 ('Oumuamua). With the goal of assessing the merits of the various scenarios that have been suggested to explain 'Oumuamua's appearance and observed properties, we report a number of new analyses and provide an up-to-date review of the current hypotheses. We consider the interpretations that can reconcile 'Oumuamua's observed non-Keplerian trajectory with the non-detection of traditional cometary volatiles. We examine the abi…
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…
The discovery of 'Oumuamua (1I/2017 U1), the first interstellar interloper, suggests an abundance of free-floating small bodies whose ejection into galactic space cannot be explained by the current population of confirmed exoplanets. Shortly after 'Oumuamua's discovery, observational results from the DSHARP survey illustrated the near-ubiquity of ring/gap substructures within protoplanetary disks, strongly suggesting the existence of a vast population of as-yet undetected wide-separation planets that are capable of efficiently ejecting debris from their environments. These planets have $a \gtr…
The origin of the interstellar object 1I/'Oumuamua, has defied explanation. In a companion paper (Jackson & Desch, 2021), we show that a body of N2 ice with axes 45 m x 44 m x 7.5 m at the time of observation would be consistent with its albedo, non-gravitational acceleration, and lack of observed CO or CO2 or dust. Here we demonstrate that impacts on the surfaces of Pluto-like Kuiper belt objects (KBOs) would have generated and ejected ~10^14 collisional fragments--roughly half of them H2O ice fragments and half of them N2 ice fragments--due to the dynamical instability that depleted the prim…
The first known interstellar object 'Oumuamua exhibited a nongravitational acceleration that appeared inconsistent with cometary outgassing, leaving radiation pressure as the most likely force. Bar the alien lightsail hypothesis, an ultra-low density due to a fractal structure might also explain the acceleration of 'Oumuamua by radiation pressure (Moro-Martin 2019). In this paper we report a decrease in 'Oumuamua's rotation period based on ground-based observations, and show that this spin-down can be explained by the YORP effect if 'Oumuamua is indeed a fractal body with the ultra-low density…
A comprehensive model of morphologically realistic Cosmic Dust particles: an application to mimic the unusual Polarization properties of the interstellar Comet 2I/Borisov
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/Boriso…
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 …
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 t…
We report the clear detection of C$_2$ and of abundant NH$_2$ in the first prominently active interstellar comet, 2I/Borisov. We observed 2I on three nights in November 2019 at optical wavelengths 4800--9300 Åwith the Multi-Unit Spectroscopic Explorer (MUSE) integral-field spectrograph on the ESO/Very Large Telescope. These data, together with observations close in time from both 0.6-m TRAPPIST telescopes, provide constraints on the production rates of species of gas in 2I's coma. From the MUSE detection on all epochs of several bands of the optical emission of the C$_2$ Swan system, a rich em…
We present an occultation study of compact radio sources by the plasma tail of interstellar Comet 2I/Borisov (C/2019 Q4) both pre- and near-perihelion using the Arecibo and Green Bank radio telescopes. The interplanetary scintillation (IPS) technique was used to probe the plasma tail at P-band (302--352 MHz), 820 MHz, and L-band (1120--1730 MHz). The presence and absence of scintillation at different perpendicular distances from the central axis of the plasma tail suggests a narrow tail of less than 6~arcmin at a distance of $\sim$10~arcmin ($\sim$$10^6$~km) from the comet nucleus. Data record…
The first discovered interstellar object (ISO), `Oumuamua (1I/2017 U1) shows a dry and rocky surface, an unusually elongated short-to-long axis ratio $c/a \lesssim 1/6$, a low velocity relative to the local standard of rest ($\sim 10$ km s$^{-1}$), non-gravitational accelerations, and tumbles on a few hours timescale. The inferred number density ($\sim 3.5 \times 10^{13} - 2 \times 10^{15}$ pc$^{-3}$) for a population of asteroidal ISOs outnumbers cometary ISOs by $\geq 10^3$, in contrast to the much lower ratio ($\lesssim 10^{-2}$) of rocky/icy Kuiper belt objects. Although some scenarios can…
Comets spend most of their lives at large distances from any star, during which time their interior compositions remain relatively unaltered. Cometary observations can therefore provide direct insight into the chemistry that occurred during their birth at the time of planet formation. To-date, there have been no confirmed observations of parent volatiles (gases released directly from the nucleus) of a comet from any planetary system other than our own. Here we present high-resolution, interferometric observations of 2I/Borisov, the first confirmed interstellar comet, obtained using the Atacama…
