Observations Of Comet Research Articles

The Spectrum of C/2023 A3 Indicates a Depleted Composition

We report a spectroscopic observation of comet C/2023 A3 using an 80 mm apochromatic refractor equipped with a custom-built spectrometer with a resolution of R ∼ 2500 on the night of 2024 October 4. Sodium D lines were detected prominently, while no other emission lines, particularly carbon-bearing species, were observed, which suggests that comet C/2023 A3 may be carbon-depleted. The mobility and flexibility of our observational setup highlight the value of amateur telescopes in observing low-altitude targets like C/2023 A3 as a complement to professional facilities.

Galileo’s microscopic and telescopic observations and their impact on how bodies are conceived

Abstract Four hundred years ago Galileo Galilei published a work that was immensely successful directly after its publication but is relatively unknown today. The Assayer (Il Saggiatore) started as a polemic reply to Orazio Grassi’s interpretation of the first telescopic observation of comets in history. The Jesuit defended the idea that comets were bodies moving in the superlunary region. However, Galileo never explicitly stated that comets are an illusion or an optical defect. Rather he aimed at undermining Grassi’s argumentation. Consequently, what announced itself initially as a controversy about the nature of comets turned out to become a controversy about the nature of scientific methodology. This paper focuses on two elements that Galileo elaborates on in this pioneering work of scientific method: his views on the qualities of bodies and the fact that “the book of nature” is written in the language of mathematics. It demonstrates how these two elements are not only connected in The Assayer (1623) but also in his earlier work. In addition, it shows how they both played a role in Galileo’s defence of mathematical demonstration and his rejection of the tools of scholastic philosophy.

The Pre-perihelion Evolution of the Activity of Comet C/2017 K2 (Pan-STARRS) during the Water Ice-line Crossover

Comets, relics from the early solar system, consist of dust and ice. The ice sublimates as comets approach the Sun, ejecting dust from their nuclei seen as activity. Different volatiles sublimate at different Sun–comet distances and eject dust of unique sizes, structures, and compositions. In this study, we present new polarimetric observations of Oort cloud comet C/2017 K2 (Pan-STARRS) in R- and I-filter domains before, during, and after its crossover of the water-ice sublimation regime at phase angles of 15.°9, 10.°5, and 20.°0, respectively. Combining multiband optical imaging data covering a wide range of heliocentric distances (∼14−2.3 au), we aim to characterize the pre-perihelion evolution of cometary activity as well as the properties of its coma dust. Two discontinuous brightening events were observed: at ∼6 au presumably associated with changes in CO-like supervolatile ice activity, and at ∼2.9 au when water ice took over. Particularly, the latter activation is accompanied by changes in coma morphology and color whose trends differ between the inner (∼103 km) and outer (∼104 km) parts of the coma. No polarimetric discontinuities on the comet were observed over the inner coma region, all epochs showing phase-angle and wavelength dependencies compatible with those of active comets observed in similar observing geometry. During this period, the underlying dust continuum overwhelmed Hα emission at around 656.3 nm, suggesting less water ice on the comet’s surface than expected. We discuss K2's coma environment by combining numerical simulations of light scattered by dust and place the observations within the context of the comet’s evolution.

Solar Wind Interactions with Comet C/2021 A1 Using STEREO HI and a Data-assimilative Solar Wind Model

Cometary tails display dynamic behavior attributed to interactions with solar wind structures. Consequently, comet-tail observations can serve as in situ solar wind monitors. During 2021 December, Comet Leonard (C/2021 A1) was observed by the STEREO-A heliospheric imager. The comet tail exhibited various signatures of interactions with the solar wind including bending, kink formation, and finally complete disconnection. In this study, we compare the timing of these events with solar wind structures predicted by the Heliospheric Upwind eXtrapolation model with a time-dependency (or HUXt) solar wind model using new solar wind data assimilation (DA) techniques. This serves both to provide the most accurate solar wind context to interpret the cometary processes, but also as a test of the DA and an example of how comet observations can be used in model validation studies. Coronal mass ejections, stream interaction regions (SIRs), and heliospheric current sheet (HCS) crossings were all considered as potential causes of the tail disconnection. The results suggest the tail disconnection at Comet Leonard was the result of it crossing the HCS and into an SIR. The timing of these features agree better with the DA model results than the non-DA model, showing the value of this approach. Case studies such as this expand our understanding of comet–solar wind interactions, and in demonstrating the utility of DA for solar wind modeling. We note that this could lead to comets acting as additional in situ measures for solar wind conditions for regions where no in situ spacecraft are available, potentially improving solar wind DA in the future.

The Impact of Cometary “Impacts” on the Chemistry, Climate, and Spectra of Hot Jupiter Atmospheres

Impacts from icy and rocky bodies have helped shape the composition of Solar System objects; for example, the Earth–Moon system, or the recent impact of comet Shoemaker–Levy 9 with Jupiter. It is likely that such impacts also shape the composition of exoplanetary systems. Here, we investigate how cometary impacts might affect the atmospheric composition/chemistry of hot Jupiters, which are prime targets for characterization. We introduce a parameterized cometary impact model that includes thermal ablation and pressure driven breakup, which we couple with the 1D “radiative-convective” atmospheric model ATMO, including disequilibrium chemistry. We use this model to investigate a wide range of impactor masses and compositions, including those based on observations of Solar System comets, and interstellar ices (with JWST). We find that even a small impactor (R = 2.5 km) can lead to significant short-term changes in the atmospheric chemistry, including a factor >10 enhancement in H2O, CO, and CO2 abundances, as well as atmospheric opacity more generally, and the near-complete removal of observable hydrocarbons, such as CH4, from the upper atmosphere. These effects scale with the change in atmospheric C/O ratio and metallicity. Potentially observable changes are possible for a body that has undergone significant/continuous bombardment, such that the global atmospheric chemistry has been impacted. Our works reveals that cometary impacts can significantly alter or pollute the atmospheric composition/chemistry of hot Jupiters. These changes have the potential to mute/break the proposed link between atmospheric C/O ratio and planet formation location relative to key snowlines in the natal protoplanetary disk.

Observations of comet C/1652 Y1 recorded in Korean histories

Abstract We report the Korean records for comet C/1652 Y1, which have not been introduced in previous studies on historical comets. According to Korean historical documents, this comet, described as bai xing (white star, in literal) or ke xing (guest star, in literal), was observed with the naked eye for 22 days from December 19, 1652 to January 9, 1653. In this study, we first cross-checked the records of comet C/1652 Y1 among Korean documents and presented the translations in the Appendix for future reference. We then compared the Korean observations with the orbital path determined from calculations using the orbital elements provided by Marsden (1983. Catalog of cometary orbit. Hillside: Enslow Publishers). We also compared the illustrations depicted by Weigelius and Schiltero (1653. Commentatio astronomica de cometa novo qui sub finem anni 1652 lumine sub obscuro nobis illuxit. Jenae: Typis Georgii Sengenvvaldi) and by Hevelius (1668. Cometographia, Totam Naturam Cometarum; Exhibens. Gedani: Typis Auctoris, & Sumptibus, Simon Reiniger). We found that the Korean observations show discrepancies with the orbital path calculated by Marsden and the illustration of Weigelius and Schiltero, particularly near the end of the observation period. In conclusion, we believe that this study will contribute to improving the orbital path calculation of comet C/1652 Y1.

Pre-perihelion observations of long-period comet C/2013 X1 (PANSTARRS)

ABSTRACT We present and analyse the results of quasi-simultaneous spectroscopic, photometric, and polarimetric observations of hyperbolic comet C/2013 X1 (PANSTARRS) obtained at the 6-m Big Telescope Alt-azimuth (BTA) telescope Special Astrophysical Observatory (SAO) and 2.6-m Shajn telescope Crimean Astrophysical Observatory (CrAO). A wide fan-shaped structure and a weak tail were detected in the comet. The mean V − R colour of the coma was estimated to be neutral compared to the solar value. The Afρ parameter, a proxy to the dust production in the comet, was about 1672 ± 21 cm in the R band. Emissions of the CN, C2, C3, and NH2 molecules were identified in the cometary spectrum, which covers the wavelength range 3800 – 7100 Å. When the comet was at a distance of 2.66 au from the Sun, the minimum degree of polarization of about −1.5 per cent was detected in the near-nucleus region of the coma, in the range up to about 10 000 km from the optocentre. Further, polarization gradually increased (in absolute value) with distance from the nucleus, reaching −6.5 per cent at about 50 000 km. To reproduce the observed values of linear polarization and the phase-angle dependence of polarization for long-period comets, we used the Sh-matrix method with conjugated Gaussian random particles as light scatters, and the chemical composition of dust particles in the coma of 74 per cent amorphous carbon, 25 per cent of Mg-rich silicates, and 1 per cent of water ice.

Faint Comet Detection with Unistellar eVscopes

We present eVscope observations of comets 12P/Pons-Brooks and C/2023 A3 (Tsuchinshan-ATLAS) when their magnitudes were greater than 16th magnitude. From a set of two observations of 12P taken on 2023 June 19 and 20, we measure an apparent magnitude of G = 16.59 ± 0.31. From four sets of observations of C/2023 A3 taken on 2023 April 14, 25, 26, and May 9, we measure apparent magnitudes of G = 16.78 ± 0.41, 16.55 ± 0.29, 16.71 ± 0.28, 16.59 ± 0.18 respectively. These images were taken from Unistellar telescope models: eVscope 1 and 2. We find an average background value of G = 17.35 ± 0.21.

Common origin of trapped volatiles in oxidized icy moons and comets

Following spacecraft encounters with comets 67P/C-G and 1P/Halley, it was surprising that O2, expected to be a very minor species in their comas, was observed to outgas at a few percent abundance during their ice sublimation phases. This challenged the direct connection suggested between comets and material in the interstellar medium (ISM), which exhibits a very low O2/H2O gas-phase abundance, leading to a number of papers suggesting novel sources for O2. Since these eccentrically orbiting comets have lost significant amounts of their evaporating surfaces over their lifetimes, the O2 observed must have been stably trapped down to significant depths in these primordial icy bodies. O2 was also seen in the coma by Rosetta, along with other volatiles, long after water ice sublimation began to subside. Here we note that the extensive observations of the icy satellites of Jupiter (Europa, Ganymede, and Callisto) exhibit radiolytic and outgassing processes that provide certain direct parallels to interpretations of recent comet observations. Given that O2 is consistently observed in the atmospheres of icy Jovian satellites, as well as stably trapped as ‘bubbles’ (Johnson and Jesser, 1997) in their water ice surfaces, their spectral observations can help constrain the environment in which Jupiter-family and Oort cloud comets formed given that the observed O2/H2O abundances at both types of comets and icy moons are nearly identical. Based on the approximate charged particle radiation required to produce the observed steady-state concentrations of O2, we suggest that comets likely formed in a far more energetic environment than the ISM. While grains can be irradiated for longer timescales in the neutral ISM, small grains are expected to erode before significant O2 formation and trapping occurs. Independent of celestial dynamics then, an unknown radiation source, may provide insight to the first population of oxidized water ice grains in the early solar system.

Optical Spectroscopy of Comets

Comets are pristine remnants of the Solar system, composed of dust and ice. They remain inactive and undetectable for most of their orbit due to low temperatures. However, as they approach the Sun, volatile materials sublimate, expelling dust and creating a visible coma. Spectroscopic observations of comets help the simultaneous study of both the gas emissions and reflected sunlight from dust particles. By implementing a long slit, the spatial variations in molecular emissions can be analysed to be further used for other computations. Additionally, spatial information aids in extracting the characteristic profile of the Afρ parameter, revealing insights into the behaviour of dust emissions. A sufficiently long slit would prove advantageous in extracting information about the emissions occurring at different parts of the coma or even the tail. We can gain an overall comprehensive understanding of a comet’s chemical composition and dust emission by constructively utilising low-resolution spectroscopy with the help of a long slit.

Activity in Centaur-like Jupiter-family Comet 2023 RN3

We present multi-filter observations of Centaur-like Jupiter-family comet 2023 RN3 conducted as part of the Las Cumbres Observatory Outbursting Objects Key Project, as well as analysis of archival data in which 2023 RN3 was expected to be present. We find clear evidence of comet-like mass loss in the form of an extended morphology on UT 2023 November 16 and 18, and an increase in intrinsic brightness of >4 mag between 2022 September and 2023 November. We also report broadband colors for the coma of , , and , giving a spectral slope of /100 nm.

Methodology of organizing astronomical observations of asteroids and comets within the extracurricular activities of students

In this article, the methodology of using a telescope and charge-coupled device camera for organizing astrometric observations of asteroids and comets, as well as photometric observations of asteroids, is presented. Since astronomy is not studied as a separate subject in schools in Romania and the Republic of Moldova (as well as in many other countries), this methodology can be used within extracurricular activities for students. The observations described in the article took place at the astronomical observatory within the ‘Răsvan Angheluță’ Natural Sciences Museum Complex in Galați, Romania.

Analysis of the first coronagraphic multi-band observations of a sungrazing comet

Context. Between 24 and 25 December 2021 a sungrazing comet (SOHO-4341) approached the Sun, being observed by “classical” visible light (VL) coronagraphs on board the SOHO and STEREO missions, and also by the innovative Metis coronagraph on board the ESA-NASA Solar Orbiter mission in the VL and ultraviolet (UV H I Lyman-α) band. Aims. We show how VL data acquired by the Metis coronagraph can be combined with those provided by other space-based coronagraphs to reconstruct the comet orbit, but also to provide information on the dust composition from the polarized VL emission. Moreover, we show how the UV emission can be employed to measure local plasma parameters of the ambient solar wind. Methods. By using the comet positions tracked with VL Metis images (with spatial resolution that is four times better than UV), the UV images (with a time cadence that is five times faster than VL) have been coaligned to maximize the signal-to-noise ratio in the UV band. The local electron density ne was measured from the observed exponential decay of the UV Lyman-α intensity along the tail, while the solar wind speed vwind was measured from the UV Lyman-α tail inclination with respect to the cometary orbital path deprojected in 3D. Moreover, the proton kinetic temperature Tk was also obtained by the aperture angle of the UV Lyman-α tail. Results. When the comet was at an average heliocentric distance of 14.3 R⊙, the comet had a radial speed of 155 km s−1 and a tangential speed of 59 km s−1. The comet had a UV Lyman-α tail extending in the anti-solar direction over more than 1.5 R⊙. From the analysis of the tail shape in UV we obtained the local solar wind speed (vwind = 190 km s−1), electron density (ne = 1.5 × 104 cm−3), and proton temperature (Tk = 1.2 × 106 K). Moreover, theoretical analysis of the measured UV Lyman-α intensity allowed us to estimate the radius of the cometary nucleus (Rcom = 65 m) and the water outgassing rate (QH2O = 4.8 × 1028 molec s−1). Conclusions. These results show that sungrazing comets are unique “local probes” for the ambient coronal plasma, providing measurements that are not as affected by the line-of-sight integration effects as those provided by remote sensing instruments, in regions of the Heliosphere that are not explored in situ by the ongoing space missions.

On the extremely low polarization in Comet C/2023 P1 (Nishimura)

ABSTRACT On September 6 and 7 of 2023, we measured the degree of linear polarization of Comet C/2023 P1 (Nishimura) with the R filter. Our observations reveal an extremely low amplitude of positive polarization. Our modelling suggests Mg-rich silicate dust particles are the dominant species in the coma; whereas, an alternative explanation is that such low positive polarization in comets is attributed to the depolarizing effect of their gaseous emission. These two alternative explanations predict dramatically different polarimetric responses at small phase angle that can be discriminated in future polarimetric observations of Comet C/2023 P1 (Nishimura).

The Return of the Rosetta Target: Keck Near-infrared Observations of Comet 67P/Churyumov–Gerasimenko in 2021

High-resolution near-infrared ground-based spectroscopic observations of comet 67P/Churyumov–Gerasimenko near its maximum activity in 2021 were conducted from the W. M. Keck Observatory, using the facility spectrograph NIRSPEC. 67P is the best-studied comet to date because of the unprecedented detail and insights provided by the Rosetta mission during 2014–2016. Because 67P is the only comet where the detailed abundances of many coma volatiles were measured in situ, determining its composition from the ground provides a unique opportunity to interpret Rosetta results within the context of the large database of ground-based compositional measurements of comets. However, previous apparitions, including in 2015, have been unfavorable for in-depth ground-based studies of parent volatiles in 67P. The 2021 apparition of 67P was thus the first-ever opportunity for such observations. We report gas spatial distributions, rotational temperatures, production rates, and relative abundances (or stringent upper limits) among seven volatile species: C2H2, C2H6, HCN, NH3, CH3OH, H2CO, and H2O. The measured abundances of trace species relative to water reveal near average or below average values compared to previous comets studied at infrared wavelengths. Both gas rotational temperatures and the spatial distributions of H2O, C2H6, and HCN measured with Keck-NIRSPEC in 2021 are consistent with the outgassing patterns revealed by Rosetta in 2015 at very similar heliocentric distance (post-perihelion). These results can be integrated with both Rosetta mission findings and ground-based cometary studies of the overall comet population, for which we encourage a wide-scale collaboration across measurement techniques.

Splitting of Long-period Comet C/2018 F4 (PANSTARRS)

Long-period comet C/2018 F4 (PANSTARRS) was observed to show duplicity of its inner region in 2020 September, suggestive of a splitting event. We here present analyses of our observations of the comet taken from the LCO Outbursting Objects Key project and the University of Hawaii 2.2 m telescope after the discovery of the splitting. The two fragments Components A and B, estimated to be ∼60 m to 4 km in radius, remained highly similar to each other in terms of brightness, color, and dust morphology throughout our observing campaign from 2020 September to 2021 December. Our fragmentation model yielded that the two components split at a relative speed of 3.00 ± 0.18 m s−1 in 2020 late April, implying a specific energy change of J kg−1, and that Component B was subjected to a stronger nongravitational acceleration than Component A in both the radial and normal directions of the orbit. The obtained splitting time is broadly consistent with the result from the dust morphology analysis, which further suggested that the dominant dust grains were millimeter-sized and ejected at a speed of ∼2 m s−1. We postulate that the pre-split nucleus of the comet consisted of two lobes resembling that of 67P, or that the comet used to be a binary system like main-belt comet 288P. Regardless, we highlight the possibility of using observations of split comets as a feasible manner to study the bilobate shape or binarity fraction of cometary nuclei.

Optical Imaging Polarimetry of Comet 21P/Giacobini–Zinner during Its 2018 Apparition

We conducted polarimetric observations of comet 21P/Giacobini–Zinner in the optical wavelength region using the polarimetric imager PICO equipped with the 50 cm telescope for Public Outreach at the Mitaka Campus of the National Astronomical Observatory of Japan on UT 2018 September 16, at a solar phase angle of 77° for the comet. We used Gunn’s i′-band filter to cover the wavelength region where sunlight reflected by dust grains dominates cometary spectra. The intensity map of comet 21P/Giacobini–Zinner showed an elongated coma structure along the east–west direction. The linear polarization degree map was almost uniform and showed no evidence of jet or arc structures in the inner coma. The linear polarization degree was 22% in the coma, which is consistent with other optical polarization observations of 21P/Giacobini–Zinner for this and previous apparitions reported in the literature. The measured polarization angles (179°) were nearly perpendicular to the position angle of the scattering plane of 271°. There is no evidence of significant changes in the dust properties (composition, porosity, and size distribution) in the comet from its 1985 to 2018 apparitions. The spectral gradient of the linear polarization degree for the comet was nearly flat in the wavelength range of ∼510–830 nm at a solar phase angle of ∼77° during the 2018 apparition.

Coma environment of comet C/2017 K2 around the water ice sublimation boundary observed with VLT/MUSE

We report a new imaging spectroscopic observation of Oort cloud comet C/2017 K2 (hereafter K2) with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope on its way to perihelion at 2.53 au, around a heliocentric distance where H2O ice begins to play a key role in comet activation. Normalized reflectances over 6500–8500 Å for its inner (cometocentric distance ρ ≈ 103 km) and outer (ρ ≈ 2 × 104 km) comae are 9.7 ± 0.5 and 7.2 ± 0.3 % (103 Å)−1, respectively, the latter being consistent with the slope observed when the comet was beyond the orbit of Saturn. The dust coma of K2 at the time of observation appears to contain three distinct populations: millimeter-sized chunks prevailing at ρ ≲ 103 km; a 105 km steady-state dust envelope; and fresh anti-sunward jet particles. The dust chunks dominate the continuum signal and are distributed over a similar radial distance scale as the coma region with redder dust than nearby. They also appear to be co-spatial with OI1D, suggesting that the chunks may accommodate H2O ice with a fraction (≳1%) of refractory materials. The jet particles do not colocate with any gas species detected. The outer coma spectrum contains three significant emissions from C2(0,0) Swan band, OI1D, and CN(1,0) red band, with an overall deficiency in NH2. Assuming that all OI1D flux results from H2O dissociation, we compute an upper limit on the water production rate QH2O of ~7 × 1028 molec s−1 (with an uncertainty of a factor of two). The production ratio log[QC2/QCN] of K2 suggests that the comet has a typical carbon chain composition, with the value potentially changing with distance from the Sun. Our observations suggest that dust chunks (>0.1 mm) containing water ice and near K2’s nucleus emitted beyond 4 au may be responsible for its very low gas rotational temperature and the discrepancy between its optical and infrared lights reported at similar heliocentric distances.

Spectroscopic identification of water emission from a main-belt comet.

Main-belt comets are small Solar System bodies located in the asteroid belt that repeatedly exhibit comet-like activity (that is, dust comae or tails) during their perihelion passages, strongly indicating ice sublimation1,2. Although the existence of main-belt comets implies the presence of extant water ice in the asteroid belt, no gas has been detected around these objects despite intense scrutiny with the world's largest telescopes3. Here we present James Webb Space Telescope observations that clearly show that main-belt comet 238P/Read has a coma of water vapour, but lacks a significant CO2 gas coma. Our findings demonstrate that the activity of comet Read is driven by water-ice sublimation, and implies that main-belt comets are fundamentally different from the general cometary population. Whether or not comet Read experienced different formation circumstances or evolutionary history, it is unlikely to be a recent asteroid belt interloper from the outer Solar System. On the basis of these results, main-belt comets appear to represent a sample of volatile material that is currently unrepresented in observations of classical comets and the meteoritic record, making them important for understanding the early Solar System's volatile inventory and its subsequent evolution.

Quasi-simultaneous photometric, polarimetric, and spectral observations of distant comet C/2014 B1 (Schwartz)

Context. We analyze the results of our comprehensive observations of the high-perihelion comet C/2014 B1 (Schwartz) with stable disk-shaped coma and jets in order to study its nature. Aims. The main objective of our study is to obtain new observational results for a unique disk-like comet C/2014 B1 (Schwartz) with a perihelion distance of 9.56 au. Methods. Quasi-simultaneous long-slit spectra, as well as photometric and polarimetric images with g-sdss and r-sdss filters, were acquired with the 6 m telescope of the Special Astrophysical Observatory on 2017 January 23. The BVR-band photometry of the comet was also performed at the 2m telescope of the Peak Terskol Observatory on 2017 January 31. We modeled the dynamics of the jets and the behavior of the color and polarization in the coma considering the dust as aggregated large particles. Results. We did not reveal any emissions in the spectra. The positions of two jets oriented along the position angles of 179° and 350° and the disk-like shape of the coma have remained unchanged for more than 4 yr. The most realistic model able to explain jets of such stable orientation includes the existence of two active sources located near the north and south poles of the rotating nucleus whose diameter was determined to be between 7.6 and 12.2 km depending on the albedo, of namely between 0.1 and 0.04, respectively. The high activity of the comet is characterized by the high dust production Afρ which varied from 4440 to 3357 cm between 2017 January 23 and 31. A significant difference between the radial surface brightness profiles of the jets and the ambient (undisturbed by the jets) coma is found. The color of the jet structures is much redder than that of the ambient coma, and the nucleus has a very red color, V − R=0.93m±0.19m. There are spatial variations of the color and polarization over the coma and jets. Conclusions. The observed trends in color and polarization, as well as the brightness profiles, can be explained by the fragmentation of aggregated particles formed by CO2/H2O ices, silicates, and organics, which are of ~1 mm in radius near the nucleus and ~10 μm in radius at the periphery of the coma.