Interstellar objects (ISOs), particularly those with cometary activity, provide unique insight into the primordial physical and chemical conditions present during the formation of the planetary system in which they originated. Observations in the sub-mm regime allow for direct measurements of several parent molecules released from the comet nucleus into the coma. Here we present observations of the third ISO, 3I/ATLAS, with the `Ū`ū heterodyne receiver on the James Clerk Maxwell Telescope (JCMT), which targeted emission from HCN(J = 3 - 2) and CO(J = 2 - 1). Our observations, taken between 16 July 2025 and 21 July 2025 (UT), when 3I/ATLAS was at a heliocentric distance between 4.01 and 3.84 au, provide the earliest sub-mm constraints on its activity. We do not detect HCN or CO in these epochs, with 3σ upper-limits on the production rates of Q(HCN) < 1.7 × 10^24 s^-1 at r_h = 4.01 - 3.97 au and Q(CO) < 1.1 × 10^27 s^-1 at r_h = 3.94 - 3.84 au, respectively. We combine this HCN limit with later JCMT observations of HCN to constrain its temporal evolution. Fitting the HCN detections with a Q(HCN) ∝ r_h^-n model and accounting for the upper-limits yields n = 12.7^+6.9_-2.5. This slope is steeper than those of typical Solar System comets, but consistent with the production rate slopes measured for other species in the coma of 3I/ATLAS.

The interstellar comet 2I/Borisov is the first interstellar object where compositional characterisation was possible throughout its entire perihelion passage. We report all 16 epochs of a comprehensive optical observation campaign with ESO Very Large Telescope's integral field spectrograph MUSE, spanning 126 days from 2019 November 14 to 2020 March 19. The spatial dust emission of 2I/Borisov was predominantly smooth, with no seasonal effect. A jetlike feature was consistently visible. The gas production morphology of its coma was also smooth and similar for C2, NH2, and CN: symmetric around the optocentre. The production rates of these species gently declined into and beyond perihelion, until 2I's outburst and splitting event in early 2020 March. C2, NH2, and CN production rates all increased, with NH2 being the most significant; the dust emission also slightly reddened. 2I/Borisov is a carbon-depleted, relatively NH2-rich comet when compared to those comets yet measured in the solar system.

We present multiepoch optical spectroscopy of the interstellar comet 3I/ATLAS obtained between 2025 December and 2026 January (heliocentric distances of 1.8-3.3 au), yielding postperihelion production rates and mixing ratios for CN, C3, C2, CH, and gaseous metals (Fe i and Ni i). Our results show that the coma is less depleted in C2 after perihelion than before, indicative of subsurface activation or compositional heterogeneity. The outgassing profiles reveal a pronounced perihelion asymmetry: the CN and metal production rates decline more gradually outbound than inbound, consistent with the reported behavior of H2O and implying a change in the comet's activity pattern across perihelion. Despite being metal rich relative to its H2O content, 3I/ATLAS follows the metal-CO correlation observed in comets of diverse origins, suggesting that gaseous metal release is more closely linked to a CO-bearing volatile reservoir than to H2O, potentially in the form of metal carbonyls. In addition, the [O i] lambda 6300 emission shows a significant residual after subtracting the expected contributions from H2O, CO2, and CO, which may reflect systematic uncertainties in the photodissociation yields of those molecules or a contribution from additional oxygen-bearing parents.

(469219) Kamo`oalewa is the most stable Earth quasi-satellite and the target of China's Tianwen-2 asteroid sample return mission. Due to its small size, fast rotation, and the limited observing geometry accessible from the ground, many physical properties of Kamo`oalewa remain poorly constrained, including the rotational status and shape. We obtained three epochs of high-cadence, high signal-to-noise photometric lightcurves of Kamo`oalewa with the Gemini North Telescope from 2026 April to May, supplemented by one lightcurve from the Lowell Discovery Telescope in 2026 May. Our analysis suggests that Kamo`oalewa is in a non-principal-axis rotation with an elongated shape. Four possible solutions exist, including a long-axis mode (LAM) solution and a short-axis mode (SAM) solution, as well as their corresponding mirrored angular momentum directions. The most preferable solution has a LAM model with a precession period P_ϕ=27.65±min, and a rotational period P_ψ=50.49±0.08min, and the angular momentum points to ecliptic coordinates (λ, β) = (226^o± 20^o, -39^o± 15^o), although we cannot rule out other solutions or other close-by periods due to aliasing. We also derived a convex shape inversion for LAM with consistent rotational parameters but could not find a satisfactory inversion for SAM. The non-principal-axis rotation provides additional constraints on the dynamic history or the internal structure of Kamo`oalewa.

Interstellar objects provide a unique view into the formation of other star systems. Here we present spectroscopic observations of the recently discovered interstellar object 3I/ATLAS between a heliocentric distance of 3.7-1.8 au on either side of its travels through perihelion. We obtained several observations with the Keck-I/LRIS, Keck-II/NIRES, Gemini/GMOS, and UH88/SNIFS spectrographs, covering a wavelength range of 0.3-2.5 mu m. We report the continued emission of both Ni and CN, along with postperihelion detections of Fe and a weak detection of C3. We determine the spectral slope across optical and near-infrared (NIR) wavelengths and find a positive spectral slope in the optical, with values ranging from similar to 21% to 27% in the blue regions (0.4-0.55 mu m) to similar to 6%-10% in the red (0.65-0.9 mu m) regions. In contrast, the NIR showed a negative spectral slope of similar to-0.9% between 0.9 and 1.5 mu m and similar to-2.3% between 1.9 and 2.5 mu m. 3I/ATLAS shows a clear turnover in its spectral shape at similar to 1.1 mu m, consistent with scattered sunlight from the dusty coma. Finally, in the NIR spectra presented in this work, we do not detect any of the water ice features that were identified in an earlier NIR observation of 3I/ATLAS. Our observations of 3I/ATLAS in the NIR show a similar shape to the NIR spectrum of 2I/Borisov as it approached perihelion.

Abstract The interstellar comet 2I/Borisov is the first interstellar object where compositional characterisation was possible throughout its entire perihelion passage. We report all 16 epochs of a comprehensive optical observation campaign with ESO Very Large Telescope’s integral field spectrograph MUSE, spanning 126 days from 2019 November 14 to 2020 March 19. The spatial dust emission of 2I/Borisov was predominantly smooth, with no seasonal effect. A jetlike feature was consistently visible. The gas production morphology of its coma was also smooth and similar for C 2 , NH 2 , and CN: symmetric around the optocentre. The production rates of these species gently declined into and beyond perihelion, until 2I’s outburst and splitting event in early 2020 March. C 2 , NH 2 , and CN production rates all increased, with NH 2 being the most significant; the dust emission also slightly reddened. 2I/Borisov is a carbon-depleted, relatively NH 2 -rich comet when compared to those comets yet measured in the solar system.

China's Tianwen-1 Mars orbiter successfully imaged the third interstellar object, 3I/ATLAS, during its close encounter with Mars using the onboard HiRIC CMOS camera. This is China's first deep-space observation of an astronomical object. These observations constitute the first imaging of this object from a vantage point significantly out of its orbital plane, providing a unique constraint on dust dynamics. Three observing epochs between 2025 September 30 and October 3 reveal clear changes in coma and tail morphology driven by the rapidly evolving viewing geometry. Comparison with Finson-Probstein dust dynamical models indicates that the coma is dominated by large grains with solar radiation pressure parameter beta approximate to 10-3-10-2, corresponding to grain sizes of a few hundreds of microns. The extent of the sunward coma implies dust ejection velocities of 3-10 m s-1. Despite the morphological evolution, the azimuthally averaged surface brightness profile remains nearly unchanged through the three epochs, transitioning from a radial slope near -1 close to the nucleus to slightly steeper than -1.5 at larger cometocentric distances, consistent with steady-state dust outflow accelerated by solar radiation pressure. Photometry yields an average Af rho similar to (2.0 +/- 0.2) & times; 104 cm and a corresponding dust mass loss rate of M similar to 103kgs-1 .

We present centimeter-wave spectroscopic observations of the OH 18-cm lines in two bright comets, 12P/Pons-Brooks and C/2023 A3 (Tsuchinshan-ATLAS), conducted with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) during their 2024 apparitions. For the Halley-type comet 12P/Pons-Brooks, five epochs of OH observations were obtained. The main OH lines at 1665 and 1667 MHz were robustly detected in absorption during one pre-perihelion epoch, while upper limits were derived for two post-perihelion epochs. For the dynamically new Oort Cloud comet C/2023 A3 (Tsuchinshan-ATLAS), six epochs of OH observations were obtained. The 1665 and 1667 MHz lines were robustly detected in absorption during two epochs immediately following the comet's closest approach to Earth. We also reported a tentative detection of the 1721 MHz satellite line in one of these two epochs. The low OH detection rates for these two bright comets are primarily attributable to their unfavorable heliocentric radial velocity during the observations, which resulted in the anti-maser negative inversion mode of the OH excitation. We calculated the OH production rates for both comets and found that, after considering the small beam size of FAST and the collisional quenching effect, production rates are consistent with the data from literature. Specifically, this matches the power-law fit of the OH production rates for comet 12P/Pons-Brooks, as well as the production rates at comparable heliocentric distances among comet C/2023 A3 (Tsuchinshan-ATLAS) and four other dynamically new Oort Cloud comets.

We report on the observation and measurement of astrometry, photometry, morphology, and activity of the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object's region of the sky during routine commissioning. Facilitated by Rubin's high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of similar to 70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of similar to 3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at similar to 0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-sectional ratio of eta greater than or similar to 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g - r = (0.657 +/- 0.013) mag, r - i = (0.235 +/- 0.018) mag, i - z = (0.147 +/- 0.042) mag, and z - y = (0.047 +/- 0.052) mag. These data represent the earliest observations of this object by a large (greater than or similar to 8 m class) telescope and illustrate the type of measurements (and discoveries) Rubin's Legacy Survey of Space and Time will provide after it begins in early 2026.

The environs of other stellar systems may be directly probed by analyzing the cometary activity of interstellar objects. The recently discovered interstellar object 3I/ATLAS was the subject of an intensive worldwide follow-up campaign in its preperihelion approach. Now, 3I/ATLAS has begun its postperihelion departure from the solar system. In this paper, we report the first postperihelion blue-sensitive integral-field unit spectroscopy of 3I/ATLAS using the Keck Cosmic Web Imager on 2025 November 16. We confirm previously reported CN, Fe, and Ni outgassing along with detections of carbon chain molecules C2 and C3. We calculate production rates for each species. We find Fe and Ni production rates of QFe = (9.55 +/- 3.96) & times; 1025 atoms s-1 and QNi = (6.61 +/- 2.74) & times; 1025 atoms s-1, resulting in a ratio of log(QNi/QFe)=-0.16 +/- 0.03 , which matches solar system comets well and continues the preperihelion trend of declining log(QNi/QFe) with rh. We investigate the radial distributions of these elemental species and find characteristic e-folding radii of 3880 +/- 39 km for Ni, 6053 +/- 68 km for CN, 4194 +/- 45 km for C2, and 3833 +/- 45 km for C3. Compared to preperihelion measurements, these radii have increased by a factor of similar to 6.5-7. Our postperihelion observations reveal that 3I/ATLAS continues to exhibit cometary behavior broadly consistent with solar system comets.

We report initial observations aimed at the characterization of a third interstellar object. This object, 3I/ATLAS or C/2025 N1 (ATLAS), was discovered on 2025 July 1 UT and has an orbital eccentricity of e ∼ 6.1, perihelion of q ∼ 1.36 au, inclination of ∼175°, and hyperbolic velocity of V _∞ ∼ 58 km s ^−1 . We report deep stacked images obtained using the Canada–France–Hawaii Telescope and the Very Large Telescope that resolve a compact coma. Using images obtained from several smaller ground-based telescopes, we find minimal light-curve variation for the object over a ∼4 day time span. The visible/near-infrared spectral slope of the object is 17.1% ± 0.2%/100 nm, comparable to other interstellar objects and primitive solar system small bodies (comets and D-type asteroids). Moreover, 3I/ATLAS will be observable through early 2025 September, then unobservable by Earth-based observatories near perihelion due to low solar elongation. It will be observable again from the ground in late 2025 November. Although this limitation unfortunately prohibits detailed Earth-based observations at perihelion when the activity of 3I/ATLAS is likely to peak, spacecraft at Mars could be used to make valuable observations at this time.

Context. The multiple outburst events of comet 12P/Pons-Brooks during its 2024 apparition offer a unique window into highly active volatile releasing processes not observable during quiescent periods. Radio observations are capable of measuring specific cometary gas species, providing valuable insights into the chemical composition and activity mechanism of comets.Aims. We performed radio observations of comet 12P/Pons-Brooks with the Tianma 65 m Radio Telescope, targeting the OH and NH3 inversion lines at 18 and 1.3 cm, respectively. By monitoring 12P at different heliocentric distances on its inbound journey, we aim to provide insights into the comet's volatile composition and outburst behavior.Methods. Four observations were carried out between December 2023 and March 2024, when the comet was approaching the Sun from 2.22 to 1.18 AU. We conducted 18 cm OH lines observations on four separate days using the cryogenically cooled receiver system of the telescope to derive the H2O production rate. During 12P's outburst on December 14, we also conducted observations targeting the NH3 emission.Results. OH 18 cm lines were clearly detected with a signal-to-noise ratio of similar to 4 sigma (peak intensity), yielding estimates on corresponding water production rates of approximately 1029 molec. s-1. A tentative detection of NH3 was made at the similar to 3 sigma level during the outburst phase, but the detection needs to be verified.Conclusions. Our observations provide information on the outgassing behavior of 12P/Pons-Brooks during its 2024 apparition. The water production rate of 12P, derived from the 18 cm OH lines, is consistent with measurements obtained in other works. The possible detection of NH3 during an outburst suggests possible connections between the subsurface volatile reservoir and the outburst mechanism. These results could further our understanding of the composition and activity of Halley-type comets.

Water ice is a fundamental building material of comets and other bodies in the outer solar system. Yet the properties of cometary water ice are challenging to study, due to its volatility and the typical distances at which comets are observed. Cometary outbursts, impulsive mass-loss events that can liberate large amounts of material, offer opportunities to directly observe and characterize cometary water ice. We present a study of comet 243P/NEAT, instigated by a −3 mag outburst that occurred in 2018 December. Optical images and a 251-day light curve were examined to characterize the outburst and the comet’s quiescent activity. Variations in the quiescent light curve appear to be dominated by coma asymmetries, rather than changing activity levels as the comet approached and receded from the Sun. Furthermore, the light curve shows evidence for one to two additional small outbursts (–0.3 mag) occurring in 2018 September. The large 2018 December outburst likely ejected water-ice grains, yet no signatures of ice were found in color photometry, a color map, or a near-infrared spectrum. We discuss possible dynamical and thermal reasons for this nondetection. In this context, we examined the comae of comets 103P/Hartley 2 and C/2013 US _10 (Catalina), and we show that a one-to-one mapping between continuum color and the presence of water ice cannot be supported. We also discuss possible causes for the large outburst, and we find that there is an apparent grouping in the kinetic energy per mass estimates for the outbursts of five comets.

We present VLT/MUSE observations of comet 67P/Churyumov-Gerasimenko during its 2021 perihelion passage, from which we generated simultaneous maps of dust, [OI], C_2, NH_2, and CN comae across 12 pre- and post-perihelion epochs. These maps reveal the evolutionary and compositional trends of 67P's coma and further enrich the context and findings of ESA's Rosetta mission. Dust and gas species displayed distinct structures, where NH_2 and CN signals were uniquely associated with known dust fans, raising the question of possible correlation to the dust and contributions of extended sources. Localised fitted NH_2 scale lengths were 1.5-1.9× larger than those fitted for the rest of the coma, which is consistent with an extended source component for northern pre-perihelion emissions. In the southern hemisphere, CN was correlated with a prominent and sharp dust structure, potentially revealing an extended source origin via larger dust particles that preserve the CN parent species, as evidenced by higher spectral slopes in the region. Gas maps depicted two distinct evolutionary regimes: (1) evolving H_2O ([OI]^1D) and C_2 emissions driven by nucleus sublimation and subsolar insolation, and (2) stable NH_2 and CN emissions associated with seasonal dynamics and possible distributed sources. Dust spectral slope maps revealed spectral slope trends consistent with Rosetta findings, while green/red [OI] ratios generally indicate a coma dominated by H_2O.

We present optical and near-infrared spectroscopy of the interstellar object 3I/ATLAS, obtained with Gemini-S/GMOS and NASA IRTF/SpeX on 2025 July 5 and 14. The optical spectrum shows a red slope of similar to 11% per 1000 & Aring; between 0.5 and 0.8 mu m, resembling typical D-type asteroids and distinct from ultrared trans-Neptunian objects. At longer wavelengths, the near-infrared continuum flattens to similar to 3% per 1000 & Aring; between 0.9 and 1.5 mu m, with a broad absorption feature near 2.0 mu m indicative of water-ice grains in the coma. Spectral modeling with a mixture of 63% amorphous carbon and 37% 1 mu m-sized water ice reproduces both the continuum and the 2.0 mu m band, while the 1.5 mu m water-ice band is not detected, likely due to limited signal-to-noise in the IRTF data and dilution by refractory material. The close agreement between the GMOS and SpeX spectra, taken 9 days apart, indicates short-term stability in the coma's optical properties. These observations demonstrate that 3I/ATLAS is an active interstellar comet containing abundant water ice, consistent with the theoretical expectation that its home planetary system had a high bulk fraction of water ice by mass.

We present optical and near-infrared (NIR) observations of the outbursting, Halley-type comet 12P/Pons-Brooks (12P). Three NIR spectra were obtained during two outbursts in 2023 October and November, with the 3 m Infrared Telescope Facility and the Palomar 200 inch Telescope, respectively. The NIR spectra exhibited absorption features at 1.5 and 2.0 μ m, consistent with the diagnostic absorption bands of water ice, superimposed on a red dust-scattering continuum. We find that the absorption bands and the red continuum can be well explained by micrometer-sized crystalline ice at 140–170 K, along with submicrometer-sized refractory grains (e.g., amorphous carbon). In addition, an optical spectrum was obtained with the Lijiang 2.4 m Telescope during the 2023 November outburst, which exhibited the emission bands of gaseous CN, C _3 , C _2 , and NH _2 . The C _3 /CN and C _2 /CN ratios suggest that 12P was “typical” in C _3 abundance but somewhat depleted in C _2 . The specific kinetic energy of the 2023 November outburst is estimated to be ∼8 × 10 ^3 J kg ^−1 , suggesting a likely triggering mechanism similar to 332P/Ikeya–Murakami and 17P/Holmes, i.e., the crystallization of amorphous water ice. A refractory-to-ice ratio of ∼1.7–3.2 is derived from the total mass loss of dust and gas, aligning with the lower-end estimates for 67P/Churyumov-Gerasimenko and 1P/Halley. This suggests either a less evolved nucleus or an outburst region enriched in icy materials relative to the bulk nucleus.

The Pallas collisional family of asteroids, named after (2) Pallas, is notable for its high orbital inclination and the distinct blue color of Pallas and a few larger B-type family members. While Pallas itself, as one of the largest asteroids, has been studied in detail, most of its smaller family members still remain unexplored. This study aims to characterize the physical properties of medium- to small-sized Pallas family asteroids to investigate the origin of their unusual blueness. Additionally, we explore the relationship between the Pallas family and the near-Earth object (NEO) (3200) Phaethon. We conducted near-infrared (NIR) spectroscopy with the NASA Infrared Telescope Facility (IRTF) to collect reflectance spectra for 22 asteroids, including one from the IRTF Legacy Archive. Spectroscopic and dynamical analyses were carried out to identify outliers, while additional data from NEOWISE and Gaia were incorporated to examine potential correlations among their physical properties. Meteorite analogs were identified through chi-square matching using samples from the RELAB database. The observed Pallas family asteroids exhibit nearly identical spectral profiles, suggesting a homogeneous composition of ejected material. Small variations in spectral slopes are observed, which may result from different levels of alteration experienced by individual asteroids, with some influence from variations in grain size. Most of the observed spectra of the Pallas asteroids, from 0.8 to 2.2 micron, closely resemble those of the CY and CI meteorites. The blueness of asteroid surfaces is likely due to the presence of magnetite, troilite, or phyllosilicates, which are products of aqueous alteration. The striking spectral similarity between (3200) Phaethon and Pallas family members of comparable sizes suggests a potential common origin.

We give a general overview of what the scientific community refers to as "exocomets". The general definition of exocomets, as presented in this work, is discussed and compared with Solar System comets and interstellar objects, addressing their detection around main-sequence stars as well as orbiting white dwarfs. We introduce the different types of exocomet observations, highlighting the difference between exocometary 'bodies' and exocometary 'material'. We provide a census of all exocometary system candidates detected so far, both via spectroscopy and photometry, including detections around white dwarfs.

We report Very Large Telescope spectroscopy of the interstellar comet 3I/Asteroid Terrestrial-impact Last Alert System (C/2025 N1), from r h ≃ 4.4 to 2.85 au, using X-Shooter (300–550 nm, R ≃ 3000) and the Ultraviolet and Visual Echelle Spectrograph (optical, R ≃ 35–80 k). The coma is dust-dominated, with a fairly constant red optical continuum slope (∼21%–22%/1000 Å). We report the detection of CN emission and also detect numerous Ni i lines, while Fe i remains undetected, potentially implying efficiently released gas-phase Ni. At r h ≃ 3.14 au, we derive 3 σ limits of Q (OH) < 1.48 × 10 26 s −1 but find no indications for [O i ], C 2 , C 3 , or NH 2 . From our latest X-Shooter measurements, conducted on 2025 August 21 ( r h = 2.85 au), we measure production rates of log Q ( CN ) = 24.81 ± 0.01 molecules s −1 and log Q (Ni) = 23.30 ± 0.07 atoms s −1 and characterize their evolution as the comet approaches perihelion. We observe a steep heliocentric distance scaling for the production rates Q ( Ni ) ∝ r h − 7.7 ± 1.0 and Q ( CN ) ∝ r h − 6.7 ± 0.2 , and we predict an Ni–CO (2) correlation if the Ni i emission is driven by the carbonyl formation channel. Energetic considerations of activation barriers show that this behavior is inconsistent with the direct sublimation of canonical metal/sulfide phases and instead favors low-activation-energy release from dust—e.g., photon-stimulated desorption or mild thermolysis of metalated organics or Ni-rich nanophases, possibly including Ni–carbonyl-like complexes. These hypotheses will be testable with future coordinated ground-based and space-based monitoring, as 3I becomes more active during its continued passage through the solar system.

2I/Borisov is the first active interstellar comet observed in the Solar Sytem, allowing for the first time to sample the composition of a planetary building block from an extrasolar system. We report on the monitoring of 2I with the FORS low resolution spectrograph of the ESO VLT at Paranal during four months, from November 19, 2019 to March 20, 2020. We collected a dozen spectra at 8 different epochs allowing to follow the evolution of the comet activity and composition around perihelion. We also observed with the same instrumental setup an Oort Cloud comet, C/2019 U6 (Lemmon), at about the same heliocentric and geocentric distance than 2I/Borisov at perihelion (rh=Delta=2 au) and with similar AfRho value and Q(CN) in order to use it as a reference for the Solar System. The usual species are detected in the optical spectrum of 2I (CN, C3, C2, and NH2) and their production rates and abundance ratios are computed. The dust production rate and colors are also derived, compared to C/2019 U6 and other comets of the Solar System, and their evolutions are followed with the heliocentric distance.