Sunho Jin

From 2025 August 1 to August 15 UT, the SPHEREx spacecraft observed interstellar object 3I/ATLAS. Using R = 40–130 spectrophotometry at λ = 0.7–5 μ m, lightcurves, spectra, and imaging of 3I were obtained. From these, robust detections of water gas emission at 2.7–2.8 μ m and CO ₂ gas at 4.23–4.27 μ m plus tentative detections of ¹³ CO ₂ and CO gas were found. A slightly extended H ₂ O coma was detected, and a huge CO ₂ atmosphere extending out to at least 4.2 × 10 ⁵ km was discovered. Gas production rates and 1 σ errors for H ₂ O, ¹² CO ₂ , ¹³ CO ₂ , and CO were Q _gas = 3.2 × 10 ²⁶ ± 20%, 1.6 × 10 ²⁷ ± 10%, 1.3 × 10 ²⁵ ± 25%, and 1.0 × 10 ²⁶ ± 25%, respectively. Coaddition of all λ = 1.0–1.5 μ m scattered light continuum images from 3I produced an image with high signal-to-noise ratio consistent with an unresolved source. The lightcurve of scattered light showed ≲15% variability over the observation period. The absolute brightness of 3I at 1.0–1.5 μ m is consistent with a nucleus of μ m continuum structure shows a strong spectral feature commensurate with water ice absorption seen in Kuiper Belt objects and distant comets. The observed cometary behavior of 3I, including its preponderance of CO ₂ emission, lack of CO output, small size, and predominance of large icy chunks of material in a flux-dominant coma, is similar to the behavior of short-period comet 103P/Hartley 2, the ”hyperactive, strongly thermally processed comet” flyby target of the NASA Deep Impact Extended mission in 2010. This correspondence suggests that interstellar objects can be significantly thermally processed before ejection into the interstellar medium, and by comparison to 1I and 2I, this processing can be widely variable in its physical outcome.

We present independent polarimetric observations of the interstellar object 3I/ATLAS, including the first near-infrared polarimetric measurements. Using imaging polarimeters, we measured the degree of linear polarization from the visible R _C band (0.64 μ m) to the near-infrared K _s band (2.25 μ m), and investigated its dependence on solar phase angle (polarization phase curve, PPC) and wavelength (polarization color curve, PCC). We confirm that the PPC of 3I/ATLAS differs significantly from those of typical solar system comets, showing an unusually large polarization amplitude. This PPC shows no significant change in the R _C band across perihelion passage, despite the perihelion lying within the water snow line. This indicates that the unusual polarimetric behavior of 3I/ATLAS is unlikely to be driven by transient volatile activity, but instead reflects intrinsic optical properties of refractory dust particles. The PCC increases with wavelength over 0.6–1.2 μ m and peaks at 1.5–2.0 μ m, suggesting that the dominant scattering units are dust aggregates composed of submicron-sized monomers, broadly consistent with interstellar dust and solar-system cometary aggregates. Taken together, our results indicate that 3I/ATLAS preserves polarimetric properties characteristic of a primitive cometary planetesimal formed in another planetary system, with a refractory dust composition that differs from that typically observed among solar system comets, despite sharing a similar size scale of the aggregate building blocks.

We present the Solar system Objects Light curve Observatory (SOLO), a wide-field, high-cadence optical survey system designed to obtain absolutely calibrated asteroid light curves, converted to the Gaia G-band photometric system, in support of the SPHEREx Solar System Object Catalog (SSOC). SOLO was installed at the Sierra Remote Observatories (SRO) in California, USA, in July 2025 and is optimized for continuous, multi-night monitoring of asteroid brightness variations. We describe the system configuration, remote operation, and data reduction pipeline, and evaluate its optical and photometric performance using commissioning data. SOLO achieves stable photometric calibration across the 11.6 deg^2 field of view and reaches a 10-sigma limiting magnitude of G ~ 17.5 for a 180 sec exposure. Sample asteroid light curves obtained over multiple nights demonstrate consistent absolute photometry at the same rotational phase, validating the estimated performance. Finally, we outline the planned operational use of SOLO in connection with NASA's SPHEREx mission. Full science operations of SOLO are scheduled to begin in January 2026. Using these data, we aim to obtain on the order of 10^3 absolutely calibrated asteroid light curves per year in the Gaia G-band, which will be used to support the construction and scientific utilization of the SPHEREx SSOC.

We present SQUIDPOL, a low-cost, multi-channel optical imaging polarimeter that performs simultaneous linear polarization measurements using a rotating half-wave plate, a non-polarizing beam splitter, and four wire-grid filters. We show that the off-the-shelf non-polarizing beam splitter introduces measurable polarization-dependent systematics, which can bias polarimetric measurements if left uncorrected. We quantify this effect for both transmitted and reflected beams and incorporate a correction scheme into the data-analysis pipeline. On-sky validation demonstrates stable and reproducible performance, achieving a polarization accuracy of about 0.15 percent for bright polarized standard stars. Mounted on the 60-cm Ritchey-Chretien telescope (focal length 4200 mm, f/7) at the Pyeongchang Observatory of Seoul National University, SQUIDPOL provides an effective common field of view of 13.5 by 8.2 arcminutes with a pixel scale of 0.45 arcseconds per pixel and supports standard B, V, R_C, and I_C filters.

Context . The surface of asteroid (25143) Itokawa shows both fresh and mature terrains, despite its short space-weathering timescale of approximately 10 ³ years, as inferred from recent studies. Seismic shaking triggered by the impact that formed the 8-meter crater Kamoi has been proposed as a possible explanation for the diversity. Aims . This study aims to examine whether the seismic shaking induced by the impact might account for the observed spatial variations in space weathering and might further constrain the internal structure of Itokawa. Methods . Assuming that the Kamoi crater was formed by a recent impact, we conducted three-dimensional seismic wave propagation simulations and applied a simplified landslide model to estimate surface accelerations and boulder displacements. Results . Our results show that even a low-energy case (1% of the nominal seismic energy) produces surface accelerations sufficient to destabilize the surface materials. The simulated boulder displacements are consistent with the observed distribution of space-weathering degrees even on the opposite hemisphere. We estimate the seismic diffusivity to be 1000–2000 m ² s ⁻¹ and the seismic efficiency to be in the range of 5.0 × 10 ⁻⁸ to 5.0 × 10 ⁻⁷ , implying that the interior of Itokawa contains blocks tens of meters across and acts as a strongly scattering medium. Conclusions . Our findings provide unique dynamical evidence, based on seismic wave propagation modeling, that supports the hypothesis that the interior of Itokawa truly is a rubble pile.

We present the ongoing effort to construct the catalog of Solar System Object (SSO) spectra obtained by the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx). NASA's SPHEREx mission was successfully launched on March 11, 2025, and has begun conducting the first all-sky spectral survey in the 0.75–5.0 µm range, with R = 41 to 135 (see e.g., Crill, B. P., et al. 2024, arXiv:2404.11017). The mission achieves sensitivity levels comparable to WISE/NEOWISE W1–W2 and operates from space, unaffected by telluric atmospheric H₂O and CO₂ absorption, using a single, stable, well-characterized, and calibrated platform. SPHEREx is expected to deliver high-quality spectra for ~100,000 SSOs (Ivezic, Z., et al. 2022, Icarus, 371, 114696). Its spectral range is highly diagnostic for identifying surface compositions of small bodies, including hydrated minerals, water ice, and organic compounds. For example, SPHEREx will significantly expand the available data on the 2.7 µm absorption band, a feature indicative of past liquid water activity on SSOs, which cannot be observed from the ground due to atmospheric absorption (Lisse C., et al., 2024, arXiv:2402.08705)However, additional calibration is required to obtain science-ready reflectance spectra of Small Solar System Objects. SPHEREx takes several days to obtain a full spectrum for individual targets, and during that time, the SSOs’ position and brightness change due to the changing target-spacecraft geometry and the SSO's rotation. Furthermore, the SSO's thermal emission will be imprinted on the observed spectrum. We thus need to account for multiple geometrical (spacecraft-target distance, phase curve) and physical effects (lightcurve, thermal emission) that depend on the dynamical and physical properties of a given SSO.To address this, a collaborative SSO team has been formed to develop the calibration pipeline for SSOs and, eventually, construct a catalog of SSO spectra. The team consists of researchers from diverse institutions, including the Korea Astronomy and Space Science Institute (KASI), Johns Hopkins University Applied Physics Laboratory (APL), and Seoul National University (SNU). Ongoing work includes the collection of existing photometric data to obtain light curves for as many asteroids as possible and the preparation to operate an 11-inch telescope at Yosemite, dedicated to obtaining light curves specifically for SPHEREx calibration. In addition, efforts are underway to acquire ground-based spectra to validate SPHEREx spectral data. In this presentation, we report on the activities of the SSO collaborative team, the current status, and the future plans of the calibration pipeline and the SPHEREx asteroid spectral catalog.Figure (a). First light images from SPHEREx, taken on March 27, 2025, each contain over 100,000 detected sources. Every exposure includes six images — one per detector — covering a 3.5° × 11.5° field of view. The top and bottom image sets show the same sky area. When routine science begins in April, SPHEREx will take ~600 exposures daily (credit: NASA),Figure (b). Spectra of large asteroids, produced using data from GAIA DR3, SMASS, MITHNEOS, and AKARI. SPHEREx will capture spectra in the 0.9 to 5 µm.

We introduce a newly developed polarimeter, the Seoul National University QUadruple Imaging Device for POLarimetry (SQUIDPOL). Polarimetry is a powerful technique in astronomy for investigating the physical properties of celestial bodies such as small Solar System objects. In the visible wavelength range, Wollaston prisms have often been used to achieve high-precision polarization measurements. However, despite their high accuracy, Wollaston prism-based polarimeters have limitations, including a narrow field of view and the need for collimating optics. SQUIDPOL employs a non-polarizing beamsplitter, four wire-grid polarizers, and four CMOS imagers (Fig. a). By using wire-grid polarizers instead of the commonly adopted Wollaston prisms, SQUIDPOL achieves a moderate field of view (15.6 × 10.7 arcmin²) with a single exposure, which is wider than that of typical Wollaston-based systems (Fig. b).In this presentation, we share the design, development process, and performance evaluation of SQUIDPOL, currently mounted on the 60-cm telescope at the Pyeongchang Observatory of Seoul National University in South Korea. SQUIDPOL is well suited for observing small Solar System objects due to its low-altitude observation capability (down to ~10° altitude), lower competition for telescope time, and single-exposure polarimetry that minimizes the rotational effects of these bodies. SQUIDPOL aims to achieve a polarization accuracy of ΔP ~ 0.1%. This presentation includes results from laboratory experiments conducted on an optical table to validate the design, as well as performance assessments based on actual observations. We also introduce our recent scientific applications of SQUIDPOL, such as the polarimetric observation of Comet Tsuchinshan–ATLAS (C/2023 A3; Lim et al. 2025, ApJL, 983, L19). Finally, we provide practical information for potential users, including operational procedures, remote observation capabilities, and telescope usage policies.. Figure (a) Optical layout of SQUIDPOL. HWP and WGF denote the half-wave plate and wire-grid filter, respectively. B1 represents the first branch, reflected by the non-polarizing beamsplitter (NPBS), while B2 is the second branch, transmitted through the NPBS.Figure (b) A set of four images captured by SQUIDPOL in a single exposure. These images show Comet C/2023 A3, observed on 2024 October 17.

We conducted contemporaneous optical and near-infrared polarimetric and spectroscopic observations of C/2023 A3 (Tsuchinshan-ATLAS, hereafter T-A) from 2024 October 16 to December 17, covering a wide range of phase angles (20°–123°) and wavelengths (0.5–2.3 μm). We paid special attention to gas contamination in the dust polarization using these data. As a result, we find the maximum polarization degree P max = 31.21 % ± 0.05 %, 33.52% ± 0.06%, 35.12% ± 0.01%, 37.57% ± 0.01%, and 35.35% ± 0.01% in the Rc, Ic, J, H, and Ks bands, respectively. Although dust polarization shows a red slope at shorter wavelengths and can peak around 1.6 μm, the phase angle at which maximum polarization occurs exhibits less dependence on wavelength ( α max ∼ 90 ° –95°). Although only a few historically bright comets, such as West, Halley, and Hale–Bopp, have undergone such extensive dust-polarization observations, our measurements are generally consistent with those of two comets that possibly originated from the Oort Cloud (West and Halley). From these results, we conjecture that the optical properties and growth processes of dust in the presolar nebula, which formed these cometary nuclei, were likely uniform.

We conducted contemporaneous optical and near-infrared polarimetric and spectroscopic observations of C/2023 A3 (Tsuchinshan-ATLAS, hereafter T-A) from 2024 October 16 to December 17, covering a wide range of phase angles (20-123 deg) and wavelength (0.5-2.3 um). We paid special attention to gas contamination in the dust polarization using these data. As a result, we find the maximum polarization degree $P_max=31.21 +- 0.05 %, 33.52 +- 0.06 %, 35.12 +- 0.01 %, 37.57 +- 0.01 %, and 35.35 +- 0.01 % in the Rc-, Ic-, J-, H-, and Ks-bands, respectively. Although dust polarization shows a red slope at shorter wavelengths and can peak around 1.6 um, the phase angle at which maximum polarization occurs exhibits less dependence on wavelength (alpha_max ~ 90 deg - 95 deg). Although only a few historically bright comets, such as West, Halley, and Hale-Bopp, have undergone such extensive dust-polarization observations, our measurements are generally consistent with those of two comets that possibly originated from the Oort Cloud (West and Halley). From these results, we conjecture that the optical properties and growth processes of dust in the presolar nebula, which formed these cometary nuclei, were likely uniform.

Context. (4015) Wilson-Harrington (hereafter, WH) was discovered as a comet in 1949 but has a dynamical property consistent with that of a near-Earth asteroid. Although there is a report that the 1949 activity is associated with an ion tail, the cause of the activity has not yet been identified. Aims. This work aims to reveal the mysterious comet-like activity of the near-Earth asteroid. Methods. We conducted new polarimetric observations of WH from May 2022 to January 2023, reanalyses of the photographic plate images taken at the time of its discovery in 1949, and dust tail simulation modelings, where the dust terminal velocity and ejection epoch are taken into account. Results. We found that this object shows polarization characteristics similar to those of low-albedo asteroids. We derived the geometric albedo ranging from p_V = 0.076 ± 0.010 to p_V = 0.094 ± 0.018 from our polarimetry (the values vary depending on the data used for fitting and the slope-albedo relationship coefficients). In addition, the 1949 image showed an increase in brightness around the nucleus. Furthermore, we found that the color of the tail is consistent with sunlight, suggesting that the 1949 activity is associated with dust ejection. From the dust tail analysis, ~9 × 10⁵ kg of material was ejected episodically at a low velocity equivalent to or even slower than the escape velocity. Conclusions. We conclude that WH is most likely an active asteroid of main belt origin and that the activity in 1949 was likely triggered by mass shedding due to fast rotation.