Jooyeon Geem

Launched in March 2025, NASA’s SPHEREx mission is currently conducting the first all-sky near-infrared spectral survey at wavelengths between 0.75μm and 5μm. Over its two-year nominal mission, SPHEREx acquires a spectrum for every 6.2” x 6.2” pixel on the sky, a transformative dataset for its primary science drivers in cosmology, galaxy formation, and interstellar ices [1]. However, this survey also provides an unprecedented opportunity for Solar System science, as exemplified by the recent SPHEREx discovery of a CO2 atmosphere around the interstellar comet 3I/ATLAS [2]. This presentation details the ongoing efforts of the SPHEREx Solar System Object Catalogue (SSOC) External Collaboration (XC) to extract, calibrate, and categorise these spectra, with a focus on the first-year asteroid results.The scientific significance of the SPHEREx dataset for minor bodies lies in its unique spectral coverage. While previous large-scale taxonomic efforts have focused on silicaceous compositions discernible in ground-based observations between 0.45μm and 2.45μm [3, 4], SPHEREx provides a critical window into the presence of volatiles, organics, and hydrated minerals identified at wavelengths beyond 2.45 μm (e.g. [5]). The sheer volume of high-quality spectra generated by this mission allows for a statistically robust ensemble analysis of the Main Belt and beyond, transitioning from individual case studies toward a holistic characterisation of the Solar System’s compositional landscape [6].Now, after more than one year of active operations and data processing, the SSOC XC presents a comprehensive status update. We showcase the first intermediate spectra derived from the mission pipeline, spanning diverse taxonomic classes including C-, S-, and M-complex asteroids. Extracting high-fidelity spectra for moving targets from a scanning mission presents unique data-reduction challenges; we discuss our solutions for these issues and address the remaining systematics currently present in the spectral products. Finally, we provide a timeline for the public release of the initial Solar System Object Catalogue. This work represents a major milestone in planetary astronomy, providing a rich, uniform spectral library that will serve as a cornerstone for Solar System studies for decades to come.

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 time series medium-band ( R ∼ 20–40) observations of the third interstellar object (ISO) 3I/ATLAS (C/2025 N1) obtained with the 7-Dimensional Telescope, which enabled spatially resolved monitoring of its gas and dust activity from 2025 July to September. The m400-band image ( λ _c = 400 nm, Δ λ ≈ 25 nm) reveals an emergence of a pronounced and spatially extended CN emission at a heliocentric distance of r _h < 3 au. This onset is consistently identified across multiple diagnostics, including a break in the light-curve evolution, excess reflectance, inward expansion of annular excess beyond 10,000–20,000 km, growth of the coma half-light radius from ∼11,000 to ∼19,000 km, and a rapid rise in the CN production rate Q CN relative to Afρ . We further separate the CN-emitting and dust-scattered components via two-dimensional surface brightness fitting into inner (dust) and outer (gas) components. We find that the outer component preserves a nearly constant profile shape, varying only in normalization, implying a relatively fast expansion of CN-bearing molecules. Together, these results reveal how a transition occurs in the optical from dust-dominated scattering at large heliocentric distances to volatile-driven, gas-dominated activity as 3I/ATLAS enters the inner solar system. The timing and characteristics of the CN activation resemble the volatile enhancement observed in 2I/Borisov, suggesting that both known active ISOs exhibit comparable activation behavior at heliocentric distances of ∼2–3 au.

We present time-series medium-band (R~20-40) observations of the third interstellar object 3I/ATLAS (C/2025 N1) obtained with the 7-Dimensional Telescope (7DT), enabling spatially resolved monitoring of its gas and dust activity from 2025 July to September. The m400-band image (lambda_c = 400 nm, Delta lambda approx 25 nm) reveals the emergence of pronounced and spatially extended CN emission at heliocentric distances r_h < 3 au. This onset is consistently identified across multiple diagnostics, including a break in the light-curve evolution, excess reflectance, inward expansion of annular excess beyond 10,000-20,000 km, growth of the coma half-light radius from ~11,000 to ~19,000 km, and a rapid rise in the CN production rate Q_CN relative to Af rho. We further separate the CN-emitting and dust-scattered components through two-dimensional surface-brightness fitting into inner (dust) and outer (gas) components. The outer component preserves a nearly constant profile shape, varying only in normalization, implying relatively fast expansion of CN-bearing molecules. Together, these results reveal a transition in the optical from dust-dominated scattering at large heliocentric distances to volatile-driven, gas-dominated activity as 3I/ATLAS enters the inner Solar System. The timing and characteristics of the CN activation resemble the volatile enhancement observed in 2I/Borisov, suggesting that both known active interstellar objects exhibit comparable activation behavior at heliocentric distances of ~2-3 au.

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.

Our study investigates the potential effects of intense solar radiation heating on the physical properties of carbonaceous asteroids, such as surface textures, particle-size distributions, and porosity. The observed number of near-Earth asteroids (NEAs) with small perihelion distances (q) is smaller than theoretically predicted (Granvik et al. 2016), indicating that NEAs are catastrophically disrupted near the Sun. One proposed destruction mechanism is that intense solar radiation induces thermal fracturing, eventually leading to breakup. Notably, (3200) Phaethon has exhibited recurrent activity near its perihelion (q = 0.14 au), such as dust and/or gas ejection over several decades (Jewitt et al. 2015), suggesting it may be experiencing thermal disruption. At this moment, the physical properties of NEA surfaces can provide insights into the mechanisms affecting them during close solar encounters.However, the effects of solar heating on the physical properties of NEA surfaces have not been extensively studied. For example, it remains unclear whether radiation heating causes sintering (leading to enlarged particles), thermal fracturing (leading to finer particles), or whether unknown mechanisms affect the resulting particle-size distributions. To address this, we simulate the near-Sun environment in the laboratory by using the Space and High-Irradiation Near-Sun Simulator located at Luleå University of Technology’s Space Campus in Kiruna, Sweden (Tsirvoulis et al., 2022), where the carbonaceous chondrite simulants (Britt et al., 2019) are irradiated under vacuum (∼10-5 atm), using a solar-like spectrum in the visible range. We have carried out the experiments at intensity levels equivalent to those at a heliocentric distance of 0.12 au, and varied the sample characteristics such as particle sizes and compositions. We find that radiation heating alters the physical properties of the simulants. For example, tamped fine-grained samples of CM simulant (samples sieved by the 351 µm sieve) show a few hundred-micrometer-sized aggregates, pits, and millimeter-scale cracks after heating (Fig. 1).We will present our experimental results. Then, by combining these with previous polarimetric laboratory studies, we interpret the polarimetric properties of NEAs observed at large phase angles, such as the maximum polarization degree and the maximum phase angle. For this discussion, we use polarimetric measurements from both previous studies and our own observations conducted by using ALFOSC on the Nordic Optical Telescope.Figure 1. Stereo microscope images of (a) the tamped fine-grained samples of CM simulant before radiation heating, and (b–f) after radiation heating. White scale bars are shown at the top left of each image, with scales of 2 mm, 500 μm, 2 mm, 2 mm, 500 μm, and 500 μm for (a), (b), (c), (d), (e), and (f), respectively. Please note that image brightness and color vary due to differences in exposure time between images and are not normalized.

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.