Tomohiko Sekiguchi

Studying the physical properties of near-Earth asteroids (NEAs) is crucial for understanding their dynamical histories and origins, and assessing impact hazards to Earth. Tiny NEAs with diameters smaller than 100 m are intrinsically faint and are typically observable only during close approaches, resulting in few well-characterized objects. Furthermore, because these objects are often fast-moving and fast-rotating, sequential multiband photometry is prone to systematic offsets in derived colors. To mitigate this effect, we performed simultaneous g -, r -, and i -band photometry of three tiny NEAs using the TriColor CMOS Camera and Spectrograph on the 3.8 m Seimei Telescope. We used high-cadence video observations with exposure times of 1 s and 5 s to investigate lightcurve variations on timescales of seconds. All three NEAs are confirmed as fast rotators with rotation periods shorter than 60 s: 15.281 ± 0.002 s for 2021 TY ₁₄ , 21.099 ± 0.003 s for 2021 UW ₁ , and 8.779 ± 0.013 s for 2022 GQ ₁ . The derived colors indicate that 2021 TY ₁₄ belongs to the X-complex, while 2021 UW ₁ and 2022 GQ ₁ belong to the S-complex. Their positions in the diameter–rotation period diagram show that all three objects belong to the small, fast-rotating NEA population, with 2022 GQ ₁ being the smallest and fastest-rotating among them with spectroscopic measurements. Analysis of the color time series suggests that the surfaces of observed NEAs are largely homogeneous, although 2021 TY ₁₄ exhibits statistically significant g – r color heterogeneity with a projected spot fraction of approximately 50%. For 2021 UW ₁ , minor localized variations of up to ∼20% in composition cannot be ruled out. Our study demonstrates that high-cadence, multicolor photometry with small to medium telescopes is effective for determining rotation periods and broadly classifying the spectral properties of tiny NEAs.

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.

Context. 2024 PT₅ is a tiny (D ≤ 10 m) near-Earth asteroid (NEA) discovered in August 2024. 2024 PT₅ was gravitationally bound to the Earth-Moon system from September to November 2024 and classified as a mini-moon. Several quick response observations suggest that 2024 PT₅ has a lunar ejecta origin, while the rotation state and albedo, essential properties for investigating its origin, are not well constrained. Aims. We aim to characterize the spectrophotometric properties of 2024 PT₅ by ground-based observations to test its taxonomic classification and origin. Methods. We performed visible to near-infrared multicolor photometry of 2024 PT₅ from data taken using the TriColor CMOS Camera and Spectrograph (TriCCS) on the Seimei 3.8 m telescope during January 4–10, 2025. The Seimei/TriCCS observations of 2024 PT₅ cover phase angles from 14 deg to 27 deg, and were obtained in the ɡ, r, i, and ɀ bands in the Pan-STARRS system. In addition, we analyzed Y, J, H, and K photometry taken with the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) on the Keck I 10-m telescope on January 16–17, 2025. Results. Our light curves show brightness variations over time periods of several tens of minutes. We infer that 2024 PT₅ is in a tumbling state and has a light curve amplitude of about 0.3 mag. Visible and near-infrared color indices of 2024 PT₅, ɡ – r = 0.567 ± 0.044, r – i = 0.155 ± 0.009, r – ɀ = 0.147 ± 0.066, Y – J = 0.557 ± 0.046, J – H = 0.672 ± 0.078, and H – Ks = 0.148 ± 0.098, indicate that 2024 PT₅ is an S-complex asteroid, largely consistent with previous observations. Using the H–G model, we derived an absolute magnitude, H_V,HG, of 27.72 ± 0.09 and a slope parameter, G_V, of 0.223 ± 0.073 in the V band. A geometric albedo of 2024 PT₅ was derived to be 0.26 ± 0.07 from the slope of its photometric phase curve. This albedo value is typical of the S- and Q-type NEAs. Conclusions. Using the albedo and absolute magnitude, the equivalent diameter of 2024 PT₅ is estimated to be 7.4 ± 1.0 m. The color properties of 2024 PT₅ derived from our observations match the ones of rock samples taken from the lunar surface, which agrees with previous studies.

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.

Context. Ch-type asteroids are distinctive among other dark asteroids in that they exhibit deep negative polarization branches (NPBs). Nevertheless, the physical and compositional properties that cause their polarimetric distinctiveness are less investigated. Aims. We aim to investigate the polarimetric uniqueness of Ch-type asteroids by making databases of various observational quantities (i.e., spectroscopic and photometric properties as well as polarimetric ones) of dark asteroids. Methods. We conducted an intensive polarimetric survey of 52 dark asteroids (including 31 Ch-type asteroids) in the R_C band to increase the size of polarimetric samples. The observed data were compiled with previous polarimetric, spectroscopic, and photometric archival data to find their correlations. Results. We find remarkable correlations between these observed quantities, particularly the depth of NPBs and their spectroscopic features associated with the hydrated minerals. The amplitude of the opposition effect in photometric properties also shows correlations with polarimetric and spectral properties. However, these observed quantities do not show noticeable correlations with the geometric albedo, thermal inertia, and diameter of asteroids. Conclusions. Based on the observational evidence, we arrive at our conclusion that the submicrometer-sized structures (fibrous or flaky puff pastry-like structures in phyllosilicates) in the regolith particles could contribute to the distinctive NPBs of hydrated asteroids.

Studying small near-Earth asteroids is important in order to understand their dynamical histories and origins as well as to mitigate the damage caused by asteroid impacts on Earth. We report the results of multicolor photometry of the tiny near-Earth asteroid 2015 RN₃₅ using the 3.8 m Seimei telescope in Japan and the TRAPPIST-South telescope in Chile over 17 nights in 2022 December and 2023 January. We observed 2015 RN₃₅ across a wide range of phase angles from 2° to 30° in the g, r, i, and z bands in the Pan-STARRS system. These lightcurves show that 2015 RN₃₅ is in a nonprincipal axis spin state with two characteristic periods of 1149.7 ± 0.3 s and 896.01 ± 0.01 s. We found that the slope of the visible spectrum of 2015 RN₃₅ is as red as asteroid (269) Justitia, one of the very red objects in the main belt, which indicates that 2015 RN₃₅ can be classified as an A- or Z-type asteroid. In conjunction with the shallow slope of the phase curve, we suppose that 2015 RN₃₅ is a high-albedo A-type asteroid. We demonstrated that surface properties of tiny asteroids could be well constrained by intensive observations across a wide range of phase angles. 2015 RN₃₅ is a possible mission-accessible A-type near-Earth asteroid with a small Δv of 11.801 km s⁻¹ in the launch window between 2030 and 2035.

Asteroid systems such as binaries and pairs are indicative of the physical properties and dynamical histories of small solar system bodies. Although numerous observational and theoretical studies have been carried out, the formation mechanism of asteroid pairs is still unclear, especially for near-Earth asteroid (NEA) pairs. We conducted a series of optical photometric and polarimetric observations of a small NEA 2010 XC₁₅ in 2022 December to investigate its surface properties. The rotation period of 2010 XC₁₅ is possibly a few to several dozen hours and the color indices of 2010 XC₁₅ are derived as g − r = 0.435 ± 0.008, r − i = 0.158 ± 0.017, and r − z = 0.186 ± 0.009 in the Pan-STARRS system. The linear polarization degrees of 2010 XC₁₅ are a few percent at the phase angle range of 58°–114°. We found that 2010 XC₁₅ is a rare E-type NEA on the basis of its photometric and polarimetric properties. Taking the similarity of not only physical properties but also dynamical integrals and the rarity of E-type NEAs into account, we suppose that 2010 XC₁₅ and 1998 WT₂₄ are of common origin (i.e., an asteroid pair). These two NEAs are the sixth NEA pair and first E-type NEA pair ever confirmed, possibly formed by rotational fission. We conjecture that the parent body of 2010 XC₁₅ and 1998 WT₂₄ was transported from the main belt through the ν ₆ resonance or Hungaria region.

We conducted optical polarimetry and near-infrared spectroscopy of JAXA’s Hayabusa2# mission target, (98943) 2001 CC₂₁, in early 2023. Our new observations indicate that this asteroid has a polarimetric inversion angle of ∼20○, absorption bands around 0.9 and 1.9 μm, and a geometric albedo of 0.23±0.04. All these features are consistent with those of S-types but inconsistent with L-types. Based on this evidence, we conclude that JAXA’s Hayabusa2# spacecraft will explore an S-type asteroid (more specifically, an L or LL chondrite-like object) with albedo and size (0.44–0.53 km when we assume the absolute magnitude of 18.8) similar to (25143) Itokawa.

Accurate estimations of the brightness of (3200) Phaethon up to lower phase angles are essential for the planning of the on-board camera for the DESTINY⁺ mission. We carried out intensive observations of Phaethon in the optical wavelength (g, r, and i) with the TriCCS camera on the Seimei 3.8 m telescope in 2021 October and November. We derived the absolute magnitude H_V and the slope parameter G of Phaethon as H_V = 14.23 ± 0.02 and G = 0.040 ± 0.008 from multiple photometric observations including lower phase angles down to ∼9° with the H–G model. Using the H_V value and the geometric albedo of Phaethon derived in previous polarimetric studies, we estimated that Phaethon’s diameter is within a range of 5.22–6.74 km, which is consistent with radar and occultation observations. With the linear model, we derived H_V = 14.65 ± 0.02, which corresponds to a diameter range of 4.30–5.56 km. Our simultaneous tricolor light curves of Phaethon indicate that no rotational spectral variations larger than 0.018 and 0.020 mag in the g − r and r − i colors, possibly related to inhomogeneity of the surface material and/or structure, are seen at the 2021 apparition.