Jin Beniyama

The small Solar System bodies (SSSBs) preserve records of the formation and evolution of the Solar System. In particular, the near-infrared wavelength region around 3 μm provides crucial insights into the origin and transport of hydrated materials and organics, although it is difficult to observe from the ground because of strong telluric absorption. We assess the observability of SSSBs as complementary science to the ESA Ariel space mission, operating in the visible to mid-infrared wavelength range (0.5–7.8 μm). Using the NASA JPL Horizons ephemeris system, we evaluate the visibility of known SSSBs from 2032 to 2035. Our analysis shows that Ariel can spectroscopically characterize up to 402 unique SSSBs under a conservative limiting visual magnitude of V = 13.6. If observations can be extended to somewhat fainter targets (V = 16), the number of observable SSSBs could increase to 2144. Assuming operational scenarios in which one SSSB is observed every 3, 7, or 30 days over a four-year mission, the total numbers of observable targets would be 357, 209, and 49, respectively. We further model the spectral energy distributions of candidate targets to evaluate their observational performance. We demonstrate that Ariel can resolve critical absorption features, such as those at 2.7 μm and 3.4 μm, with a higher spectral resolution than the previous infrared survey with AKARI. Observations of SSSBs with Ariel can provide insights into the distribution and evolution of life-forming materials in the Solar System and constitute a scientifically compelling component of Ariel’s complementary science program.

Context. The potentially hazardous asteroid (153814) 2001 WN ₅ will pass inside the lunar distance on June 26, 2028, offering a rare opportunity to characterize a kilometer-scale near-Earth asteroid at high angular resolution. However, previous taxonomic classifications have been inconsistent and its physical properties remain insufficiently consolidated. Aims. We aim to constrain the rotation state, shape, visible colors, geometric albedo, and taxonomy of 2001 WN ₅ prior to its 2028 close approach. Methods. We combined new photometry from the 1.54 m Danish Telescope (DK154) with archival and survey observations from the Transiting Exoplanet Survey Satellite (TESS), Dark Energy Camera (DECam), Zwicky Transient Facility (ZTF), and Asteroid Terrestrial-impact Last Alert System (ATLAS). These data were used to refine the rotation period, investigate the spin-shape solution space, derive visible colors, and estimate the geometric albedo from phase curve slopes. Results. We measured a synodic rotation period of 4.263 ± 0.002 h and 4.259 ± 0.009 h from DK154 and TESS, respectively, consistent with results from previous light curve observations. The available light curves do not uniquely constrain the sidereal rotation period, but the preferred pole solutions lie in the southern hemisphere in ecliptic coordinates. Visible colors from multiple independent datasets are consistent with the C-complex, while the TESS phase curve slopes give p _V = 0.13 ± 0.04, consistent with previous thermal-infrared albedo estimates. Combining the visible colors, albedo, and published near-infrared spectra, we classify 2001 WN ₅ as a likely B-type asteroid. The effective diameter is estimated to be D = 0.81 ± 0.13 km using the H - G model, while the linear model yields a slightly smaller value of 0.74 ± 0.11 km. Dynamical models indicate that 2001 WN ₅ likely originated from the inner main belt via the ν ₆ resonance, with the Flora–Baptistina and Nysa–Polana complexes identified as the parent candidates. Conclusions. During the 2028 encounter, 2001 WN ₅ should reach an apparent diameter of about 0.5 arcsec, making it an excellent target for coordinated photometric, spectroscopic, and high-angular-resolution observations. Observations during the 2026–2027 apparition will be essential for improving its spin and shape model before its 2028 close approach.

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.

D-type asteroids are among the most primitive, organic-rich objects in the Solar System and are widely thought to have originated in the outer regions of Solar System, being them key witnesses to early Solar System chemistry and volatile delivery. (162998) 2001 SK162 is a Near-Earth Asteroid (NEA) belonging to the Apollo group and designated as a Potentially Hazardous Asteroid (PHA). Discovered by the LINEAR program in 2001, the object follows an eccentric orbit (e ~ 0.47) with a small orbital inclination of 1.7° and a perihelion distance of ~1.01 au. Some previous studies showed that this asteroid is possible D/T types [1,2,3,4]. These orbital and spectral characteristics led to its selection as the D-type primary target of the Japanese Next Generation Small-body Sample Return mission (NGSR-D) [5]. Precise knowledge of the target’s fundamental physical properties—in particular its geometric albedo, diameter, rotation state, and shape—is essential for mission planning, including spacecraft navigation, approach trajectory design, and sample acquisition operations. However, these properties remain poorly constrained: previous photometric and thermal-infrared observations [7,6] have yielded significant uncertainties that could directly impact mission design and operations.To address this, we conducted a coordinated, multi-technique observing campaign of 2001 SK162 between October 2025 and March 2026, using a worldwide network of facilities: lightcurve photometry with the Two-meter Twin Telescope (TTT) at Teide Observatory; visible multi-band photometry with MuSCAT3/4 on the Las Cumbres Observatory (LCO) 2-m telescopes at Haleakala Observatory and with the Pirka 1.6-m telescope at Nayoro Observatory; visible spectroscopy with OSIRIS on the 10.4-m Gran Telescopio Canarias (GTC) at Roque de los Muchachos Observatory (ORM); NIR multi-band photometry with MOSFIRE on Keck I and polarimetry with the Subaru Telescope at Maunakea Observatories and the 2.5-m Nordic Optical Telescope (NOT) at ORM. In parallel, we reanalyzed archival WISE and Spitzer thermal-infrared data to independently constrain the size and albedo.Lightcurves were obtained with TTT over 37 nights between 2025-10-21 and 2025-12-22. Periodogram analysis yields a rotational period of ~178 h, much longer than the previous estimate [8] and placing 2001 SK162 firmly in the slow-rotator regime. The lightcurve shows a peak-to-peak amplitude of 0.34 mag, indicating a moderately elongated shape. Combining our TTT photometry with archival ZTF data, we fit the H–G phase function and obtain H_V = 18.04 mag and G = 0.10, consistent with the photometric behavior of dark, primitive asteroids.Visible multi-band photometry with MuSCAT3/4 (LCO 2-m) and Pirka 1.6-m telescope and visible spectroscopy with OSIRIS/GTC consistently reveal a featureless, red visible reflectance spectrum. Within the recent taxonomic framework of Mahlke et al. [9], the spectrum is best classified as D- or Z-type, although the color variation with different rotational phase is little. This confirms the spectral classification on which the NGSR-D target selection was based and reinforces the scientific case for sampling 2001 SK162. Moreover, we also obtained the near infrared reflectance at Y, J, and K bands by MOSFIRE/Keck I.Because the geometric albedo of 2001 SK162 has been poorly constrained, we performed polarimetric observations with Subaru and NOT to discriminate between high- and low-albedo solutions, and reanalyzed archival WISE and Spitzer thermal-infrared data to provide an independent estimate of the geometric albedo and effective diameter. Joint analysis of these complementary techniques is ongoing and will be presented at the meeting, with the goal of delivering a self-consistent set of physical parameters for the NGSR-D mission. Together, these results provide the first comprehensive physical characterization of (162998) 2001 SK162. The combination of an unusually long rotation period, elongated shape, and primitive D/Z-type spectral classification makes this object a scientifically compelling target for NGSR-D. [1] Binzel et al. (2004) MAPS 39, 351, [2] Ye (2011) AJ 141, 32, [3] Thomas et al. (2014) Icarus 228, 217, [4] Marsset et al. (2022) AJ 163, 165, [5] Okada, T. et al. (2026) EPSC 2026, [6] Mainzer et al. (2011) AJ 743, 156, [7] Mueller et al. (2011) AJ 141, 109, [8] Pravec, P. et al. (2005) https://space.asu.cas.cz/~ppravec/neo.htm, [9] Mahlke, M., Carry, B., & Mattei, P.A. (2022) A&A 665, A26.

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.

One of the major achievements of planetary science in the past century has been the discovery of a population of asteroids and comets that have the potential to impact our planet. These objects are collectively known as Near-Earth Objects (NEOs), which include both asteroids and comets whose orbits bring them within 1.3 astronomical units (au) of the Sun. Among them, a subset is classified as Potentially Hazardous Asteroids (PHAs) — objects that have an Earth Minimum Orbit Intersection Distance (MOID) of 0.05 au or less and an absolute magnitude (H) of 22 or brighter, indicating a size large enough to cause significant damage in the event of an impact. Imminent Earth impactors have been shown to "come" from two principal directions in the sky, one towards local midnight and the other towards the local midday [1].While midnight impactors are well covered by existing optical survey telescopes, midday impactors present a major observational challenge. From the ground, observations in the direction of the Sun are nearly impossible due to its overwhelming brightness, which prevents the detection of faint asteroid signals. The Chelyabinsk event of 2013 highlighted this vulnerability, as the ~20-meter asteroid responsible for the airburst was completely undetected before impact, approaching from a region of the sky that was effectively unobservable by existing surveys [2]. A promising solution to this problem is to place an infrared telescope at the Sun-Earth L1 Lagrange point. From this vantage point, such an observatory would be able to scan the region of space closest to the Sun with unprecedented sensitivity, detecting asteroids that would otherwise remain hidden. This is the fundamental concept behind NEOMIR (Near-Earth Object Mission in the Infrared), a proposed ESA concept mission, designed to provide early warning for asteroids approaching from the daytime sky [3]. NASA's Near-Earth Object (NEO) Surveyor will also be at L1 Lagrange point [4], but NEOMIR will be complementary in terms of survey region. By observing in the infrared at small solar elongations, NEOMIR would complement existing ground-based surveys and significantly enhance our ability to detect and respond to imminent impact threats. By positioning NEOMIR at the Sun-Earth Lagrange point L1, ESA aims to maximize early detection capabilities for hazardous asteroids approaching from the Sun’s direction. However, these asteroids are typically detected at high phase angles (angle between the Sun and the observer as seen from the asteroid) as seen from NEOMIR at L1 Lagrange point.Hence, the mission’s success will depend on addressing the technical and observational challenges posed by high phase-angle detections, ultimately improving our ability to provide timely warnings for potential impactors. Unlike visible-light observations, which rely on sunlight reflected off an asteroid’s surface, in the medium infrared it is possible to measure the thermal radiation emitted by the asteroid itself. This advantage becomes especially crucial at high phase angles — when the Sun, the asteroid, and the observer form a nearly straight line. At such angles, only a small fraction of the asteroid’s illuminated surface is visible in reflected light, making it difficult to detect the body. However, in the thermal infrared, an asteroid's emission originates from most of its surface, which remains warm and radiates detectable heat. Despite these advantages, observing asteroids in the thermal infrared at high phase angles presents significant challenges. The extreme observing geometry complicates flux predictions, as both standard and sophisticated thermal models have not been fully validated for such conditions. Additionally, space-based detectors operating near the Sun must contend with high background noise from zodiacal dust emission and stray light, which can limit sensitivity. Overcoming these obstacles requires careful optimization of telescope location, observation strategy, and data processing techniques.To simulate NEOMIR observations of imminent Earth impactors, we employ a ThermoPhysical Model (TPM; implementation based on [5]) to compute infrared fluxes from synthetic asteroid populations. These fluxes will then be analyzed with simple thermal models, which output the estimated diameter of the synthetic asteroid. The error between these derived diameters and the original ones (used as input for the TPM) will then be discussed. In this presentation, we provide an overview of the NEOMIR mission and present a simulation framework to estimate the infrared fluxes of synthetic asteroids, using a TPM and physically plausible assumptions about their properties (e.g., thermal inertia and pole orientations).AcknowledgmentsThis work was supported by the French government through the France 2030 investment plan managed by the National Research Agency (ANR), as part of the Initiative of Excellence Université Côte d’Azur under reference number ANR-15-IDEX-01. This work was supported by JSPS KAKENHI grant Number JP23KJ0640.References:[1] Veres et al., 2009, Icarus, Vol. 203, 472.[2] Müller et al., submitted to Advances in Astronomy.[3] Conversi et al., 2024, Proceedings of the SPIE, Vol. 13092, 130922H.[4] Mainzer et al., 2023, PSJ, Vol. 4, 224.[5] Delbo’ et al., 2007, Icarus, Vol. 190, 236.

Near-Earth object (NEO) 1998 KY₂₆ is a target of the Hayabusa2# spacecraft, which it will rendezvous with in 2031 July. The asteroid has been noted to rotate rapidly and has a large out-of-plane nongravitational acceleration. We present observations consisting of deep-g- and R-band imaging obtained with the Keck I/Low Resolution Imaging Spectrometer (LRIS) and visible spectroscopy from Gemini North/Gemini Multi-Object Spectrograph (GMOS) taken of 1998 KY₂₆ on 2024 June 8–9 when the asteroid was ∼0.037 au from the Earth. The asteroid does not show evidence of a dust coma and has a surface brightness profile similar to nearby background stars in the deep images. The spectrum of 1998 KY₂₆ from the combined LRIS and GMOS observations most closely resembles Xe-type asteroids, possessing a spectral slope of 6.71% ± 0.43% 100 nm⁻¹, and color indices g – r = 0.63 ± 0.03, r – i = 0.15 ± 0.03, i – z = 0.05 ± 0.04, and implies a diameter of ∼10 m. From our deep image stacks, we compute a 3σ upper limit on the dust production of 1998 KY₂₆ of −5 kg s⁻¹, −2 kg s⁻¹, and −1 kg s⁻¹ assuming μm, mm, and cm size dust particles. In addition, we compare the orbit of 1998 KY₂₆ and other known asteroids with large nongravitational parameters to NEO population models and find that the majority, including 1998 KY₂₆, likely originated from the inner Main Belt, while the second most numerous group originates from the outer main belt, followed by a third group possibly originating from the Jupiter Family Comet population. Given its inner Main Belt origin, its Xe-type spectrum, and rapid rotation, we hypothesize that the nongravitational acceleration of 1998 KY₂₆ may be caused by the shedding of large dust grains from its surface due to its rotation rather than H₂O vapor outgassing.

1998 KY₂₆ is a tiny near-Earth asteroid (H = 26.1) discovered in 1998. It has been selected as the target of the Hayabusa2 extended mission, which will rendezvous with 1998 KY₂₆ in 2031. However, one of the most basic physical properties, size, remains poorly constrained, posing potential challenges for spacecraft operations. We aimed at constraining the size of 1998 KY₂₆ by means of thermal infrared observations. We performed thermal infrared observations of 1998 KY₂₆ using the ESO Very Large Telescope/VISIR on three consecutive nights in 2024 May. After stacking all frames, we find no apparent detection of 1998 KY₂₆ on the resulting images. The upper-limit flux density of 1998 KY₂₆ is derived as 2 mJy at 10.64 μm. From this upper-limit flux density obtained via non-detection, we conclude that the diameter of 1998 KY₂₆ is smaller than 17 m with thermophysical modeling. This upper limit size is smaller than the radar derived 30 (±10) m. Our size constraint on 1998 KY₂₆ is essential for the operation of the Hayabusa2 spacecraft during proximity operations using remote sensing instruments as well as a possible impact experiment using the remaining projectile.