arXiv subjects

Jian-Yang Li

Publications and source records attributed to Jian-Yang Li.

At least 19 recordsLinked to original sources

arXiv2026

Spectral Evolution of Ceres' Surface and Implications for Space Weathering

The surface of Ceres exposed to the space environment will be gradually altered, but the evolution of its spectral properties remains poorly understood. Here we analyze visible and infrared spectra acquired by the Dawn mission to investigate spectral variations across different surface units. We identify local shifts of a few nm toward shorter wavelengths in the 2.7 $μ$m band center, predominantly associated with freshly exposed slumping and impact materials. Our analysis suggests that compositional variations in Ceres' regolith are unlikely the explanation for the band center shift, and therefore it may represent a spectral evolution within 10$^6$ years. Combining previous studies about the spectral reddening on Ceres, we propose a two-stage spectral evolution on Ceres. The blue-shifted regions are interpreted as the youngest materials. Within 10$^6$ years after exposure, these materials become spectrally bluer, the 2.7 $μ$m band weakens, and its center shifts toward longer wavelengths. Then the evolution is characterized by spectral reddening and strengthening of the 2.7 $μ$m band without the band center shift. Based on the previous laboratory work and observations on other C-complex asteroids, we suggest that the observed spectral variations may be attributed to the chemical and/or physical changes of the regolith, possibly associated with space weathering.

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arXiv2026

The Non-Principal-Axis Rotation and Convex Shape Model of Earth Quasi-Satellite and the Target of China's Tianwen-2 Mission (469219) Kamo`oalewa

(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 state 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 $P_ϕ=27.65\pm0.03~\text{min}$ and a rotational period $P_ψ=50.49\pm0.08~\text{min}$, and the angular momentum points to ecliptic coordinates $(λ, β) = (226^\mathrm{o}\pm20^\mathrm{o}, -39^\mathrm{o}\pm20\mathrm{o})$, although we cannot rule out other solutions or other close-by periods due to aliasing. We also derived a convex shape inversion for the LAM models with consistent rotational parameters but could not find a satisfactory inversion for the SAM models. The corresponding angular momentum points to $(λ, β)=(225^\mathrm{o}, -43^\mathrm{o})$, and the periods are $P_ϕ=27.90~\text{min}$ and $P_ψ=49.66~\text{min}$. The non-principal-axis rotation provides additional constraints on the dynamic history or the internal structure of Kamo`oalewa.

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arXiv2026

Centimeter-wave OH observations of comets 12P/Pons-Brooks and C/2023 A3 (Tsuchinshan-ATLAS) with FAST

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.

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arXiv2026

Interstellar Object 3I/ATLAS Observed from Mars by China's Tianwen-1 Spacecraft

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 $β\approx 10^{-3} $ - $10^{-2}$, corresponding to grain sizes of a few 100s $μ$m. 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 $Afρ\sim (2.0\pm0.2)\times10^4$ cm and a corresponding dust mass loss rate of $\dot{M} \sim 10^3$ kg s$^{-1}$.

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arXiv2026

Thermal and Dielectric Properties of Juno's Regolith at One Millimeter Wavelength

We present the modeling results of the thermal lightcurve of asteroid (3) Juno at the wavelength of $λ$ = 1.3 mm measured by the Atacama Large Millimeter-submillimeter Array. A thermophysical model together with a radiative transfer model suggest a thermal inertia of 13$\pm$10 [J m$^{-2}$ K$^{-1}$ s$^{-0.5}$], an equivalent emissivity of 0.8$\pm$0.1, a loss tangent of 0.4$\pm$0.3, and an index of refraction 1.8$\pm$0.3. Based on previous laboratory measurements, the modeled index of refraction suggests a regolith porosity of about 45%. However, thermal inertia models using the material parameters of ordinary chondrite indicate a grain size of 10s $μ$m and require a high porosity of $\sim$90% to explain the low thermal inertia. In order to explain such a contradiction, we postulate that some repulsive mechanism might be in effect to reduce the contact of grains and therefore the thermal inertia. The loss tangent of Juno's regolith corrected for the modeled thermal skin depth is in the order of 0.5, much higher than that of the lunar regolith and indicating an electrical skin depth of L = 0.1 - 1.4 mm that is within the thermal skin depth. The shape of the rotational lightcurve of Juno in the mm wavelengths is dominated by its irregular shape, but rotational variations in the thermal and/or dielectric properties cannot be ruled out. Our results demonstrate that mm-wavelength observations of asteroids provide an extra dimension of constraints to the porosity and grain size of asteroid regolith compared to the thermal infrared observations.

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arXiv2026

Perihelion Asymmetry in the Water Production Rate of the Interstellar Object 3I/ATLAS

3I/ATLAS is an interstellar object whose activity provides critical insights into its composition and origin. However, due to its orbital geometry, the object is too close to the Sun near perihelion to be observed from the ground, and space-based measurements are therefore required. Here we characterize the water production rate of 3I/ATLAS using SOHO/SWAN Lyman-$α$ observations from 2025 November to December (heliocentric distances 1.4 to 2.2 au) with 3D Monte Carlo modeling. We report a peak post-perihelion water production rate of $Q_{\mathrm{H_2O}} \approx 4 \times 10^{28}$ molecules~s$^{-1}$, corresponding to a minimum active fraction of $\sim$30\% (assuming a maximum nucleus radius of 2.8 km). Comparison of our post-perihelion measurements with published pre-perihelion results reveals a heliocentric asymmetry, with an $r^{-5.9 \pm 0.8}$ scaling for the inbound rise, followed by a shallower $r^{-3.3 \pm 0.3}$ scaling during the outbound decline, where $r$ is heliocentric distance. The post-perihelion behavior indicates that the water production of 3I/ATLAS was driven primarily by the varying solar insolation acting on a stable active area. Combined with other evidence, including comparison with the hyperactive comet 103P/Hartley 2, our findings suggest that its water production is likely dominated by a distributed source of icy grains. Furthermore, it displayed remarkable stability in the activity with no signs of outbursts or rapid depletion of water production.

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arXiv2026

Composition and Space Weathering Characteristics of Tianwen-2 Mission's First Target Near-Earth Asteroid (469219) Kamo`oalewa

The near-Earth asteroid Kamo`oalewa, a quasi-satellite of the Earth and the target for sample return by China's Tianwen-2 mission, exhibits distinctive spectral characteristics. This study re-analyzes the visible and near-infrared reflectance spectrum of Kamo`oalewa published by B. N. L. Sharkey et al. (2021), obtained using the Large Binocular Telescope, to infer its mineral composition and space weathering characteristics. Spectral similarity analysis is performed by comparing the spectrum of Kamo`oalewa to the mean spectra of various types in the Bus-DeMeo taxonomy to make a preliminary constraint on the combined characteristics of surface mineralogy and space weathering effects. To further characterize the mineral composition, a detailed analysis of the 1 μm band center is conducted based on spectral data below 1.25 μm that have higher signal-to-noise ratios. Empirical models for normalized spectra are developed to estimate the Is/FeO content. The results suggest that asteroid Kamo`oalewa has higher olivine abundance than that of typical S-type asteroids and the Moon, exhibiting an immature to submature degree of space weathering. These findings enhance our understanding of the evolution of similar quasi-satellites and provide important implication for the future exploration of Tianwen-2 mission.

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arXiv2025

Pre-perihelion radio observations of comet 12P/Pons-Brooks with Tianma radio telescope

{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. We performed radio observations of comet 12P/Pons-Brooks with the Tianma-65m radio telescope, targeting the OH and NH$_3$ inversion lines at 18-cm 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. Four observations were carried out between December 2023 and March 2024 when the comet was approaching the Sun from 2.22 AU to 1.18 AU. We conducted 18-cm OH lines observations on 4 single days using the cryogenically cooled receiver system of the telescope to derive $\rm H_{2}O$ production rate. During 12P's outburst on December 14, we also conducted observations targeting the $\rm NH_{3}$ emission. OH 18-cm lines were clearly detected with a signal-to-noise ratio of $\sim$4$σ$ (peak intensity). A tentative detection of $\rm NH_{3}$ was made at the $\sim$$3σ$ level during the outburst phase, but the detection needs to be further verified. 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 $\rm NH_{3}$ during an outburst suggests possible connections between subsurface volatile reservoir and the outburst mechanism. These results could further our understanding of the composition and activity of Halley-type comets.

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arXiv2025

High-Speed Boulders and the Debris Field in DART Ejecta

On 26 September 2022 the Double Asteroid Redirection Test (DART) spacecraft collided with Dimorphos, the moon of the near-Earth asteroid 65803 Didymos, in a full-scale demonstration of a kinetic impactor concept. The companion LICIACube spacecraft documented the aftermath, capturing images of the expansion and evolution of the ejecta from 29 to 243 s after the impact. We present results from our analyses of these observations, including an improved reduction of the data and new absolute calibration, an updated LICIACube trajectory, and a detailed description of the events and phenomena that were recorded throughout the flyby. One notable aspect of the ejecta was the existence of clusters of boulders, up to 3.6 m in radius, that were ejected at speeds up to 52 m/s. Our analysis of the spatial distribution of 104 of these boulders suggests that they are likely the remnants of larger boulders shattered by the DART spacecraft in the first stages of the impact. The amount of momentum contained in these boulders is more than 3 times that of the DART spacecraft, and it is directed primarily to the south, almost perpendicular to the DART trajectory. Recoil of Dimorphos from the ejection of these boulders has the potential to change its orbital plane by up to a degree and to impart a non-principal axis component to its rotation state. Damping timescales for these phenomena are such that the Hera spacecraft, arriving at the system in 2026, should be able to measure these effects.

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arXiv2025

Shape index of Yarkovsky effect on irregularly shaped asteroids

The Yarkovsky effect on real asteroids is complicated to calculate either by analytical or numerical methods, since they are generally irregular in shape. We propose an index to properly characterise the shape of any asteroid, through which the Yarkovsky effect can be easily calculated without the heavy computations of surface temperatures. By analysing the energy absorbed and then emitted by a surface element, we find that the effective working power produced by the radiation recoil force on this surface element and its contribution to the Yarkovsky effect are both proportional to the double projected area of the surface element. The normalized total projected area over the asteroid's surface is defined as the shape index ($S_1$). We model the Yarkovsky effects of different asteroids using multiphysics software COMSOL, and take the rate of semi-major axis drift ($d a/d t$) obtained in these numerical simulations as the measurement of the strength of Yarkovsky effect. A linear relationship between $d a/d t$ and $S_1$ is confirmed. The shape index is then improved by taking the shadowing effect into account. A much better linear relationship is found between $d a/d t$ and the improved index $S_2$. This linear relationship is obeyed very well in a wide range of thermal parameter values. The influences of scattering and self-heating effects on the linear relationship are found ignorable. Using the shape index and the linear relation obtained in this paper, the rate of semi-major axis migration due to the Yarkovsky effect can be calculated accurately. Compared with the full numerical modeling of surface temperature and then the thermal radiation on an irregularly shaped asteroid, it is very easy to compute the shape index, which brings great convenience to the estimation of Yarkovsky effect.

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arXiv2024

FAST Observations of Four Comets to Search for the Molecular Line Emissions between 1.0 and 1.5 GHz Frequencies

We used the Five-hundred-meter Aperture Spherical radio Telescope (FAST) to search for the molecular emissions in the L-band between 1.0 and 1.5 GHz toward four comets, C/2020 F3 (NEOWISE), C/2020 R4 (ATLAS), C/2021 A1 (Leonard), and 67P/Churyumov-Gerasimenko during or after their perihelion passages. Thousands of molecular transition lines fall in this low-frequency range, many attributed to complex organic or prebiotic molecules. We conducted a blind search for the possible molecular lines in this frequency range in those comets and could not identify clear signals of molecular emissions in the data. Although several molecules have been detected at high frequencies of great than 100 GHz in comets, our results confirm that it is challenging to detect molecular transitions in the L-band frequency ranges. The non-detection of L-band molecular lines in the cometary environment could rule out the possibility of unusually strong lines, which could be caused by the masers or non-LTE effects. Although the line strengths are predicted to be weak, for FAST, using the ultra-wide bandwidth receiver and improving the radio frequency interference environments would enhance the detectability of those molecular transitions at low frequencies in the future.

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arXiv2023

VLT/MUSE Characterisation of Dimorphos Ejecta from the DART Impact

We have observed the Didymos-Dimorphos binary system with the MUSE integral field unit spectrograph mounted at the Very Large Telescope (VLT) pre and post-DART impact, and captured the ensuing ejecta cone, debris cloud, and tails at sub-arcsecond resolutions. We targeted the Didymos system over 11 nights from 26 September to 25 October 2022, and utilized both narrow and wide-field observations with and without adaptive optics, respectively. We took advantage of the spectral-spatial coupled measurements and produced both white-light images and spectral maps of the dust reflectance. We identified and characterized numerous dust features, such as the ejecta cone, spirals, wings, clumps, and tails. We found that the base of the Sunward edge of the wings, from 03 to 19 October, consistent with maximum grain sizes on the order of 0.05-0.2 mm, and that the earliest detected clumps have the highest velocities on the order of 10 m/s. We also see that three clumps in narrow-field mode (8x8'') exhibit redder colors and slower speeds, around 0.09 m/s, than the surrounding ejecta, likely indicating that the clump is comprised of larger, slower grains. We measured the properties of the primary tail, and resolved and measured the properties of the secondary tail earlier than any other published study, with first retrieval on 03 October. Both tails exhibit similarities in curvature and relative flux, however, the secondary tail appears thinner, which may be caused by lower energy ejecta and possibly a low energy formation mechanism such as secondary impacts.

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arXiv2023

Ejecta Evolution Following a Planned Impact into an Asteroid: The First Five Weeks

The impact of the DART spacecraft into Dimorphos, moon of the asteroid Didymos, changed Dimorphos' orbit substantially, largely from the ejection of material. We present results from twelve Earth-based facilities involved in a world-wide campaign to monitor the brightness and morphology of the ejecta in the first 35 days after impact. After an initial brightening of ~1.4 magnitudes, we find consistent dimming rates of 0.11-0.12 magnitudes/day in the first week, and 0.08-0.09 magnitudes/day over the entire study period. The system returned to its pre-impact brightness 24.3-25.3 days after impact through the primary ejecta tail remained. The dimming paused briefly eight days after impact, near in time to the appearance of the second tail. This was likely due to a secondary release of material after re-impact of a boulder released in the initial impact, through movement of the primary ejecta through the aperture likely played a role.

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arXiv2023

ALMA Observations of the DART Impact: Characterizing the Ejecta at Sub-Millimeter Wavelengths

We report observations of the Didymos-Dimorphos binary asteroid system using the Atacama Large Millimeter/Submillimeter Array (ALMA) and the Atacama Compact Array (ACA) in support of the Double Asteroid Redirection Test (DART) mission. Our observations on UT 2022 September 15 provided a pre-impact baseline and the first measure of Didymos-Dimorphos' spectral emissivity at $λ=0.87$ mm, which was consistent with the handful of siliceous and carbonaceous asteroids measured at millimeter wavelengths. Our post-impact observations were conducted using four consecutive executions each of ALMA and the ACA spanning from T$+$3.52 to T$+$8.60 hours post-impact, sampling thermal emission from the asteroids and the impact ejecta. We scaled our pre-impact baseline measurement and subtracted it from the post-impact observations to isolate the flux density of mm-sized grains in the ejecta. Ejecta dust masses were calculated for a range of materials that may be representative of Dimorphos' S-type asteroid material. The average ejecta mass over our observations is consistent with 1.3--6.4$\times10^7$ kg, with the lower and higher values calculated for amorphous silicates and for crystalline silicates, respectively. Owing to the likely crystalline nature of S-type asteroid material, the higher value is favored. These ejecta masses represent 0.3--1.5\% of Dimorphos' total mass and are in agreement with lower limits on the ejecta mass based on measurements at optical wavelengths. Our results provide the most sensitive measure of mm-sized material in the ejecta and demonstrate the power of ALMA for providing supporting observations to spaceflight missions.

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arXiv2023

Characterization of the ejecta from NASA/DART impact on Dimorphos: observations and Monte Carlo models

The NASA/DART (Double Asteroid Redirection Test) spacecraft successfully crashed on Dimorphos, the secondary component of the binary (65803) Didymos system. Following the impact, a large dust cloud was released, and a long-lasting dust tail was developed. We have extensively monitored the dust tail from the ground and from the Hubble Space Telescope (HST). We provide a characterization of the ejecta dust properties, i.e., particle size distribution and ejection speeds, ejection geometric parameters, and mass, by combining both observational data sets, and by using Monte Carlo models of the observed dust tail. The differential size distribution function that best fits the imaging data was a broken power-law, having a power index of --2.5 for particles of r$\le$ 3 mm, and of --3.7 for larger particles. The particles range in sizes from 1 $μ$m up to 5 cm. The ejecta is characterized by two components, depending on velocity and ejection direction. The northern component of the double tail, observed since October 8th 2022, might be associated to a secondary ejection event from impacting debris on Didymos, although it is also possible that this feature results from the binary system dynamics alone. The lower limit to the total dust mass ejected is estimated at $\sim$6$\times$10$^6$ kg, half of this mass being ejected to interplanetary space.

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arXiv2023

Momentum Transfer from the DART Mission Kinetic Impact on Asteroid Dimorphos

The NASA Double Asteroid Redirection Test (DART) mission performed a kinetic impact on asteroid Dimorphos, the satellite of the binary asteroid (65803) Didymos, at 23:14 UTC on September 26, 2022 as a planetary defense test. DART was the first hypervelocity impact experiment on an asteroid at size and velocity scales relevant to planetary defense, intended to validate kinetic impact as a means of asteroid deflection. Here we report the first determination of the momentum transferred to an asteroid by kinetic impact. Based on the change in the binary orbit period, we find an instantaneous reduction in Dimorphos's along-track orbital velocity component of 2.70 +/- 0.10 mm/s, indicating enhanced momentum transfer due to recoil from ejecta streams produced by the impact. For a Dimorphos bulk density range of 1,500 to 3,300 kg/m$^3$, we find that the expected value of the momentum enhancement factor, $β$, ranges between 2.2 and 4.9, depending on the mass of Dimorphos. If Dimorphos and Didymos are assumed to have equal densities of 2,400 kg/m$^3$, $β$= 3.61 +0.19/-0.25 (1 $σ$). These $β$ values indicate that significantly more momentum was transferred to Dimorphos from the escaping impact ejecta than was incident with DART. Therefore, the DART kinetic impact was highly effective in deflecting the asteroid Dimorphos.

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arXiv2023

Ejecta from the DART-produced active asteroid Dimorphos

Some active asteroids have been proposed to be the result of impact events. Because active asteroids are generally discovered serendipitously only after their tail formation, the process of the impact ejecta evolving into a tail has never been directly observed. NASA's Double Asteroid Redirection Test (DART) mission, apart from having successfully changed the orbital period of Dimorphos, demonstrated the activation process of an asteroid from an impact under precisely known impact conditions. Here we report the observations of the DART impact ejecta with the Hubble Space Telescope (HST) from impact time T+15 minutes to T+18.5 days at spatial resolutions of ~2.1 km per pixel. Our observations reveal a complex evolution of ejecta, which is first dominated by the gravitational interaction between the Didymos binary system and the ejected dust and later by solar radiation pressure. The lowest-speed ejecta dispersed via a sustained tail that displayed a consistent morphology with previously observed asteroid tails thought to be produced by impact. The ejecta evolution following DART's controlled impact experiment thus provides a framework for understanding the fundamental mechanisms acting on asteroids disrupted by natural impact.

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arXiv2022

The diurnal Yarkovsky effect of irregularly shaped asteroids

The Yarkovsky effect plays an important role in the motions of small celestial bodies. Increasingly improving observations bring the need of high-accuracy modelling of the effect. Using a multiphysics software COMSOL, we model the diurnal Yarkovsky effect in three dimensions and compare the results with that derived from the widely adopted theoretical linear model. We find that the linear model presents a high accuracy for spherical asteroids in most cases. The ranges of parameters in which the relative error of the linear model is over 10\% are explored. For biaxial ellipsoidal asteroids (particularly oblate ones), the linear model systematically overestimates the transverse Yarkovsky force by $\sim$10\%. The diurnal effect on triaxial ellipsoids is periodic for which no linear model is available. Our numerical calculations show that the average effects on triaxial ellipsoids are stronger than that on biaxial ellipsoids. We also investigate the diurnal effect on asteroids of real shapes and find it be overestimated by the linear model averagely by 16\%, with a maximum up to 35\%. To estimate the strength of Yarkovsky effect directly from the shape, we introduce a quantity "effective area" for asteroids of any shapes, and find a significant linear relationship between the Yarkovsky migration rate and the effective area. This brings great convenience to the estimation in practice.

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