Simone Ieva

On 2022 September 26, the NASA/DART spacecraft performed the first-ever test of the kinetic impactor technique by hitting Dimorphos, the moon of the asteroid (65803) Didymos. The Italian Space Agency (ASI)/Light Italian CubeSat for Imaging of Asteroids (LICIACube) witnessed the event in situ. In this work, we used images from both Didymos Reconnaissance and Asteroid Camera for Opnav (DRACO) and LICIACube Explorer Imaging for Asteroid (LEIA) instruments on board the two spacecraft to search for small objects that may be present around the primary before the impact, to provide hints on the complete dynamical characterization of the system. This will be compared to the future environment the European Space Agency (ESA)/Hera mission will encounter in late 2026. We found no unknown objects larger than ∼50 m in the entire Hill sphere. When considering closer regions with respect to the primary object, the limiting size gets smaller, reaching a minimum diameter equal to ∼2 m within a radial distance of ∼2 km from Didymos. Despite the dynamical environment in the system being extremely perturbed and unstable, there is still a small chance that large objects could find a stable niche in the phase space of the Didymos–Dimorphos system, but the null result of our work is a strong indication that this niche is either nonexistent or empty.

We report on the observation and measurement of astrometry, photometry, morphology, and activity of the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object’s region of the sky during routine commissioning. Facilitated by Rubin’s high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of ∼70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of ∼3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at ∼0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-sectional ratio of η ≳ 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g − r = (0.657 ± 0.013) mag, r − i = (0.235 ± 0.018) mag, i − z = (0.147 ± 0.042) mag, and z − y = (0.047 ± 0.052) mag. These data represent the earliest observations of this object by a large (≳8 m class) telescope and illustrate the type of measurements (and discoveries) Rubin’s Legacy Survey of Space and Time will provide after it begins in early 2026.

We present spectroscopic observations of the third interstellar comet 3I/ATLAS obtained with the DOLORES instrument on the 3.58 m Telescopio Nazionale Galileo on 2025 August 16 ( r _h = 2.98 au). The spectrum revealed a red slope of 12.68% ± 0.05%/1000 Å across the 5000–8000 Å range, consistent with previous findings. Observations show the CN outgassing violet band, which yields a production rate of (4.28 ± 0.83) · 10 ²³ mol s ⁻¹ . We derived upper limits for C ₂ and C ₃ production rates since no clear detection was possible and used those to compute C ₂ /CN and C ₃ /CN ratios. The resulting, notably low, values of −0.86 and −1.47, respectively, for C ₂ /CN and C ₃ /CN ratios classify 3I as a strongly carbon-chain depleted comet in the preperihelion orbit. Notably, recent postperihelion observations found a typical solar system cometary composition, probably suggesting new innermost material being exposed to solar radiation.

We present a 3D reconstruction of the ejecta plume generated by the DART impact on Dimorphos based on LUKE images acquired by LICIACube. Using adaptive histogram equalization and geometric coregistration from multiple vantage points, we identified and tracked extended ejecta features and reconstructed their 3D spatial distribution with a voxel-based method. The reconstructed ejecta field shows strong anisotropy. An azimuth-matched comparison with the published elliptical cone highlights coherent, direction-dependent departures from a simple conic surface, despite broad agreement on the global cone orientation. This quantifies nonaxisymmetric structures that cone models cannot capture. We derived lower limits to ejecta velocities ranging from 20 to 50 m s ⁻¹ , consistent with numerical simulations of hypervelocity impacts into weak, porous targets. Compared to laboratory experiments, these values correspond to few-meter launch depths, suggesting a significant role of near-surface boulders in the ejecta distribution. Complementing this ejecta filament analysis, we studied 105 isolated comoving diffuse ejecta features, tracked up to 7 km from the impact site. These features followed highly tangential (85 ^∘ –92 ^∘ from the impact direction) trajectories relative to Dimorphos’s surface. Their velocity distribution peaks around 50 m s ⁻¹ , indicating that crater excavation persisted throughout the imaging sequence. Our results challenge and refine previous cone-based ejecta distribution models, offering new insights into the complex nature of impact-induced ejecta and improving constraints relevant for planetary defense strategies. Given these new insights, we urge the designers of future planetary defense missions to take into consideration anisotropic ejecta models for more realistic estimates of the imparted momentum via a kinetic impactor technique.

We present the first polarimetric observations of the third discovered interstellar object (ISO), 3I/ATLAS (C/2025 N1, or 3I), obtained preperihelion with FORS2 at the Very Large Telescope, ALFOSC at the Nordic Optical Telescope, and FoReRo2 at the 2 m Ritchey-Chrétien-Coudé telescope, over a phase angle range of 7 . ° 7–22 . ° 4. This marks the second-ever polarimetric study of an ISO, the first distinguishing 2I/Borisov from most solar system comets by its higher positive polarization. Our polarimetric measurements as a function of phase angle reveal that 3I is characterized by a deep and narrow negative polarization branch, reaching a minimum value of −2.7% at phase angle 7^∘, and an inversion angle of 17^∘—a combination unprecedented among asteroids and comets, including 2I/Borisov. At very small phase angles, the extrapolated slope of the polarization phase curve is consistent with that of certain small trans-Neptunian objects and Centaur Pholus, consistent with independent spectroscopic evidence for a red, possibly water-ice-bearing object. Imaging confirms a diffuse coma present from our earliest observations, though no strong polarimetric features are spatially resolved. These findings may demonstrate that 3I represents a distinct type of comet, expanding the diversity of known interstellar bodies.

We present the first polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1), obtained pre-perihelion with FORS2/VLT, ALFOSC/NOT, and FoReRo2/RCC, over a phase angle range of 7.7-22.4°. This marks the second ever polarimetric study of an interstellar object, the first distinguishing 2I/Borisov from most Solar System comets by its higher positive polarisation. Our polarimetric measurements as a function of phase angle reveal that 3I is characterised by an deep and narrow negative polarisation branch, reaching a minimum value of -2.7% at phase angle 7°, and an inversion angle of 17° -- a combination unprecedented among asteroids and comets, including 2I/Borisov. At very small phase angles, the extrapolated slope of the polarisation phase curve is consistent with that of certain small trans-Neptunian objects and Centaur Pholus, consistent with independent spectroscopic evidence for a red, possibly water-ice-bearing object. Imaging confirms a diffuse coma present from our earliest observations, though no strong polarimetric features are spatial resolved. These findings may demonstrate that 3I represents a distinct type of comet, expanding the diversity of known interstellar bodies.

The classical distinction between asteroids (rocky and inert), and comets (ice-rich and active) has been blended in the last 15 years, leading to a more nuanced picture. Both classes are now believed to be simply end-members of a physical and dynamical continuum [1]. In particular, transient objects show the characteristics of both classes (asteroids exhibiting intense activity or comets in asteroidal orbits) and they have often double cometary/asteroidal designation.We started a project called TRANSNEO, devoted to the physical characterization of TRANSient NEOs (Near-Earth Objects showing characteristics of both classes). We targeted TRANSNEO because: i) their repeated passages around the Sun make it in principle easier to detect a potential activity; ii) their proximity with Earth makes them accessible for observations and future space mission; iii) activity on NEO surfaces has been recently discovered even on apparently inactive places, thus attracting the interest of various space agencies. The study of their physical attributes (colors, spectra, activity…) is very promising, showing intermediate characteristics between asteroids and comets. This could strengthen furthermore the idea of a “continuum of small bodies” paradigm, with TRANSNEOs potentially be the missing link to understand the asteroid-comet transition.Indeed, one of these extremely intriguing bodies is (3200) Phaethon, which will be the target of the future DESTINY+ mission. This JAXA-financed mission will perform a high-speed (36 km/s) flyby of Phaethon at a distance of approximately 500 km. During the flyby, physical and chemical properties of the dust near Phaethon will be measured with a dust analyzer (DDA) [2]; surface characterization will be conducted with a Telescopic Camera (TCAP) and Multiband Camera (MCAP) to better understand the active asteroid surface compositional variation [3].Phaethon is a very interesting object. Originally discovered as an asteroid in 1983, over the past three decades the situation regarding its physical nature has become less and less clear. It has been identified as an active asteroid, with recurrent dust ejecting phenomena near its perihelion [4]. Moreover, it has an extremely small perihelion distance (q = 0.14 au) and an unusually large eccentricity (0.89), making it one of the largest (> 6 km) near-Sun and near-Earth objects. Due to the extreme temperatures experienced on Phaethon’s surface, particularly at perihelion (up to 1000 K), it is possible that certain minerals, unstable at high temperatures, become volatile, causing dust ejection at small heliocentric distances [5]. Several mechanisms have been proposed over the years to explain Phaethon’s activity, or they are reasonably plausible based on the extreme thermal environment. Among others: volatilization of some elements [6], thermal fracturing [7], meteoroid collisions [8], radiation pressure [9]. Thermal fracturing should be more efficient for larger rocks (i.e. boulders) present on its surface, and could be more relevant in the northern equatorial regions, where boulders should be more abundant. Finally, it is also possible that small particles are lost via radiation pressure, and transported from northern areas to southern latitudes. Recently, using thermophysical modeling, [10] found evidence of potential different grain sizes on Phaethon surface: the southern hemisphere should be dominated by fine-grained material, while the northern hemisphere is probably abundant in coarse-grained regolith and boulders. In order to unveil the potential mechanism beyond Phaethon’s activity and assist the proper design of the DESTINY+ mission (i.e. identify the best regions to investigate during the flyby) we obtained new spectral observations during the last year from TNG, NOT and IRTF. During this window, Phaethon was observed close to the equatorial latitudes, up until the southern hemisphere, while during the previous passages the sub-observer's latitude was much closer to the northern pole. We will present this new data, and compare them with previously available spectra in literature, in order to tackle Phaeton’s variability, and put these results in the wider context of other TRANSNEO characterizations made by our group. References:[1] Jewitt & Hsieh (2024) in Comets III [2] Kobayashi M. et al. (2018), 49th LPSC, #2050 [3] Ishibashi K. et al. (2018), LPSC 49th, #2126 [4] Jewitt, D. (2013), AJ, 145, 133 [5] MacLennan, E. M. et al. (2021), Icarus, 366, 114535 [6] Springmann, A. et al. (2019), Icarus, 324, 104 [7] Delbo, M. et al. (2014), Nat., 508, 233 [8] Szalay, J.R. et al. (2019), PSS, 165, 194 [9] Bach, Y.P. & Ishiguro, M. (2021), A&A, 654, A113 [10] MacLennan et al. (2022), Icarus, 388, 11522

The NEO Physical Observations and Properties Simulation (NEOPOPS) is a project, financed by the European Space Agency (ESA) and led by INAF - OAR (Observatory of Rome) dedicated to the physical characterization of Near-Earth Objects (NEOs). The project, started in February 2025, will be completed by August 2028, and it is built upon the legacy of prior large programs financed by the EU commission (NEOShield-2, NEOROCKS) devoted to the comprehension and the physical characterization of the NEO population. Those past surveys have made clear that only a joint effort is able to address the current major issues related to NEO impact monitoring and mitigation. In particular, extensively coordinated scientific research and innovative technical developments are necessary to improve our knowledge of the physical nature of small NEOs (the ones with the highest statistical likelihood to impact our planet in the next centuries) and to significantly reduce the risk of unannounced impacts. Therefore, the main objective of the NEOPOPS program is to tackle this scenario, providing a radically new approach:organise follow-up astronomical observations of NEOs efficiently, in order to obtain high-quality data to derive their physical properties, giving priority to timely addressing potentially hazardous asteroids (PHAs) and potential impactors; improve dramatically the statistical analysis, modelling and computer simulations to better understand the physical nature of NEOs. This would be of uttermost importance, for designing effective mitigation measures; foster European and international cooperation on NEO physical characterization, providing scenarios and roadmaps, with the potential to scale-up at a global level the experience gained during the project; cooperate with ESA to significantly enhance public understanding and perception of the asteroid hazard, counteracting the spreading of fake news and unjustified alarms.

On 2022 September 26 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 Light Italian Cubesat for Imaging of Asteroids (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 of up to 52 m s⁻¹. 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.

Kinetic deflection is a planetary defense technique delivering spacecraft momentum to a small body to deviate its course from Earth. The deflection efficiency depends on the impactor and target. Among them, the contribution of global curvature was poorly understood. The ejecta plume created by NASA’s Double Asteroid Redirection Test impact on its target asteroid, Dimorphos, exhibited an elliptical shape almost aligned along its north-south direction. Here, we identify that this elliptical ejecta plume resulted from the target’s curvature, reducing the momentum transfer to 44 ± 10% along the orbit track compared to an equivalent impact on a flat target. We also find lower kinetic deflection of impacts on smaller near-Earth objects due to higher curvature. A solution to mitigate low deflection efficiency is to apply multiple low-energy impactors rather than a single high-energy impactor. Rapid reconnaissance to acquire a target’s properties before deflection enables determining the proper locations and timing of impacts.