Stellar occultations by Near Earth Asteroids: challenges and results 

The observation of stellar occultation by asteroids is an intrinsically challenging activity in the case of Near Earth Objects, that produce very short events on narrow occultation paths. From prediction to observation, and even to data reduction, the whole process requires tackling important challenges. If successful, however, the reward can be very relevant. In this contribution we report some results and lessons learned from the exploitation of occultations by the smallest asteroids observed with this technique.A few years ago, the possibility to reach sub-km asteroids by stellar occultations was not even contemplated for extensive observation efforts, since it had to face unrealistically large ephemeris uncertainties, resulting in poorly exploitable prediction. The situation has evolved dramatically starting with the first large data release (DR2, 2018) by the Gaia mission of ESA [1] disclosing the access to sub-mas absolute astrometry. The impact on stellar occultations has come from (1) the availably of a new, highly precise and very dense reference catalogue for traditional ground-based astrometry; (2) the exploitation of the same catalogue to better derive the position of stars target of occultations and (3) the direct observations of asteroids themselves by Gaia [2][3].The combination of these factors has resulted in much improved asteroid orbits in general [4], an increasing number of exploitable predictions for smaller and smaller asteroids and, as an obvious consequence, an asteroid astrometry at the level of the precision of the star position [5], i.e. of Gaia. While DR2 did contain the astrometry of 14.000 asteroids only (for 22 months of observations), the sample has increased to 150,000 in DR3 (2022) over 5 years and will reach the complete data set of the nominal mission (350,000 asteroids) in DR4 (2026). The final DR5 (2030) will include the whole extended mission (10.5) for about the same number of objects. The accuracy of stellar proper motions, and the detection of binary stars in the astrometry, will benefit from the extended time base and provide even better-quality predictions for stellar occultations.Over the recent years, we have actively worked to the prediction, the observation and the exploitation of occultations for specific NEOs (in particular, targets of space missions). Recent campaigns on (99942) Apophis, (3200) Phaeton, (65803) Didymos and other NEOs have clearly demonstrated the potential of this technique for both their physical and dynamical characterization [6][7].As an outstanding example, we illustrate here in more detail the case of (65803) Didymos, target of the DART (NASA) and Hera (ESA) missions. In the frame of the ACROSS collaboration, we have exploited stellar astrometry and star colors provided by Gaia to boost the improvement of the orbit quality before the impact by DART (September 26, 2022). By combining this effort to the other data available, the nominal uncertainty on the ephemeris has thus gradually collapsed, over several months in 2022, from ~800 km to ~1600 m. In the meantime, we have worked to motivate the observers and organize large campaigns trying to catch the first positive events [8].Today, with the contribution of 20 stellar occultations, the measurement of the heliocentric deviation of the orbit of Didymos has been possible [9][10][11]. The post-fit residuals of stellar occultations from the orbital fit are of the order of ~1 mas.Moreover, the photometry of the occultation chords for one of the best observed events is shown to sample particularly well the diffraction pattern of the star light at the edge of Dimorphos. This signature is both size- and shape-dependent. With a suitable model we are thus able to provide constraints on the projected shape of Dimorphos, observed after the impact by DART. We present an updated analysis of this result. The accurate knowledge of the occultation path, and the capability to place the observers to capture both Didymos and Dimorphos, are a direct consequence of the impact of Gaia on this observation method. To date, Dimorphos is the smallest asteroid, and the smallest satellite, ever observed by occultations.Finally, we stress here that these results would not have been possible without the commitment of unpaid non-professional astronomers, whose contribution is fundamental to occultation activities, and further demonstrates the extend of the impact that Gaia has on all levels of activities related to astrometry. [1] Brown, A. G. A., et al., “Gaia Data Release 2 - Summary of the Contents and Survey Properties.” A&A 616 (2018): A1.[2] Gaia Collaboration, F. Spoto, P. Tanga, F. Mignard, J. Berthier, B. Carry, A. Cellino, et al., “Gaia Data Release 2. Observations of Solar System Objects.” A&A 616 (2018): A13.[3] Tanga, P., T. Pauwels, F. Mignard, K. Muinonen, A. Cellino, P. David, D. Hestroffer, et al. “Gaia Data Release 3: The Solar System Survey.” A&A 674 (2023): A12.[4] Gaia Collaboration, P. David, F. Mignard, D. Hestroffer, and P. Tanga. “Gaia Focused Product Release: Asteroid Orbital Solution. Properties and Assessment.” A&A, 680, A37 (2023)[5] Ferreira, J. F., P. Tanga, F. Spoto, P. Machado, and D. Herald. “Asteroid Astrometry by Stellar Occultations: Accuracy of the Existing Sample from Orbital Fitting.” A&A, 658, A73 (2022)[6] Senshu, H., et al. “Yarkovsky and YORP Effects Simulation on 3200 Phaethon.” Phil. Trans. of the RASC A 383, 2291 (2025)[7] Dunham, D. W., J. B. Dunham, F. Yoshida, T. Hayamizu, D. Herald, D. Farnocchia, R. Venable, et al. “New NEA and Other Asteroid Results from Occultations Recorded by IOTA Observers.” Asteroids, Comets, Meteors Conference 2851 (2023): 2363.[8] Tanga et al. “The ACROSS campaign”, in preparation[9] Makadia, R., et al. “Measurability of the Heliocentric Momentum Enhancement from a Kinetic Impact: The Double Asteroid Redirection Test (DART) Mission.” The Planetary Science Jour., 5, 2, 38, (2024).[10] Makadia, R., et al. “First detection of an asteroid’s heliocentric deflection: The Didymos system after DART”, submitted[11] Chesley et al. “First detection of an asteroid heliocentric deflection: The Didymos system after DART”, EPSC-DPS2025-133