Didymos and Dimorphos: characterisation by stellar occultations, and plans before the Hera fly-by

The capability to caracterize small bodies by stellar occultations has expanded enormously thanks to the Gaia mission, whose accurate astrometry has improved by order of magnitude the accuracy of star positions and proper motions, and asteroid orbits. More and more occultations by minor planets as small as 5-10~km in the Main Belt are reported by observers often equipped with mobile telescopes of modest size. Appropriate techniques to recover the absolute timing of the observed events and software tools that are now widely available permit to recover asteroid astrometry (relative to the star positions) with accuracies comparable to Gaia, especially at the smallest asteroid sizes.After some successful observations of occultations by Near Earth Asteroids such as Phaethon and Apophis, we took the challenge to push the limit at smaller sub-km NEAs. Several months before the impact by the DART probe on Dimorphos, satellite of (65803) Didymos, we established a networking activity of collaboration among amateur and professional astronomers, named ACROSS (Asteroid Collaborative Research via Occultation Systematic Survey), initially supported by the European Space Agency.One of the primary goals of ACROSS is to obtain very accurate astrometry over a few years, in order to evaluate the effect of the(September 2022)DART impact on the heliocentric orbit of Didymos. However, just a few months before the DART arrival to the target the uncertainty in the ephemeris was more than an order of magnitude larger than the size of Didymos, making predictions and telescope deployment practically not feasible.After a long work of astrometric reduction of available CCD images collected during the DART/Hera ground-based campaign (fully exploited for photometry but not for astrometry) and careful weighting of the observations, we were able to gradually converge to an exploitable orbit accuracy very close to the date of the impact. Following several failed attempts, the first successful event was observed just after the DART impact.This positive detection was followed by several others (19 in total), over 6 months, providing an astrometric accuracy comparable to the DART telemetry. At the end of the campaign astrometry by occultations was able to constrain the orbit at the same level as the use of the DART telemetry.However, the success of the observations was also enhanced by the detection in 4 events of occultations by the satellite Dimorphos. While their contribution to our knowledge about the position of Dimorphos along its orbit is not major at this stage, we found that the occultation signal, modulated by diffraction effects, can provide relevant information about its size and shape.The new ongoing campaign of stellar occultation in 2024 can thus provide an important contribution not only to constraint the dynamical properties of the Didymos system, but also to understand the post-impact properties of Dimorphos, before the arrival of Hera, including its shape. A first occultation in May 2024 proved the high quality of the orbit, resulting in the first positive of the new sequence. In this talk, we will discuss the results obtained in 2023 and present some preliminary results from the ongoing 2024 campaign.