Occultation astrometry for NEA study and risk mitigation

The powerful method of stellar occultations is an unbeatable technique uniquely approaching, in some aspects, the performances of planetary space missions. It allows km-level accuracies on the determination of shapes and sizes of objects. Furthermore, it allows to derive, from ground using small aperture telescopes, asteroid positions at Gaia-level accuracy [1].

The 2021 occultations by the sub-km sized (99942) Apophis [2] have marked the beginning of a new era, thus showing occultation-derived astrometry can be used to drastically reduce orbital uncertainties and significantly improve NEA risk assessment. For a good mitigation strategy in planetary defence one must combine different techniques to constrain the dynamical (orbital) and the physical properties of the asteroids. An extremely good quality orbit can be derived from several high quality classical astrometric data covering the entire orbit and over a long time interval, or by combining different types of measurements including "occultation derived astrometry" and radar when possible. While equivalent to radar data in terms of accuracy, occultation derived astrometry is complementary to it (in the orthogonal direction). Finally, unlike radar observations which are limited in range, occultations can be recorded at any distance along the orbit.

Although challenging, occultation by NEAs and in particular by sub-km NEAs are feasible under some conditions. They require an initial astrometric follow-up to predict reliable occultation events. They also require the deployment of mobile stations equipped with fast cameras across the prediction path. For most our targets, the maximum expected duration of the event is <0.5s.

We present here the strengths and limitations of such an approach as well as preliminary results obtained over a few month, in particular in the case of (65803) Didymos to support both the DART (NASA) and Hera (ESA) planetary defence missions.

Acknowledgments: This work is carried out under the ACROSS (Asteroid Collaborative Research via Occultation Systematic Survey) umbrella, which is an ESA funded project. It aims at supporting the first planetary defence missions DART and Hera.

References [1] Ferreira, J.F., Tanga, et al.: 2022, A&A 658, A73.

[2] Souami, D., Desmars, J., et al.: 2022, IAU General Assembly e-talk.