Letter to the Editor

Asteroid modelling by starlight diffraction: The shape of Dimorphos, the satellite of (65803) Didymos

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
² Aristotle University of Thessaloniki (AUTh), 54124 Thessaloniki, Greece
³ LIRA, CNRS UMR8254, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, Meudon 92190, France
⁴ naXys, Department of Mathematics, University of Namur, Rue de Bruxelles 61, 5000 Namur, Belgium
⁵ Association Française d’Astronomie, 17, rue Emile Deutsch de la Meurthe, 74014 Paris, France
⁶ Uranoscope de l’Ile de France, Allée Camille Flammarion, 77220 Gretz-Armainvilliers, France
⁷ LTE, Observatoire de Paris, Paris Sciences et Lettres University, 77 av. Denfert-Rochereau, 75014 Paris cedex, France
⁸ Charles University, Faculty of Mathematics and Physics, Institute of Astronomy, V Holešovičkách 2, 18 000 Prague, Czech Republic
⁹ SETI Institute, Carl Sagan Center, 189 Bernado Avenue, Mountain View, CA 94043, USA
¹⁰ Unistellar, 5 allée Marcel Leclerc, bâtiment B, 13008 Marseille, France
¹¹ International Occultation Timing Association/European Section, Am Brombeerhag 13, 30459 Hannover, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 March 2026
Accepted: 9 April 2026

Abstract

Context. The DART spacecraft impacted Dimorphos, the satellite of (65803) Didymos, in September 2022. Evidence of crater formation and possible global reshaping has been obtained indirectly from spacecraft and ground-based data.

Aims. Since the impact, several stellar occultations by Didymos have been observed, but only one in particular, on January 21, 2023, can provide useful constraints on the size and shape of Dimorphos.

Methods. We modelled the diffraction signatures recorded on multiple occultation chords to constrain the projected shape and size of Dimorphos, assuming an ellipsoidal model. This is the first time diffraction observed simultaneously on several chords of a single event has been used for such a purpose.

Results. The projected shape at the epoch of the event is well constrained and consistent with recent post-DART determinations. When extended to a full three-dimensional ellipsoidal solution, the result remains compatible with previous studies, suggesting an equatorially elongated post-impact shape.