Didymos is a binary near-Earth asteroid. It is the target of the DART and HERA space missions. The primary body, Didymos, rotates close to the spin at which it is expected to shed mass. The secondary body, Dimorphos, is a 140 meters moon that orbits the primary body in about 12 hours. Here we investigate the possible origin of Dimorphos. Using 1D models of ring/satellite interactions, we study the evolution of material lost from Didymos' surface and deposited as a ring at its equator. We find that due to viscous spreading, the ring spreads outside the Didymos' Roche limit forming moonlets. A fraction of the mass will form Dimorphos and a set of objects near the Roche limit, while most of the ring's mass falls back on Didymos. To match the properties of today's Dimorphos, the total mass that must be deposited in the ring is about 25% of Didymos' mass. It is possible that a fraction of the material travelled several times between the ring and the surface of Didymos. The models produce an orbit similar to that observed for a Didymos tidal parameter k2/Q<1e-5. If the ring deposition timescale is long (>100 yr) (so the material flux is small) Dimorphos could be irregularly shaped as it forms from the collision of similar-sized satellitesimals. However, the top-shape of Didymos is expected to be achieved due to a fast spin-up of the asteroid, which would result in a short deposition timescale (<yr). In that case, the satellite would form from progressively accreting material at the Roche Limit, resulting in an ellipsoidal Dimorphos constructed of small pieces with sizes of the order of meters, which is apparently in agreement with the recent images of Dimorphos obtained by DART mission.
Asteroid modelling by starlight diffraction: The shape of Dimorphos, the satellite of (65803) Didymos
Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange（蔚蓝海岸大学，蔚蓝海岸天文台，法国国家科学研究中心，拉格朗日实验室）；Aristotle University of Thessaloniki (AUTh)（塞萨洛尼基亚里士多德大学）；LIRA, CNRS UMR8254, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université（巴黎天文台 LIRA 实验室，法国国家科学研究中心，巴黎文理研究大学，索邦大学，巴黎西岱大学，CY 塞尔吉巴黎大学）；University of Namur（那慕尔大学）；International Occultation Timing Association/European Section（国际掩食计时协会欧洲分会）；Association Française d’Astronomie（法国天文学会）；LTE, Observatoire de Paris, Paris Sciences et Lettres University（巴黎天文台 LTE，巴黎文理研究大学）；Charles University, Faculty of Mathematics and Physics, Institute of Astronomy（查理大学数学物理学院天文学研究所）；SETI Institute（搜寻地外文明研究所）；Unistellar
A change of the rotation period of asteroid (65803) Didymos caused by the DART impact
Charles University（查理大学）；Academy of Sciences of the Czech Republic（捷克科学院）；Georgia Institute of Technology（佐治亚理工学院）；University of Maryland, College Park（马里兰大学帕克分校）；Université Côte d’Azur（蔚蓝海岸大学）
Dynamical evolution and surface accretion of DART impact ejecta in the (65803) Didymos system
University of Liverpool（利物浦大学）；Politecnico di Milano（米兰理工大学）；IFAC-CNR（意大利国家研究委员会应用研究所）；Universidad de Alicante（阿利坎特大学）；European Space Agency (ESA)（欧洲空间局）
