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Significance Statement
The study of binary systems is essential for space missions like the proposed Asteroid Impact Mission, which target primary and secondary asteroidal bodies. Didymos is a binary system that consists of a larger primary and a smaller secondary. The attributes of the main body in the Didymos system are characterized through radar observations and photometric methods. It has been found that, of all secondary orbital periods identified as of the moment, that of Didymos is the shortest with an approximate value 11.9h. While the main body’s rotational period is determined as 2.6h, the secondary rotational period could not be derived easily. A synchronous rotation however is compatible with observations, in the sense that the rotational period is equal to the secondary orbital period. Modeling daily temperature variations of Didydmos’ moon in the course of the mission is of great importance when it comes to investigating the thermal survivability of the MASCOT-2 lander and designing the thermal imager, two of the payloads of the Asteroid Impact Mission.
Thermal emission of the secondary in the binary system is dependent on the shape, spin vector, thermal inertia, solar distance, surface roughness and size of the secondary as well as shadowing and thermal radiation from the primary. All these parameters can be incorporated in the thermophysical model to estimate the thermal emission of the moon. Ivanka Pelivan and colleagues at the DLR Institute of Planetary Research and DLR-MUSC, Space Operations and Astronaut Training in Germany performed the thermosphysical modeling of the Didymos’ secondary based on the current understanding of the physical characteristics of the moon. Owing to the unknown thermal inertia, they covered a broad range of thermal inertia in their simulation. Their work is published in Advances in Space Research.
The authors modelled the Didymos’ secondary through a simplified approach owing to the several assumptions and unknowns that had to be adopted. Neglected effects however were shown to be small at least when compared to the uncertainty in thermal inertia. While the full thermophysical model included direct and diffuse solar radiation and direct as well as diffuse self-heating, in this study the diffuse terms as well as direct self-heating do not play a role due to the ellipsoidal model shape assumption.
The research team was able to establish a thermophysical model of the secondary in the Didymos system. They selected a case study that entailed a wide range of thermal inertia with and without shadowing effects of the primary. Their computations were based on analytical formulations of an ellipsoid, but the model could also accommodate complex shape models with quadrilateral and triangular facets allowing an in-depth investigation of thermal conditions of realistic shapes.
Using the thermophysical model results for a thermal infrared imager performance study, it was found that the proposed thermal imager covers the range of peak emission for the likely range of thermal inertia. Furthermore, applying the thermophysical model with high as well as low thermal inertia, the authors were able to evaluate the performance of the MASCOT-2 lander for a number of possible landing sites as well as specific settings. For all cases considered, they found that the temperatures were mostly within operating limits except for one case of latitude +45 degrees, which was, fortunately, outside the desired landing range.
Reference
Ivanka Pelivan, Line Drube, Ekkehard Kuhrt, Jorn Helbert, Jens Biele, Michael Maibaum, Barbara Cozzoni, Valentina Lommatsch. Thermophysical modeling of Didymos’ moon for the Asteroid Impact Mission. Advances in Space Research, volume 59 (2017), pages 1936–1949.
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