Geodesy of Small Bodies: From Gravitation to Interior Structure

• Applicant: Dr. Xuanyu Hu, Ph.D.
• Subject Area: Geodesy, Photogrammetry, Remote Sensing, Geoinformatics, Cartography ; Astrophysics and Astronomy
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 500329796

Small extraterrestrial bodies, such as asteroids, comets, and small natural moons, have preserved important clues on the formation of solar and planetary systems. They may be fragmented monoliths or porous rubble piles, whose composition reflects their origin and evolution. Small bodies were targets of various dedicated space missions and more will be explored in the near future. The most direct constraint on a body’s interior structure or mass distribution is its gravitational field, which can be determined from radio and optical tracking data of spacecraft motion around the body mass. However, the task is ultra challenging, mostly because the weak gravitation of a small body is easily overwhelmed by other perturbations on the spacecraft, such as solar radiation. Active bodies have been frequently and, on occasion, unexpectedly encountered. In the presence of activity, the spacecraft further experiences an atmospheric drag and/or impacts by ejected dust. This project sets out to address the challenges in gravitational field determination arising from complex non-gravitational effects, especially around active bodies. We investigate the case of comet 67P/Churyumov-Gerasimenko (67P) using observations from ESA's Rosetta mission, which to date is the most comprehensive dataset for any small body. We develop and implement elaborate gas and dust coma models, calibrated to in situ measurements of gas pressure and dust impacts, for determining spacecraft orbit and 67P’s gravitational field. With nongravitational effects accurately characterized, we aim to improve the estimation of 67P's gravitational field to the extent possible. The body’s activity is also a blessing in disguise in that the outgassing and dust emission induce variations of the body’s rotation. We explore the interior structure of 67P using the activity-induced rotation variability as additional constraints. As the distinction between active and inactive bodies becomes less definitive and the nature of their activity more varied, this research is intended to bring important insight into the general strategy of future small body exploration.

• DFG Programme: Independent Junior Research Groups
• International Connection: China, Japan, USA
• Cooperation Partners: Professor Dr. Koji Matsumoto; Dr. Xian Shi; Professor Dr. C.K. Shum