Dynamics of dm-sized Particles in the Coma of Comet 67P/Churyumov–Gerasimenko

During the visit of the European Space Agency’s Rosetta mission to comet 67P/Churyumov-Gerasimenko, the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) on the spacecraft took several image sequences of 67P’s nucleus that highlight its activity against the dark backdrop of space. Especially during the later phase of the mission when 67P was outbound from perihelion and its activity ceasing, Rosetta was able to record sequences from less than 100 km away, which resulted in m-sized resolutions at the distance of the nucleus. At this distance, thousands of point-source-like particles can be distinguished from the diffuse coma.

Following Agarwal et al. (2016), who successfully tracked such particles manually, we developed a fully automated tracking algorithm (Pfeifer et al. 2021) which we have now applied to several datasets and performed an in-depth analysis of the results.

We for example selected sub-sets of particles and extrapolated their 2D-trajectories in the image plane back in time to relatively confined areas on the nucleus. Assuming that the particles originated from the identified location and that they have not moved too far away from it, we can estimate their distance to the spacecraft and thus their (projected) dynamics and size distributions.

In particular, we can compare the measured accelerations to a (simplified) version of the particle’s equation of motion:

where ⃗a is the particle acceleration, m_p, the particle mass, ⃗ F_G the nucleus gravity, ⃗ F_D the gas drag, G the gravitational constant, M the nucleus mass, d the particle-nucleus distance, ⃗n_G the gravity unit vector, C_D the drag coefficient, m_g the mass of a water molecule, n_g the gas number density, ρ_p the particle density, ⃗v_g the gas and ⃗v_p the particle velocity, and r the particle radius. By fitting this equation to our data we try to constrain the parameters that go into it.

We present our latest results from tracking mostly dm-sized particles that were recorded by OSIRIS near the surface of comet 67P in December 2015 and January 2016. While the particle dynamics themselves already present new information, the properties that we can derive from them also help us to better understand the gas flow and other circumstances at the time of the observed activity.

 

Acknowledgements

We acknowledge the operation and calibration team at MPS and the Principal Investigator Holger Sierks on behalf of the OSIRIS Team for providing the OSIRIS images and related datasets. OSIRIS was built by a consortium of the Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany; the CISAS University of Padova, Italy; the Laboratoire d’Astrophysique de Marseille, France; the Instituto de Astrofísica de Andalucia, CSIC, Granada, Spain; the Research and Scientific Support Department of the European Space Agency, Noordwijk, The Nether- lands; the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain; the Universidad Politéchnica de Madrid, Spain; the Department of Physics and Astronomy of Uppsala University, Sweden; and the Institut für Datentechnik und Kommunikationsnetze der Technischen Universität Braunschweig, Germany. The support of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledged. We thank the Rosetta Science Ground Segment at ESAC, the Rosetta Missions Operations Centre at ESOC and the Rosetta Project at ESTEC for their outstanding work enabling the science return of the Rosetta Mission.
MP and JA acknowledge funding by the ERC Starting Grant No. 757390 Comet and Asteroid Re-Shaping through Activity (CAstRA). JA acknowledges funding by the Volkswagen Foundation.

 

References

Agarwal, J., A’Hearn, M. F., Vincent, J.-B., et al. 2016, MNRAS, 462, S78
Pfeifer, Marius, Agarwal, Jessica, & Schröter, Matthias. 2022, A&A, 659, A171