Orbital elements of the material surrounding comet 67P/Churyumov-Gerasimenko

Context. We investigate the dust coma within the Hill sphere of comet 67P/Churyumov-Gerasimenko.
Aims. We aim to determine osculating orbital elements for individual distinguishable but unresolved slow-moving grains in the vicinity of the
nucleus. In addition, we perform photometry and constrain grain sizes.
Methods. We performed astrometry and photometry using images acquired by the OSIRIS Wide Angle Camera on the European Space Agency
spacecraft Rosetta. Based on these measurements, we employed standard orbit determination and orbit improvement techniques.
Results. Orbital elements and effective diameters of four grains were constrained, but we were unable to uniquely determine them. Two of the
grains have light curves that indicate grain rotation.
Conclusions. The four grains have diameters nominally in the range 0.14–0.50 m. For three of the grains, we found elliptic orbits, which is
consistent with a cloud of bound particles around the nucleus. However, hyperbolic escape trajectories cannot be excluded for any of the grains,
and for one grain this is the only known option. One grain may have originated from the surface shortly before observation. These results have
possible implications for the understanding of the dispersal of the cloud of bound debris around comet nuclei, as well as for understanding the
ejection of large grains far from the Sun.