Comet 67P/Churyumov–Gerasimenko is past its perihelion and is currently visible in telescopes in the morning hours. The picture is taken from Tenerife by the Bradford robotic telescope, where I submitted the request. The tail is extending hundred thousands kilometers into space and consists of dust particles emitted from the cometary nucleus, which measures just a few kilometers. In a recent work just published in the Astrophysical Journal Letters (arxiv version), we have explored how dust, which does not make it into space, is whirling around the cometary nucleus. The model assumes that dust particles are emitted from the porous mantle and hover over the cometary surface for some time (<6h) and then fall back on the surface, delayed by the gas drag of gas molecules moving away from the nucleus. As in the predictions for the cometary coma discussed previously, we are sticking to a minimal-assumption model with a homogeneous surface activity of gas and dust emission.
The movements of 40,000 dust particles are tracked and the average dust transport within a volumetric grid with 300 m sized boxes is computed. Besides the gas-dust interaction, we do also incorporate the rotation of the comet, which leads to a directional transport.
The Rosetta mission dropped Philae over the small lobe of 67P/C-G and Philae took a sequence of approach images which reveal structures resembling wind-tails behind boulders on the comet. This allowed Mottola et al (Science 349.6247 (2015): aab0232) to derive information about the direction of impinging particles which hit the surface unless sheltered by the boulder. Our model predicts a dust-transport inline with the observed directions in the descent region, it will be interesting to see how wind-tails at other locations match with the prediction. We put an interactive 3d dust-stream model online to visualize the dust-flux predicted from the homogeneous surface model.