Neutral sodium emission is observed at many comets. The evolution of neutral sodium in comets is unintuitive due to the Swings and Greenstein effects, therefore modelling is required to allow interpretation of observations. We have developed the first orbital, fully heliocentric distance and velocity dependent model of neutral sodium evolution in comets, known as COMPASS (Cometary Orbital Motion at Perihelion: An Adaptable Sodium Simulation).

The source of sodium in comets is still disputed, but an understanding of the processes that release sodium in comets may lead to greater understanding of the physical processes occurring in the coma. Ellinger et al (2015) suggested that sodium detected in ice grains could be evidence of aqueous alteration in comet nuclei. Neutral sodium has been detected in dust grains at comet 67P/Churyumov-Gerasimenko (67P) using COSIMA (Schulz et al, 2015).

A distinct neutral sodium tail was first imaged by Cremonese et al (1997) at comet Hale-Bopp (C/1995 O1) using the CoCam instrument. We present the results of our application of COMPASS to the large cometary sodium dataset available for the test case of comet Hale-Bopp. We then present a prediction of the likely morphology of the sodium emission detected on large scales at comet 67P.