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Deriving properties of cometary nuclei from coma data is of significantimportance for our understanding of cometary activity and has implicationsbeyond. Ground-based data represent the bulk of measurements available forcomets. Yet, to date these observations only access a comet's gas and dust comaat rather large distances from the surface and do not directly observe itssurface. In contrast, spacecraft missions are one of the only tools that gaindirect access to surface measurements. However, such missions are limited toroughly one per decade. We can overcome these challenges by recognising thatthe coma contains information about the nucleus's properties. In particular,the near-surface gas environment is most representative of the nucleus. It caninform us about the composition, regionality of activity, and sources of comafeatures and how they link to the topography, morphology, or other surfaceproperties. The inner coma data is a particularly good proxy because it has notyet, or only marginally, been contaminated by coma chemistry or secondary gassources. Additionally, the simultaneous observation of the innermost coma withthe surface provides the potential to make a direct link between comameasurements and the nucleus. If we hope to link outer coma measurementsobtained by Earth-based telescopes to the surface, we must first understand howthe inner coma measurements are linked to the surface. Numerical models thatdescribe the flow from the surface into the immediate surroundings are neededto make this connection. This chapter focuses on the advances made tounderstand the flow of the neutral gas coma from the surface to distances up toa few tens of nuclei radii. We describe both simple/heuristic models andstate-of-the-art physically consistent models. Model limitations and what theyeach are best suited for are discussed.

Neutral gas coma dynamics: modeling of flows and attempts to link inner coma structures to properties of the nucleus