Cometary activity arises from the sublimation of surface ices into the gas phase. Dust is entrained in the gas and is accelerated by gas drag as the gas escapes into interplanetary space. Previous observations [1, 2] of cometary nuclei have shown remarkable, diverse structures in the near‐nucleus dust distribution. The gas from the comet expands into vacuum and the flow therefore passes through a wide range of densities and pressures and hence though different flow regimes. While a direct simulation Monte Carlo (DSMC) simulation [3] should give, in all cases, accurate results if applied correctly, the calculation becomes slow in denser regions. Solving the Euler equations (EE) is in these cases faster, but they are only an approximation for dilute gas. A direct comparison between DSMC and EE was made by Lukianov et al. [4] for water vapor subliming from an ice sphere into vacuum. He found that the Euler equation with correct boundary conditions gives a good approximation to the velocity and density field in the supersonic region even at global Kn>10−3. We have performed further comparisons of the DSMC and EE methods for a case with an ‘active cap’: A region with a half opening angle of 10° has twice the production rate compared with the rest of the sphere. We found that far away from the active cap, the flow is like one from a sphere and our results are in a very good agreement with Lukianov et al.’s [4]. However the Euler equations start to fail close to and at the sides of the jet produced by the active area at lower Knudsen numbers.

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