DSMC(Direct Simulation Monte Carlo)直接模擬蒙地卡羅法(Bird, 1994)，可用於計算彗星和衛星稀薄氣體噴發的流場，我們使用一套平行化運算的三維DSMC程式(PDSC++)( Wu et al., 2005) (Su et al., 2010) (Su, 2013)，來模擬羅賽塔彗星任務的目標彗星67P/Churyumov–Gerasimenko(67P/C-G)表面氣體的昇華與塵埃噴流。在我們的計算中，可以從天體表面有碰撞的流體區域延伸到遠離表面的無碰撞的流體區域。為了計算這些氣體昇華數量的多寡，我們同時也利用了熱傳導模型在彗星67P的形狀上，假設彗星67P表面有一小塊面積的物質是由純水冰所組成，其大小約是0.056 km2 (大約是表面積的0.14 %)，由於氣體的牽引作用，若有大小1μm的塵埃粒子，會以最大到9m/s的速度垂直飛離彗星表面。

The application of a parallel 3D Direct Simulation Monte Carlo (DSMC) code, named PDSC++ (Wu et al., 2005) (Su et al., 2010) (Su, 2013), allows us to study the expansion of outgassing from surface or subsurface sublimation of volatile ices from comet 67P/Churyumov–Gerasimenko (67P/C-G) which is the target of ESA’s Rosetta mission. Our calculations can automatically link the hydrodynamic flow region close to the nucleus surface to the Knudsen flow regime with infrequent collisions. Another computational advantage is that the geometry model of comet 67P can be used to produce the surface temperature distribution and its diurnal variation. The global case has to do with the global structure of the gas and dust coma if most of the outgassing is concentrated at an active region of 0.056 km2 (about 0.14 % of the total surface area). It is found that the (1-10 μm sized) dust distribution is characterized by a sunward jet with radial speed of 1-9 m/s plus a disc of slow-moving dust particles which are nearly perpendicular to the sun-comet direction.

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