I understand that the interstellar medium (ISM) consists mostly of neutral hydrogen gas, plus a smaller fraction of helium gas. Furthermore, I gather that between 0.1 and 1.0 atoms per cm3 correspond to the average density values pertaining in our "Local Bubble". The above, however, refers exclusively to the gas content of the ISM; that's to say it doesn't include dust particles (or "presolar grains") - never mind more massive objects, like micrometeoroids. True, these objects comprise a tiny percentage of the ISM, but they are in individual terms disproportionately more massive when compared to the interstellar gas. How much more massive, I simply don't know. All that can be said that their average distribution in the ISM is claimed to be around 10-6/m3 (0.00001/m3). None of this would be an issue were it not for a desire to work out how the average density of the ISM (as it applies to our Local Bubble) compares with conditions here on Earth. Given that Earth's air density at sea level is about 1.29x10-3 g/cm3, I've calculated that this density could be as much as 300 million, trillion times greater than that pertaining to the ISM. The above is significent from an SF writing point of view. It might be expected that the interstellar gas itself wouldn't prove too hazardous for a sufficiently shielded spacecraft travelling at 3%c. Encountering a dust particle at this speed, however, could spell instant doom - e.g. a single 30mg dust speck would have a kinetic energy of around 0.3 kg TNT (a little under that of a modern hand grenade). There are SF workarounds even here, of course. Nevertheless, I would like to have a clearer idea what the "average" density of the ISM is (or could be) - a density figure too that includes material objects like dust particles etc. More than anything else, though, any comments or criticisms about my calculations would be extremely welcome. Many thanks.