If two rarefied masses of gas collide at a sufficiently high speed, their relative motion will be stopped by a collective plasma instability, rather than by collisions of individual particles. This is true even if the gases are initially non-ionized. In the present note, the possible influence of magnetic fields is ignored for the most part, but a criterion is found to decide under what conditions the collective interaction remains dominant even when they cannot be ignored. It seems that the

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... lect of magnetic effects may be justified, for example, in the case of two galaxies in collision at a relative speed of 1000 km/sec. The magnetic energy density here is (1/8π)H2∼4X10−12 erg/cm3 (with H∼10−5 gauss), which is much less than the kinetic energy density 1/2NmU2∼8X10−9 erg/cm3 (with N∼1 particle per cm3).