The movie "Alien" used the line "In space, no one can hear you scream" as its advertising campaign. The reason? Outer space is a vacuum. That is, it is empty of matter and sound does not travel through a vacuum.

Or, at least, space is a vacuum by earth standards. We live at the bottom of a very thick and dense ocean of air (surface density is about 0.074 lb/cu.ft. or 1.184 g/liter). But this atmosphere thins out considerably with increasing altitude until outer space is reached (by definition, about 200 miles or 300 kilometers above the earth's surface). Even though gas densities are several orders of magnitude lower, in the immediate outer space around our planet, there are perhaps 100 molecules per cubic meter. Between our solar system and the nearest star, this density drops to a few atoms per cubic meter which is a vacuum far better than...