text: why do astronauts on the moon have to communicate by radio even when close to one another?   sound travels through air in longitudunal waves, it vibrates the air molecules as it moves.  it is these vibrations that vibrate your ear drum so you hear the sound.  space is a vacuum (ie. no air), without any air there are no molecules to vibrate as so the sound cannot travel.  the more molecules there are per unit area, the faster sound can travel; this is why sound travels faster in water and even faster in a solid.