Sound travels in pressure waves through a medium of matter such as air or water. When a sound is made, the surrounding molecules vibrate. The eardrum then vibrates in sympathy, which the brain interprets as sound. Sound travels faster if the medium is dense (in other words, when the molecules are closer together). Sound therefore travels faster through steel than it does through air.

Sound cannot travel in a vacuum, and this is what makes up most of space. However, gas clouds called nebulae do offer a medium for sound to propagate. A nebula contains far fewer atoms per cubic centimetre than, for example, the air on Earth. Such sounds would go undetected by the human ear. What we consider as sound in every day life does not exist in space, even if science fiction would have us believe otherwise.

Many movies and TV shows often have viewpoint space scenes with audible engines and explosions. This is inaccurate but almost certainly deliberate in some cases. In the book Bad Astronomy, published by John Wiley and Sons, Inc. (pp 246-247), author Phil C. Plait points out that a spacecraft zooming past in silence may seem odd to a viewer because they’re used to associating sounds with moving vehicles in everyday life.

Is sound a problem for astronauts then? No – all space craft (and the space station) have air. For an astronaut to communicate with people on Earth, they rely on radio waves – electromagnetic radiation that will happily travel through the vacuum of space. The astronaut’s radio device has a microphone with an electric current running through it, turning sound waves into electromagnetic radio waves, which get picked up by antennae on Earth.

What would happen if a small chunk of rock hit a space craft? The astronauts in the space craft would be able to hear it because they have a medium – air. Astronauts witnessing the event in the vacuum of space wouldn’t hear the rock clunking against the craft. However, if an astronaut pressed his helmet against the outside of the craft, they would hear the clunk because there is a continuous medium between the source of the sound and the astronaut’s ear:

• The air within the craft
• The craft’s hull
• The astronaut’s helmet
• The air in the suit

For more information on radio waves, the NASA’s Imagine the Universe website has a page devoted to explaining the Electromagnetic Spectrum. To learn more about both sound and radio, visit the NASA’s Cosmicopia General Physics: Basics website.


Space Physics, Energy Travelling Through Space, NASA Cosmicopia website, accessed 5th October 2008, author: Christian, E et al

Enterprise, Bad Astronomy website, published 21st September 2001, accessed: 5th October 2008, author: Plait, P.C

Cosmology: The Origin and Evolution of the Universe, Universe (8th ed), published by W.H. Freedman and Company in 2008, authors: Freedman, A and Kaufmann, W III