NASA's Voyager 1 spacecraft, which left our solar system almost 10 years ago, has detected a faint monotonous hum caused by vibrations from the trace amounts of gas in deep space. These vibrations are known as plasma waves and are far too faint to be heard with an unaided ear, but they are detectable as electromagnetic waves.

This sound is actually the noise of electrically charged plasma, which makes up much of the so-called "dust" of interstellar space. With a frequency of around 3 kilohertz, it sounds like a slightly varying, somewhat eerie, low hum when amplified to levels detectable by a human ear. This is the background noise of deep space. Voyager 1, which was launched in 1977 to gather information about the planets, continues to measure and send back data from its present location some 14 billion miles from Earth. It is the most distant thing ever produced by humans.

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