79: Voyager Hears A Hum

Our farthest-flung object, little Voyager 1, is still hurtling through the cosmos, 21 light hours away in interstellar space. And decades into its mission it's still measuring stuff! Voyager 1 sent back measurements of the background hum of the interstellar medium, the incredibly diffuse plasma that fills the void between our solar system and the next.