What Can We Hear in Space?
On a daring flight in 1836 from London across the English Channel and beyond, the pioneering balloonist Charles Green and his companions found themselves flying at night over the city of Liège in Belgium, at that time one of Europe’s major industrial centers, and were overwhelmed by the thunderous machine noise below. “There was,” records the historian Richard Holmes, “disembodied shouting, coughing, swearing, metallic banging, and sometimes, weirdly, sharp echoing bursts of laughter.” From a balloon, the world beneath becomes not just a panorama but a panacousticon, in which everything is audible.
Float higher than a few hundred feet, however, and most sounds on the ground start to become too faint for the human ear. At a height of 21 kilometers,
