In 1964, astronomers Arno A. Penzias and Robert W. Wilson found themselves cleaning pigeon poop out of the Holmdel Horn Antenna, a radio telescope in New Jersey. The data from the instrument had weird, persistent noise that they couldn’t get rid of. They tried looking for places where errant radiation signals could sneak in, and even redesigned a part of the telescope, but the noise endured. When nothing else seemed to work, they trapped two pigeons that had taken up roost in the telescope and scrubbed out their droppings, yet the noise remained. Unbeknownst to them, they were trying to remove a fundamental signature of our universe — the cosmic microwave background.

Just over a decade later, Wilson and Penzias won the Nobel Prize in Physics for their serendipitous discovery of cosmic microwave background (CMB) radiation. Although the pair initially had been searching for a halo around the Milky Way, they instead found the first light of the universe, left over from right after the Big Bang, when photons of light were just bursting forth.

Wilson and Penzias aren’t alone in making fortuitous breakthroughs. Indeed, unexpected discoveries are almost a hallmark of astronomy. William Herschel discovered Uranus in 1781 while looking for binary stars, originally identifying the planet as a comet. Engineer Linda Morabito found volcanoes on Io while fiddling with image contrast to better see the background stars behind the jovian moon. Physicist Karl Jansky stumbled upon radio waves emanating from the center of the Milky Way while trying to improve trans-Atlantic phone calls. A U.S. spy satellite originally detected gamma-ray bursts while looking for covert nuclear bomb explosions in the 1960s. And these are just a few examples.

While astronomy has progressed through dogged reconciliation of theory and observation, it has also greatly benefited from things no one could have expected at the time. Two decades on, the Hubble Space Telescope has fulfilled its key goals, but it has also discovered proplyds (a type of planet-forming disk around a young star), unveiled dark energy, and showed that a seemingly empty section of the night sky was actually burgeoning with untold numbers of galaxies. Nobody expected these discoveries when Hubble launched.