Riding an Atlas-Agena rocket, Lunar Orbiter 1 carried a film-based camera system that developed images onboard and scanned them for transmission to Earth. Its photographs of candidate landing zones, along with a famous accidental image of Earth rising over the lunar horizon, directly informed NASA's planning for the crewed Apollo missions that followed within three years.
The Apollo program needed more than confidence to put astronauts on the Moon—it needed maps. No one knew for certain whether the lunar surface could support a landing craft without swallowing it in dust, or which flat plains might hide boulder fields lethal to a descending module. Lunar Orbiter 1, launched atop an Atlas-Agena rocket from Cape Canaveral, was built to answer those questions.
Its imaging system was a marvel of Cold War-era ingenuity: rather than transmitting digital images (video technology wasn't yet capable of the needed resolution), the spacecraft carried a miniature photo lab that exposed film, developed it chemically in flight, then scanned the negatives with an electron beam for radio transmission to Earth. The process was slow and finicky but produced remarkably detailed pictures of the Sea of Tranquility and other candidate Apollo sites.
Along the way, on August 23, 1966, the spacecraft's camera happened to capture Earth hanging above the Moon's horizon—the first such image taken by a spacecraft from lunar distance, predating the more famous "Earthrise" photo from Apollo 8 by two years. Four more Orbiter missions followed, together mapping 99 percent of the lunar surface and clearing the path, quite literally, for Neil Armstrong's boots.