The Palomar Observatory Sky Survey, formally the National Geographic Society–Palomar Observatory Sky Survey and now usually called POSS-I, was a photographic atlas of the sky from the north celestial pole to about 30 degrees south. Funded by a National Geographic Society grant to Caltech, it was made with the 48-inch Schmidt telescope, now the Samuel Oschin telescope, at Palomar Observatory. Each of 936 fields, about six degrees on a side, was photographed twice, on blue-sensitive and red-sensitive glass plates.

The first plate was exposed in November 1949, nearly all were finished by mid-1956, and the last was taken in December 1958. Issued as 1,872 photographic prints, with glass copies for major observatories, the survey served for decades as a standard reference atlas, and a southern extension followed in 1962. The Space Telescope Science Institute's Digitized Sky Survey, released in 1994, put the plates into digital form, making it possible to compare the 1950s sky with modern surveys.

That comparison underlies the VASCO project led by astronomer Beatriz Villarroel, which searches the plates for transients, star-like sources seen on one exposure and never again. In October 2025 her team reported aligned groups of transients and a shortage of them inside Earth's shadow, consistent, they argued, with sunlight glinting off reflective objects in high orbit before Sputnik; a companion Scientific Reports paper associated the transients with nuclear-test dates and UFO reports while disclaiming causation. Nigel Hambly and Adam Blair had concluded in 2024 that such sources are probably emulsion flaws, possibly introduced in copying, and a critique by Wesley Andrés Watters, Kevin Knuth and colleagues, accepted by the Publications of the Astronomical Society of Australia, found no significant shadow deficit and traced the nuclear link to Palomar's observing schedule.

In September 2026 the team posted preprints modeling the reflectors at roughly 20,000 to 35,000 kilometers and mapping apparent hotspots beneath the transients over the Pacific off Central America, the southern Gulf of Mexico and the American Southwest, while acknowledging simplified assumptions. Villarroel also said that original plate material, examined with Harvard's Jun Yang, showed the best-known candidates as sharp and compact, but that such inspection cannot rule out emulsion defects. An atlas made to map stars and galaxies has become a contested record of what the sky held before the first satellite.