VASCO, short for Vanishing & Appearing Sources during a Century of Observations, is an international astronomy project led by Beatriz Villarroel of Nordita in Stockholm. It compares historical sky-survey images, chiefly the first Palomar Observatory Sky Survey of 1949-1958, with modern surveys to find objects that vanish or appear. It began around 2017 as a search for disappearing stars, a possible technosignature, and is now best known for its claim that star-like flashes on pre-Sputnik plates may record reflective objects in Earth orbit.

The project's first major paper, in the Astronomical Journal in 2019, compared about 600 million catalogued objects with the modern Pan-STARRS survey and found roughly 100 point sources seen only once. In 2021, a paper in Scientific Reports described nine sources that appeared within about 30 minutes on an April 12, 1950 plate from Palomar Observatory and proposed two explanations: contamination of the plates, or sunlight glinting off flat objects near geosynchronous orbit. Astronomers Nigel Hambly and Adam Blair answered in 2024 that the features were most likely emulsion flaws. VASCO also opened a citizen-science program in 2022.

The work drew wider attention in October 2025. A paper in the Publications of the Astronomical Society of the Pacific reported aligned groups of transients and a shortage of them inside Earth's shadow, where sunlight could not reach an orbiting object. A companion paper led by statistician Stephen Bruehl in Scientific Reports found transients about 45 percent more likely within a day of a nuclear test between 1949 and 1957, and associated with reports of unidentified objects, while calling the associations small and not proof of cause. In 2026 the team added a machine-learning analysis of 107,875 candidates, models placing the reflectors 20,000 to 35,000 kilometers up, and a paper mapping apparent hotspots over the Pacific, the Gulf of Mexico, and the American Southwest.

The claims remain contested in the literature. A critique by Watters and colleagues, published in July 2026, argued that the Earth-shadow deficit and the nuclear link disappear once the background is modeled properly and results are normalized by observing schedules. An independent pipeline by Zachary Hayes recovered most of VASCO's catalog but found no timing association. Supporting analyses came from Ivo Busko, who found similar transients on Hamburg Observatory plates and argued in a 2026 peer-reviewed paper that their shapes show the telescope's own optical distortion, implying the light passed through it rather than arising on the plate.

VASCO's significance is that it moved a UAP-adjacent question into refereed astronomy, where it has drawn replication attempts, rebuttals, and replies rather than only media attention. Its central question remains open: whether the transients are artifacts of mid-century photographic plates, or a record of something real above Earth before the first satellite launched.