In astronomy a transient is a source that appears briefly; in UAP discussion the word usually means star-like point sources on photographic plates of the first Palomar Observatory Sky Survey, taken from 1949 to 1957, that appear on one exposure but not on others or in modern surveys. Astronomer Beatriz Villarroel and the VASCO project she leads have catalogued more than 100,000 of them and propose that some may be sunlight glinting off flat, reflective objects in high orbit before Sputnik 1's launch in October 1957.

A 2021 VASCO paper described nine sources appearing within about half an hour on an April 12, 1950 plate from Palomar Observatory. In October 2025 the team reported aligned groups of transients and a shortage of them inside Earth's shadow, where an orbiting reflector could not catch sunlight. A companion paper in Scientific Reports, led by Vanderbilt's Stephen Bruehl, found transients on 15.6 percent of dates within a day of a nuclear test against 10.8 percent of other dates, plus a small association with civilian UAP reports; the authors wrote that the findings "do not definitively indicate what transients are." arXiv declined to host preprints of both papers, drawing a protest letter from Disclosure Foundation chairman Christopher Mellon.

Critics argue the transients are artifacts. Nigel Hambly and Adam Blair concluded in 2024 that they are likely spurious features of the photographic emulsion, possibly introduced when the glass copy plates were made, and a July 2026 critique by Wesley Andrés Watters, Kevin Knuth and colleagues questioned the image validation and statistics; Villarroel's team replied in an unreviewed preprint. Others propose physical causes, from nuclear-test debris, suggested by physicist Michael Wiescher, to ball-lightning-like effects of hypothetical axion quark nugget dark matter, proposed in August 2026 by physicist Ariel Zhitnitsky.

Work in 2026 strengthened the team's case without settling it. An August Scientific Reports paper used machine learning to rank 107,875 candidates, judged only about a fifth likely to be real, and found the most probable transients showed a 55 percent shortfall inside Earth's shadow and the strongest nuclear-test association on the day before and the day of a test. The authors noted that the original plates are not public and the transients' nature remains unexplained. NBC News and NewsNation then featured the pair and their maps of apparent hotspots beneath the transients, including White Sands. The transients matter because they bring a UAP-adjacent question into refereed astronomy, where archival data rather than testimony can confirm or refute it.