Scientists may have found a powerful new space object: 'It doesn't fit comfortably into any known category' “You don’t get those kinds of solar reflections from round objects like asteroids or dust grains in space, which leave streaks during a 50-minute exposure, but only if something is very flat and very reflective and reflects the sunlight with a short flash,” said Villarroel. Transients studied by the VASCO project, which attempts to pin down the sources for these mysterious flashes of light seen in archival sky survey photographs. (Image credit: Beatriz Villarroel et al 2025/CC BY 4.0 (DOI 10.1088/1538-3873/ae0afe)) Villarroel and Bruehl propose another possible explanation, however: that nuclear weapons tests triggered some unknown atmospheric phenomenon that went unnoticed at the time. But Villarroel and Bruehl are skeptical that such a phenomenon would stand still in the atmosphere for 24 hours between the weapons test and when the plate was exposed at Palomar in California. The transients do not seem to be particles of nuclear fallout that have drifted down onto the photographic plate either, since such particles would produce foggy, diffuse spots, not pinpoint, star-like objects. The explanation Villarroel and Bruehl focus on most in their papers is that these transients are UAP of some kind. Their study connects the nuclear tests to sightings, which have been reported in the vicinity of nuclear sites for decades . "Significantly more UAP sightings were reported within nuclear weapons testing windows (test date + /- 1 day) than outside of testing windows," they report in their study. There are, of course, many caveats. Critics have claimed that the transients could be photographic defects, or contamination, especially as the plates are quite old and were stored away for many decades before being digitized. Villarroel and Bruehl perhaps also give too much credit to reports of UFO sightings. Their reported correlation of 8.5% between the appearance of the transients and flying saucer sightings is only relevant if it can be assumed those UAP sightings are credible in the first place. There may also be an observation bias – the 1950s were the heyday of UFO sightings , so it is perhaps not too surprising that there were sightings coinciding with the appearance of transients, since UAP sightings were reported on many different days. Ultimately, correlation does not necessarily mean causation, and Villarroel and Bruehl do acknowledge this in their study. In SETI, the search for extraterrestrial intelligence, researchers tend to assume that any unexplained phenomena isn’t aliens, and to exhaust every possible natural explanation before invoking an extraterrestrial one. This approach would be helpful here, although what those alternative explanations might be are not yet certain. Because of the nuclear test-ban treaty there is, quite rightly, no way to test the hypothesis that the transients are related to atmospheric phenomena caused by nuclear explosions, of which there were at least 124 above the ground between 1951 and 1957. For now, the discovery of the transients remains an intriguing puzzle. One possible way forward that has been suggested is to try and repeat the observations on the modern day sky. If geosynchronous satellites that we know about produce similar patterns of transients on photographic plates, then that would strengthen the hypothesis that the transients on the Palomar plates could depict metallic objects reflecting sunlight in high orbit. Source: https://www.space.com/space-exploration/search-for-life/were-unexplained-flashes-of-light-in-70-year-old-sky-surveys-caused-by-ufos-or-nuclear-testing-why-not-both-researchers-say