'Who put them there?' Scientists struggle to explain UFO-like objects captured in 1950s astronomy photos.

The National Geographic Society undertook a massive astronomical survey between 1949 and 1958 at the Palomar Observatory in California, snapping thousands of photographs of the sky from the north celestial pole to 33 degrees south of the celestial equator.

According to a 1959 leaflet issued by the Astronomical Society of the Pacific, the result was a "map of the sky, one that can be used like any road map, to help the astronomer find his way to objects too faint to see directly at the eye-piece of a telescope."

The Palomar Observatory Sky Survey images captured a multitude of inexplicable star-like objects that astronomers had reportedly seen appear then quickly vanish. The objects, which flashed in the sky several years before the October 1957 launch of Sputnik, supposedly cannot be chalked up to gravitational lensing, gamma ray bursts, fragmenting asteroids, and/or various non-astronomical effects.

"We've ruled out some of the prosaic explanations, and it means we have to at least consider the possibility that these might be artificial objects from somewhere," Stephen Bruehl, a professor at the Vanderbilt University School of Medicine, told Live Science.

In a peer-reviewed study published in October in the journal Scientific Reports, Bruehl and co-author Beatriz Villarroel, a Swedish astronomer, found that there are "associations beyond chance between occurrence of transients and both nuclear testing and [unidentified anomalous phenomenon] sightings."

The duo analyzed the transient data available for the time period Nov. 19, 1949, to April 28, 1957, and tested for possible associations between the occurrence of 107,875 transients, which were observed on 310 of the 2,718 days in this period, and above-ground nuclear weapons tests, which were conducted by the U.S., the U.K., and the former USSR on 123 days during the study period.

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Photo by © CORBIS/Corbis via Getty Images

The researchers found that a "transient was 45% more likely to be observed on dates within a nuclear test window compared to dates outside of a nuclear test window."

The duo also linked the transients to unidentified flying object/unidentified anomalous phenomenon reports, noting that "for days on which at least one transient was identified, significant associations were noted between total number of transients and total number of independent UAP reports per date."

'Why do they seem to show interest in nuclear testing?'

"For every additional UAP reported on a given date, there was an 8.5% increase in number of transients identified," Bruehl and Villarroel wrote.

When it came down to hypothesizing what the transients might be, the duo came up with two possibilities that could account for associations of transients with both nuclear testing and UAP reports.

"The first involves an unexpected and previously undocumented atmospheric phenomenon triggered by nuclear detonations or related to nuclear fallout that may serve as a stimulus for some UAP reports and appear as transients on astronomical images," they wrote.

The duo noted, however, that this first hypothesis is problematic, as effects in the atmosphere "would be likely to result in a streak on the image over the 50 min exposure, yet all transients appear as distinct point sources rather than streaks."

Additionally, the researchers noted that transients were "most often observed one day after a nuclear test; such atmospheric phenomena would have to be sustained and remain localized in one location for approximately 24 h to account for the visual appearance of transients."

After poking holes in their first hypothesis, the duo propped up their second hypothesis on the "well-known strand of UAP lore suggesting that nuclear weapons may attract UAP."

"Within this latter hypothesis, our results could be viewed as indicating that transients are artificial, reflective objects either in high-altitude orbits around Earth or at high altitudes within the atmosphere," they added.

Bruehl said to Live Science, "If it turns out that transients are reflective artificial objects in orbit — prior to Sputnik — who put them there, and why do they seem to show interest in nuclear testing?"

Michael Wiescher, a nuclear astrophysicist at the University of Notre Dame in France, suggested to Scientific American that nuclear tests alone might be the simpler explanation for the transients as they "obviously have an impact on the atmosphere" and can leave "a lot of junk in the outer atmosphere."

Sean Kirkpatrick, former head of the Department of War's UAP-investigating All-Domain Anomaly Resolution Office, similarly suggested that the explanation likely has to do with nuclear tests and the sun, noting that the "first thing that comes to mind is solar flare radiation or ionized particle radiation from nuclear testing."

Kirkpatrick also suggested that high-altitude balloons, which were used for nuclear monitoring, could account for some of the UAP reports.

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