Not of this Earth? On the night of July 19th, 1952 something rather strange happened on two fronts in North America. At that time, one of the most puzzling UFO flaps in history was happening in the Washington, DC area including multiple radar returns and visual sightings from people in several locations. At Mount Palomar observatory in Southern California, a pair of photographic plates were taken about one hour apart, one just before 9 pm, the other just before 10 pm local time. In these plates, one showed a cluster of three star-like points, essentially indistinguishable from stars. But in the second plate, the points had abruptly and inexplicably disappeared.

"Is there a background population of high-albedo objects in geosynchronous orbits around Earth?" Villarroel et al, 2022https://arxiv.org/pdf/2204.06091.pdf

"A bright triple transient that vanished within 50 minutes" Solano et al., 2023https://arxiv.org/pdf/2310.09035.pdf

"A glint in the eye: photographic plate archive searches for non-terrestrial artefacts" Villarroel et al., 2021https://arxiv.org/abs/2110.15217

"Exploring nine simultaneously occurring transients on April 12th 1950" Villarroel et al., 2021https://arxiv.org/abs/2106.11780

"The Vanishing & Appearing Sources during a Century of Observations project: I. USNO objects missing in modern sky surveys and follow-up observations of a "missing star"" Villarroel et al., 2019https://arxiv.org/abs/1911.05068