Objects in Geosynchronous Orbit in the 1950s?

The Palomar Observatory Sky Survey (POSS), conducted by the National Geographic Society, was a massive effort to survey the entire sky visible from Palomar Observatory in California. A total of 1872 photographic plates, each 14 inches (35.6 cm) square, were exposed by the 48-inch Schmidt camera at Palomar observatory. The faintest objects captured depends on the region of the sky, but is typically around magnitude 22. Each region of the sky was photographed twice, once on a red-sensitive plate and again on a blue-sensitive plate, allowing estimation of the spectral type of stars.

The first plate was exposed on November 11, 1949, with 99% of plates completed by June 20, 1956. A few recalcitrant plates where weather had been uncooperative were completed by December, 1958.

The POSS has been an essential resource for astronomy since its initial “publication” as a collection of photographic negative prints, and possession of a copy was a (very expensive) asset highly valued by observatories and university astronomy departments. In the early 1990s, Fourmilab funded the creation of a microfiche edition of the POSS to make it available at a price affordable by amateur astronomers. In 1994, a digitised version was made available on a collection of 102 CD-ROMs, and in 1996, the Astronomical Society of the Pacific (who had earlier distributed the microfiche edition), released a compressed version of the POSS called RealSky.

The overwhelming majority of POSS plates were taken before the launch of the first Earth satellite in October, 1957, and all plates were complete before the first satellite was placed in geosynchronous orbit in 1963. So while glints from the Sun reflecting from satellites in geosynchronous orbits are commonplace on images captured by Earth-based telescopes today, there ought to be exactly zero on the POSS plates.

In a 2021 paper, “Exploring nine simultaneously occurring transients on April 12th 1950” Beatriz Villarroel and eight other researchers reported the presence of nine objects on a POSS plate taken in 1950 which were absent from subsequent surveys and could not be found on follow-up observations with a 10.4 metre telescope and modern CCD detector. The authors note:

If contamination as an explanation can be fully excluded, another possibility is fast (t < 0.5 s) solar reflections from objects near geosynchronous orbits. An alternative route to confirm the latter scenario is by looking for images from the First Palomar Sky Survey where multiple transients follow a line.

Now, that follow-up study has been done, posted on arXiv on 2022-04-12, “Is there a background population of high-albedo objects in geosynchronous orbits around Earth?”. Here is the abstract:

Old, digitized astronomical images taken before the human spacefaring age offer a unique view of the sky devoid of known artificial satellites. In this paper, we have carried out the first optical searches ever for non-terrestrial artifacts near the Earth following the method proposed in Villarroel et al. (2022). We use images contained in the First Palomar Sky Survey to search for simultaneous (during a plate exposure time) transients that in addition to being point-like, are aligned. We provide a shortlist of the most promising candidates of aligned transients, that must be examined with the help of a microscope to separate celestial sources from plate defects with coincidentally star-like brightness profiles. We further explore one possible, but not unique, interpretation in terms of fast reflections off high-albedo objects in geosynchronous orbits around Earth. If a future study rules out each multiple transient candidate, the estimated surface density becomes an upper limit of <10^{−9} objects {\rm km}^{−2} non-terrestrial artifacts in geosynchronous orbits around Earth. Finally, we conclude that observations and analysis of multiple, simultaneously appearing and vanishing light sources on the sky merit serious further attention, regardless of their origin.

Bright, flashing objects in geosynchronous orbit before the space age? I’m not saying it’s aliens, but…. Beatriz Villarroel discusses the observations and possible interpretations with John Michael Godier in this Event Horizon interview.