Is there a background population of high-albedo objects in geosynchronous orbits around Earth?

TL;DR

This study searches for non-terrestrial artifacts in geosynchronous orbit using archival sky survey images, proposing a method to identify potential high-albedo objects and setting upper limits on their population.

Contribution

First optical search for non-terrestrial artifacts near Earth using archival images, introducing a method to identify aligned transient signals and estimating their upper population limit.

Findings

No definitive non-terrestrial artifacts found

Estimated upper limit of <10^{-9} objects km^{-2} in geosynchronous orbit

Highlighted the importance of analyzing simultaneous transient light sources

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 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.