A glint in the eye: Photographic plate archive searches for non-terrestrial artefacts

In this paper, we present a simple strategy to identify Non-Terrestrial artefacts [NTAs; Haqq-Misra and Kopparapu (2012)] in or near geosynchronous Earth orbits (GEOs). We show that even the small pieces of reflective debris in orbit around the Earth can be identified through searches for multiple transients in old photographic plate material exposed before the launch of first human satellite in 1957. In order to separate between possible false point-like sources on photographic plates from real reflections, we present calculations to quantify the associated probabilities of alignments. We show that in an image with nine “simultaneous transients” at least four or five point sources along a line within a 10∗10 arcmin2 image box are a strong indicator of NTAs, corresponding to significance levels of 2.5 to 3.9σ. This given methodology can then be applied to set an upper limit to the prevalence of NTAs with reflective surfaces in geosynchronous orbits. •Our paper is motivated by the curious finding in Villarroel et al. (2021).•Photographic plate surveys from pre-satellite times are ideal to use in searches for non-terrestrial artefacts.•The methodology proposed is cheap and powerful as all image data sets already exists, are public and only need to be analysed.