Impact of satellite constellations on astronomical observations with ESO telescopes in the visible and infrared domains
Authors: Hainaut Olivier R. , Williams Andrew P. 
Published: March 2020

The effect of satellite mega-constellations on astronomical observations in the visible, near-infrared, and thermal infrared domains is estimated using a simple methodology, which is applied to ESO telescopes and instruments as examples (radio and (sub-)millimetre domains are not considered here). The study considers a total of 18 constellations in development by SpaceX, Amazon, OneWeb, and others, with over 26 thousand satellites, constituting a representative distribution. This study uses a series of simplifications and assumptions in order to obtain conservative, order-of-magnitude estimates of the effects: the satellites are assumed to be uniformly spread over the Earth’s globe, and their magnitude is estimated using a simplistic model calibrated on actual observations. The effect on various types of ground-based telescopic observations is estimated using a geometric probabilistic approach. The “trains” of very-low altitude satellites typically observed immediately after launch are extremely bright due to their very low orbit. They also fall very quickly in the shadow of the Earth after sunset. However, this initial bright state is not considered further, as the satellites quickly disperse into their higher altitude orbits. The number of illuminated satellites from the constellations above the horizon of an observatory ranges from approximately 1600 immediately after sunset, decreasing to 1100 at the end of astronomical twilight, most of them (∼85%) close to the horizon (below 30° of elevation). The large majority of these satellites will be too faint to be seen with the naked eye: at astronomical twilight, 260 would be brighter than magnitude 6 (i.e. visible in exceptional conditions), 110 brighter than 5 (i.e. visible in good conditions). Again, most of them (∼95%) will be close to the horizon (below 30° of elevation). The number of naked-eye satellites plummets as the Sun reaches 30−40° of elevation below the horizon. Specular flares and occultations by satellites are expected to cause only negligible effects on telescopic astronomical observations. The light trail caused by the satellite would ruin a small fraction (below the 1% level) of telescopic exposures using narrow to normal field imaging or spectroscopic techniques in the visible and near-infrared during the first and last hours of the night. Similarly, the thermal emission of the satellite would affect only a negligible fraction of the observations in the thermal infrared domain. However, wide-field exposures and long medium-field exposures would be affected at the 3% level during the first and last hours of the night. Furthermore, ultra-wide imaging exposures on a very large telescope (where saturation of the satellite trails has a ruinous effect on the detectors, such as those from the National Science Foundation’s Vera C. Rubin Observatory, formerly known as LSST), would be significantly affected, with 30–40% of such exposures being compromised during the first and last hours of the night. Coordination and collaboration between the astronomical community, satellite companies, and government agencies are therefore critical to minimise and mitigate the effect on astronomical observations, in particular on survey telescopes.