Page:SATCON2 Observations Report.pdf/6



The apparent brightness of visible satellites is critically dependent on the reflectivity of their surfaces. However, this is highly dynamic because it depends on the altitude, attitude (orientation), albedo, size, surface characteristics, specular versus diffuse reflections, self-shadowing, and the solar phase angle. To date, constellation operators do not provide comprehensive brightness models for their satellites. However, several measurements of LEOsat visual magnitudes have been made by different astronomical teams (e.g., Pomenis Observatory, Ckoirama Observatory, the Visible and Infrared Survey Telescope for Astronomy, and more) to assess their impact on optical and near-infrared astronomy.

Observations of Starlink satellites with no darkening mitigations show that they have typical apparent brightness in the magnitude 4–5 range (Mallama, 2020a; Otarola et al., 2020) and are easily visible with telescopes or even the unaided human eye. Observations of OneWeb satellites show typical brightness fainter than magnitude 6–7, but they are placed at a higher orbital height, around 1200 km (Mallama, 2020c; Zamora et al., 2020). Limited observations of DarkSat and VisorSat Starlink satellites indicate that the brightness-reduction mitigation measures implemented in the modified designs are effective, but do not achieve the SATCON1 brightness recommendation of 7th V magnitude at an orbital height of 550 km (see, e.g., Tyson et al., 2020; Tregloan-Reed et al., 2020; Mallama, 2020b). Observations of Starlink satellites in multiple spectral bands further show the satellites are brighter at longer wavelengths, and the efficacy of the DarkSat mitigation strategy experiment decreases in the near-infrared (Tregloan-Reed et al., 2021; D&QS Report).

While these ad hoc observing campaigns have been crucial for understanding the initial impacts of LEOsat constellations on astronomy and observers worldwide, in the longer-term, SATCON1 Recommendation 8 calls for a “comprehensive satellite constellation observing network [...] for feedback to operators and astronomical programs.” It notes that the regular launching, maneuvering, on-orbit aging, and deorbiting of LEOsats creates an ever-evolving population of satellites, which require ongoing monitoring. Current estimates indicate there will be numerous satellites visible to the unaided eye under dark skies in the next several years, sometimes to the point of saturating telescope CCD detectors, unless darkening measures are implemented. This conclusion is sound well beyond the level of uncertainty of Rh