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In this Note, we discuss the rationale behind the IAU CPS’s recommendation on satellite brightness for objects in low Earth orbit (LEO). Specifically, we clarify the reasoning behind the chosen altitude dependence and the limitations of this choice. We further discuss some approaches toward brightness limits for objects beyond LEO. This Note is intended for both astronomers and satellite operators in the spirit of furthering mutual cooperation. 

Ground-based astronomical surveys such as the much anticipated Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) [1] face new challenges due to the increasing number of satellites in Low and Medium Earth Orbit (LEO & MEO). With hundreds of thousands of artificial satellites likely to be launched in the near future as estimated by the IAU CPS, satellite trails in astronomical images will become frequent enough to require some form of mitigation. Limiting the maximum brightness of artificial satellites to below 7th magnitude is one of the proposed means to reduce satellite impact on Rubin data, which prevents the most severe systematic errors and widespread data loss in LSST camera sensors [2]. Glints caused by satellites and space debris are also expected to pollute LSST alerts [3].Brighter objects may require active satellite-avoidance scheduling schemes such as investigated by Hu et al. [4]. The IAU Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference (IAU CPS) in cooperation with the Aerospace Corp. is currently developing online services with global API access, such as SatChecker that enable satellite-avoidance [5,6].  However, dodging satellites would effectively decrease observing time in large sky surveys such as the LSST. In this contribution we report updates on satellite brightness mitigation efforts including technologies developed by both satellite constellation operators to darken their constellations as well as new IAU CPS tools for astronomers intended to help tackle this challenge. We also discuss the potential satellite constellation related losses in Solar System Object (SSO) discovery based on recent Sorcha simulations [7].Acknowledgments: The authors acknowledge support from the National Science Foundation through the award Collaborative Research: SWIFT-SAT: Minimizing Science Impact on LSST and Observatories Worldwide through Accurate Predictions of Satellite Position and Optical Brightness NSF Award Numbers 2332736 and 2332735. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.References:  [1] Ivezić, Ž., et al. (2019) “LSST: From science drivers to reference design and anticipated data products.” The Astrophysical Journal 873.2 : 111. [2] Tyson, J.A. et al. (2020) “Mitigation of LEO satellite brightness and trail effects on the Rubin Observatory LSST,” The Astronomical Journal, 160(5), p. 226. [3] Tyson, J.A., Snyder, A., Polin D., Rawls, M.L. and Ivezić Ž. (2024) Expected Impact of Glints from Space Debris in the LSST. The Astrophysical Journal Letters 966, no. 2 : L38. [4] Hu, J.A. et al. (2022) “Satellite constellation avoidance with the Rubin Observatory Legacy Survey of Space and Time,” The Astrophysical Journal Letters, 941(1). [5] IAU CPS SatChecker (https://satchecker.readthedocs.io/en/latest/fov.html), accessed 2025 May 2. [6] Skinner, M. A., Coursey, C.D., and George, E.R. (2023). Dark and Quiet Skies: A predictive technique to mitigate the impact of satellite reflections on astronomical observatories. 74th International Astronautical Congress (IAC), Baku, Azerbaijan, 2-6 October 2023. [7] Schwamb, M. et al. “Sorcha: A Solar System Survey Simulator for the Legacy Survey of Space and Time”, The Planetary Science Journal (in press). 

We examine the simple model put forth in a recent note by Loeb regarding the brightness of space debris in the size range of 1–10 cm and their impact on the Rubin Observatory Legacy Survey of Space and Time (LSST) transient object searches. Their main conclusion was that “image contamination by untracked space debris might pose a bigger challenge [than large commercial satellite constellations in Low-Earth orbit].” Following corrections and improvements to this model, we calculate the apparent brightness of tumbling low-Earth orbit (LEO) debris of various sizes, and we briefly discuss the likely impact and potential mitigations of glints from space debris in LSST. We find the majority of the difference in predicted signal-to-noise ratio (S/N), about a factor of 6, arises from the defocus of LEO objects due to the large Simonyi Survey Telescope primary mirror and finite range of the debris. The largest change from the Loeb estimates is that 1–10 cm debris in LEO pose no threat to LSST transient object alert generation because their S/N for detection will be much lower than estimated by Loeb due to defocus. We find that only tumbling LEO debris larger than 10 cm or with significantly greater reflectivity, which give 1 ms glints, might be detected with high confidence (S/N > 5). We estimate that only one in five LSST exposures low on the sky during twilight might be affected. More slowly tumbling objects of larger size can give flares in brightness that are easily detected; however, these will not be cataloged by the LSST Science Pipelines because of the resulting long streak.