Optical Characterization of Artificial Satellites and Assessing the Impact on Astronomy

Abstract

The proliferation of low-Earth orbit (LEO) with thousands of new satellites poses to impact astronomy in ways not yet fully understood. Although astronomers have worked around satellites appearing in the night sky for decades, until recently sighting one was a familiar but uncommon event. Now, and in the future, visible satellites are increasingly common, and their appearance will impact all activities utilizing the night sky, including research astronomy. Observers will see more satellites and discard more images or expend more effort to recover data affected by the appearance of satellites. Ultimately, observing projects will require more time, more effort, and more funding to complete. The community calls for research to understand how satellites will impact astronomy and to create solutions to mitigate the impacts. Key to this is characterizing the satellites’ brightness and visibility to observers.The Space Situational Awareness (SSA) community has a long and rich history of optical characterization of satellites with techniques and instrumentation that overlap and parallel those of astronomy. SSA optical characterization concentrates on determining information about the satellite shape, size, and structure. Techniques rely on understanding and exploiting that satellites are reflecting bodies with variations in brightness related to the illumination geometry and the physical characteristics of the satellite itself. Existing methodology focuses on in-depth characterization of individual satellites, most often those in higher orbits that are easier to observe. There is no established methodology for characterizing the brightness of thousands of LEO satellites, particularly with respect to their appearance in the night sky. However, the same understanding that satellites are reflecting bodies with variable appearance is vital to characterizing their brightness and assessing the impact on astronomy. Building on experience with satellite characterization, we started the Steward Observatory LEO Satellite Photometric Survey with particular intent to observe satellites in all visible geometries and capture the variations in brightness. With over 16,000 independent measurements of nearly 2,800 individual satellites from over 400 nights of observation. The Starlink v1.0 satellites exhibited a typical brightness range of 4.50 – 7.08 V magnitude. The Starlink v1.0 VisorSat satellites and Starlink v1.5 satellites exhibited typical brightness ranges of 5.25 – 8.15 and 4.99 – 7.58 V magnitude respectively, demonstrating the effectiveness of brightness-reducing design changes. The OneWeb satellites exhibited a typical brightness range of 7.05 – 9.10 V magnitude, notably fainter than the Starlink satellites. For each population of satellites we show how the range of apparent brightness correlates with on-sky position relative to the below-horizon Sun Beyond reporting the visual magnitude, we assess the impact on research astronomy by calculating the expected photon flux, i.e. how bright the satellites will appear in an image. With a complete description of satellite brightness, we can determine if current brightness-reducing mitigations are working, and explore opportunities for additional mitigations such as observation scheduling to avoid the brightest and most impactful satellites.