Page:The Case for Space Environmentalism.pdf/8

 ''Starlink satellites were clumped together during the orbit raising phase shortly afer launch. Credit: CTIO/NOIRLab/NSF/AURA/DECam DELVE Survey.''

Only some satellites are visible above the horizon at a time. For our 2030-era population, roughly 4,300 are above the horizon at any one time, and they cross the sky in about 13 minutes. For a small field of view, there may be only a few percent chance of being affected by a streak, but the observation could be completely wasted and need to be repeated [18]. The more serious impact will be on wide field survey instruments. The Zwicky Transient Facility has already seen an increase in affected images from 0.5% in late 2019 to 18% in August 2021 [17]. The 3.5 degree wide field imager of the Vera C. Rubin Observatory nearing completion in Chile, will contain at least one streak in the majority of exposures [19]. Laboratory experiments using the Rubin Observatory camera detectors show that electronic cross-talk causes streaks to cascade and create additional fainter streaks; this effect can render some scientific analyses impossible because the statistics of the background sky brightness are irrevocably altered. To avoid the crosstalk problem, the satellites would need to be no brighter than 7th magnitude, fainter than the faintest stars visible to the unaided eye at the darkest sites [19].

Furthermore, as an object in space rotates, a brief bright flash or “glint” can occur as a facet or particularly reflective component of the satellite briefly reflects more sunlight to an observer on the ground [20, 21]. For example, Starlink satellites have been seen to change rapidly from fainter than 6th magnitude to almost 3rd magnitude [22]. These extremely bright and short duration (transient) events can mimic some of the most exciting phenomena in modern astronomy. A study in 2020 identified such a flash as the sign of a gamma-ray burst at the edge of the Universe — potentially an extremely exciting discovery. However, a year later it was found that this flash was actually caused by sunlight reflecting off an old Russian Proton rocket part [23]. We do not yet know how frequent this kind of problem will become as the LEO population grows.

When the Earth eclipses a satellite, the satellite is no longer illuminated from the perspective of an observer on Earth. (However, ASOs do emit thermal photons so affect IR sensing even when in eclipse.) As a result, the impact of satellite constellations on astronomical observations is worst near the beginning and end of the night. However, some types of observation simply have to be done at those times; and the fraction of the night affected depends strongly upon the height of the constellation, the geographic latitude of the observatory and the time of year [3,17, 24]. In addition, observations near twilight will see the most streaks, and that is the same time when it is preferable to search for near-Earth objects. As a result, our 2030-era satellite population would yield fewer discoveries of nearEarth asteroids, including ones that may cross Earth's orbit. These are all factors that must be considered carefully in an environmental assessment.

Radio astronomy

Radio astronomy is affected by satellites using radio signals to relay data back and forth with ground stations and end-user antennas. Detecting faint celestial objects against this anthropogenic background can be potentially very problematic, as the emissions from satellites can be easily a trillion times louder than the astronomical targets [3, 4]. In some observations, finely detailed maps are made by combining signals from many interlinked