At a combined total of natural plus space objects background of ~ 700 μcd m-2, the brightness of the night sky at the zenith in this scenario would rival that at a site moderately impacted by terrestrial skyglow: 20.7 V magnitudes per square arcsecond, a value three times higher than the natural background alone. This condition is described by Class 4 on the qualitative Bortle Scale of night sky quality.[1] Only half the number of stars would be visible in the night sky relative to what would be visible in the absence of space-object light pollution.[2] This reduction in the visibility of stars is akin to a global view of the night sky that lies somewhere between typical suburban and rural skies.[3] We emphasize that this is only a lower limit to the stars being erased, assuming that crowded conditions in LEO lead to more frequent debris-generating collisions. This estimate further assumes that future satellites will have optical properties broadly like those of today. Although SpaceX has demonstrated a reduction in the total reflectivity of its Starlink objects through engineering innovations,[4][5] the long-term choices made by industry regarding mitigating solutions are not guaranteed. Without binding legal regulations that impose mitigation targets, it remains a purely voluntary matter whether operators pursue these solutions.
Other than the loss of stars, there is also the potential for increased target observation times for professional astronomy as higher backgrounds require longer integration times to reach a specific signal to noise ratio. Last but not least, there will be reduced viewing of celestial phenomena that have united human observations across the ages, including, e.g., the Milky Way, meteor showers and aurorae.
The brightest parts of the Milky Way become just visible to the unaided eye at the zenith around a brightness of 2000 μcd m-2 (~ 19.5 V magnitudes per square arcsecond, or mV arcsec-2). At 800 μcd m−2 (20.5 mV arcsec-2), depending on the presence of light domes on the horizon, most of the Milky Way is visible from horizon to horizon. But the visual appearance of the Milky Way with richness of detail does not begin until the zenith brightness is around 400 μcd m−2, (~ 21.2 mV arcsec-2). In terms of factors above the assumed natural background of ~ 200 μcd m−2 (~ 21.9 mV arcsec-2), these represent thresholds of about 10, 4 and 2 times, respectively.[6]
Observing meteor showers and aurorae are also popular activities at dark-sky sites. While the brightest meteors are visible from even the most light-polluted cities, dark sites excel at providing the opportunity to see relatively large numbers of meteors during a given night. Faint meteors tend to dominate these numbers, and so the resulting effect is rather dependent on night-sky brightness. Keeping in mind that every step brighter in sky brightness in terms of magnitudes per square arcsecond is a factor of approximately 2.5 toward higher backgrounds, and given the brightness distribution of meteors in major
- ↑ For a description of the Bortle Scale, see Bortle, John E. (February 2001), Gauging Light Pollution: The Bortle Dark-Sky Scale, Sky & Telescope. Sky Publishing Corporation.
- ↑ This assumes ~ 9000 stars brighter than the canonical unaided eye limit of magnitude +6.5 spread over the entire sky (Hoffleit, D.; Jaschek, C., eds. 1991. The Bright Star Catalogue. New Haven: Yale University Observatory) and the relationship between the luminance of the night sky and limiting visual magnitude given in Schaefer, B. E. (1990). Telescopic limiting magnitudes. Publications of the Astronomical Society of the Pacific, 102, 212. https://doi.org/10.1086/132629.
- ↑ Note that Kocifaj et al. assumed the pre-Starlink rate of growth for new satellite launches to estimate a zenith brightness of 25 μcd m−2 in 2030 — some 20 times less than what we might more realistically expect in the age of mega-constellations.
- ↑ Horiuchi, T., Hanayama, H. & Ohishi, M. (2020). Simultaneous Multicolor Observations of Starlink’s Darksat by the Murikabushi Telescope with MITSuME. The Astrophysical Journal, 905(1), 3. https://doi.org/10.3847/1538-4357/abc695.
- ↑ Mallama, A. (2021). The Brightness of VisorSat-Design Starlink Satellites, arXiv:2101.00374.
- ↑ Conversions between SI (cd m−2) and ‘astronomer’ luminance units (mV arcsec-2) were made here according to the calibrations in Bará, S., et al. (2020). Magnitude to luminance conversions and visual brightness of the night sky. Monthly Notices of the Royal Astronomical Society, 493(2), 2429–2437. https://doi.org/10.1093/mnras/staa323 and Fryc, I., et al. (2021). On the Relation between the Astronomical and Visual Photometric Systems in Specifying the Brightness of the Night Sky for Mesopically Adapted Observers. LEUKOS, 1–12. https://doi.org/10.1080/155-02724.2021.1921593.
