Dark skies and bright satellites

Most ground-based observatories require a dark night sky to uncover answers to some of the most fundamental questions about the nature of our Universe. However, a number of companies and governments are in various stages of planning or deploying bright satellites in low-Earth orbit (or LEOsats) in greater numbers than ever before. These “megaconstellations” will fundamentally change astronomical observing at visible wavelengths. Nighttime images will be contaminated by streaks caused by the passage of Sun-illuminated satellites. If proposals calling for 100,000 or more LEOsats are realized, no combination of mitigations will be able to fully avoid the negative impact on astronomy. This threat comes at a time when new technology offers unprecedented scientific opportunities, all requiring access to dark skies. One example is the Vera C. Rubin Observatory, which is nearing completion. Its Legacy Survey of Space and Time (LSST) will soon offer a dramatic new view of the changing sky. Rubin Observatory will employ the 8.4-m Simonyi Survey Telescope and the 3200-megapixel LSST Camera to capture about 1000 images of the sky, every night, for 10 years. A single 30-s exposure will reveal distant objects that are about 40 million times fainter than those visible with the unaided eye. The observatory's combination of a large light-collecting area and field of view is unparalleled in the history of astronomy, which is why the project was the top ground-based priority for U.S. astronomers in the 2010 National Academies Decadal Survey of Astronomy and Astrophysics. LSST six-color images will contain data for about 20 billion ultrafaint galaxies and a similar number of stars, and will be used for investigations ranging from cosmological studies of the Universe to searches for potentially hazardous Earth-impacting asteroids. However, the discoveries anticipated from Rubin and other observatories could be substantially degraded by the deployment of multiple LEOsat constellations. The most exciting science to come out of current and planned astronomical facilities may be the discovery of types of objects and phenomena not yet observed or predicted. Such profound surprises have the potential to revolutionize our understanding of every field from exobiology to cosmology. Rubin Observatory's LSST, for example, opens the prospect of observing how ultrafaint objects change over time. It is precisely this kind of astronomy that is most at risk from image artifacts arising from LEOsat megaconstellations. These satellites scatter sunlight for several hours after sunset or before sunrise, are relatively close and bright, and thus can affect ground-based telescopes observing at visible wavelengths. Constellations in orbits well above 600 km will be illuminated by the Sun all night long. Astronomers worldwide are seeking ways to diminish the satellites' most damaging effects—the focus of a recent virtual workshop[*][1] sponsored by the U.S. National Science Foundation—and are collaborating with SpaceX (in particular, the Rubin Observatory), the first operator to launch a substantial constellation of LEOsats. SpaceX has shown that satellite operators can reduce reflected sunlight through satellite orientation, Sun shielding, and surface darkening. A joint effort to obtain higher-accuracy public data on the predicted location of individual satellites could help astronomers point their instruments to avoid some of the interference. Although all of these measures are helpful, there are no guarantees, and the research community is left to hope for good corporate citizenship. Future constellations owned and operated by foreign governments pose a different sort of challenge. Although there are international regulations covering radio-frequency interference, there are no such regulations in place for visible-frequency light pollution from space. Earth orbit is a natural resource without environmental protections, and we are now witnessing its industrialization. Currently there are about a thousand bright LEOsats, but that may be just the beginning. Proposals to expand telecommunications and data relay to serve new technologies like self-driving cars could lead to a 100-fold increase in the number of LEOsats in the next decade. The American Astronomical Society is working with astronomy stakeholders, commercial satellite operators, and international organizations to begin to forge policy on light pollution from space. It is unclear how long this will take and how effective it can be. What is clear is that without productive industry-observatory collaboration, voluntary operator compliance with best practices for mitigation, and subsequent regulatory action, we are slated to lose a clear view of the Universe and its secrets. [1]: #fn-1