In June 2019, the International Astronomical Union expressed concern about the negative impact that the planned mega-constellations of communication satellites may have on astronomical observations and on the pristine appearance of the night sky when observed from a dark region. We here present a summary of the current understanding of the impact of these satellite constellations.
Following the statement of June 2019, IAU’s Commission B7 Protection of Existing and Potential Observatory Sites and the Executive Committee Working Group Dark and Quiet Sky Protection were asked by the IAU Executive Committee to assess the situation and to start discussions with the companies that are responsible for launching and operating the mega-constellations in order to study measures to mitigate their interference.
Commission B7 has requested the input of astronomers from different organisations (Vera C. Rubin Observatory, U. Michigan, CAHA, ESO and ESA) skilled in modeling the frequency, location and brightness of satellite mega-constellations. Some of those results are presented below. The results of the simulations, given the large number of parameters involved and the associated assumptions and uncertainties, are to be considered preliminary.
While there is large uncertainty about the future number of satellites, some simulations were conducted on the basis of a large sample of over 25 000 satellites from representative satellite constellations from different companies. With this sample, the number of satellites above the horizon at any given time would be between ~1500 and a few thousand, depending on the latitude. Most of these will appear very close to the horizon, only a few of them passing directly overhead; for instance, about 250 to 300 would have an elevation of more than 30 degrees over the horizon (i.e. where the sky is clear from obstructions, and where most of the astronomical observations are performed). The vast majority of these will be too faint to be visible to the naked eye   .
When the Sun is 18 degrees below the horizon (i.e. when the night becomes dark), the number of illuminated satellites above the horizon would be around 1000 (with around 160 at elevations higher than 30 degrees). The numbers decrease further towards the middle of the night, when more satellites are in the Earth's shadow (e.g., no reflected sunlight)    .
At the moment it is difficult to predict how many of the illuminated satellites will be visible to the naked eye, because of uncertainties in their actual reflectivity (also since experiments are being carried out by SpaceX to reduce the reflectivity of a Starlink satellite by adopting different coatings). The appearance of the pristine night sky, particularly when observed from dark sites, will nevertheless be altered, because the new satellites could be significantly brighter than existing orbiting man-made objects. The interference with the uncontaminated view of the night sky will be particularly important in the regions of the sky close to the horizon and less evident at high elevation  .
The prominent trains of satellites (“strings of pearls”), often seen in images and videos, are significant immediately after launch and during the orbit-raising phase when they are considerably brighter than they are at their operational altitude and orientation. The global effect depends on how long the satellites are in this phase and on the frequency of launches .
Apart from their naked-eye visibility, it is estimated that the trails of the constellation satellites will be bright enough to saturate modern detectors on large telescopes. Wide-field scientific astronomical observations will therefore be severely affected. For instance, in the case of modern fast wide-field surveys, like the ones to be carried out by the Rubin Observatory (formerly known as LSST), it is estimated that up to 30% of the 30-second images during twilight hours will be affected. Instruments with a smaller field of view would be less affected. In theory, the effects of the new satellites could be mitigated by accurately predicting their orbits and interrupting observations, when necessary, during their passage. Data processing could then be used to further “clean” the resulting images. However, the large number of trails could create significant and complicated overheads to the scheduling and operation of astronomical observations  .
A summary of the findings and of the actions that have so far been undertaken is presented in a specific IAU Theme.
The focus of this Statement has been on the optical wavelengths. This is not to underplay the effect on the radio and submillimetre wavelength ranges, which is still under investigation. The IAU considers the consequences of satellite constellations worrisome. They will have a negative impact on the progress of ground-based astronomy, radio, optical and infrared, and will require diverting human and financial resources from basic research to studying and implementing mitigating measures.
A great deal of attention is also being given to the protection of the uncontaminated view of the night sky from dark places, which should be considered a non-renounceable world human heritage. This is one of the main messages communicated on the dedicated IAU–UNESCO web site on astronomical heritage.
In order to mitigate the impacts of satellite constellations that may interfere with professional and amateur astronomical observations, the IAU, in close collaboration with the American Astronomical Society (AAS), will continue to initiate discussions with space agencies and private companies that are planning to launch and operate currently planned and future satellite constellations.
The IAU notes that currently there are no internationally agreed rules or guidelines on the brightness of orbiting manmade objects. While until now this was not considered a priority topic, it is now becoming increasingly relevant. Therefore the IAU will regularly present its findings at the meetings of the UN Committee for Peaceful Uses of Outer Space (COPUOS), bringing the attention of the world Government representatives to the threats posed by any new space initiative on astronomy and science in general. In addition, the specific theme of the mega-satellites will be included in the Programme of the IAU/UNOOSA/IAC Conference Dark and Quiet Skies for Science and Society, which will be held in Santa Cruz de La Palma, Canary Islands, Spain, on 5–8 October 2020.
The IAU stresses that technological progress is only made possible by parallel advances in scientific knowledge. Satellites would neither operate nor properly communicate without essential contributions from astronomy and physics. It is in everybody’s interest to preserve and support the progress of fundamental science such as astronomy, celestial mechanics, orbital dynamics and relativity.
 Hainaut, Olivier (ESO), 2020, On the Impact of Satellite Constellations on Astronomical Observations with ESO telescopes in the Visible and Infrared Domains, accepted for publication in Astronomy & Astrophysics. Also see this ESO press release.
 Seitzer, Pat (University of Michigan), 2020, Presentation to the US National Science Foundation Astronomy and Astrophysics Advisory Committee
 Galadí-Enríquez, David (Calar Alto Observatory), 2020, Geometric simulation of the visibility of Starlink satellite constellation from ground-based optical observatories: LSST as a case study, progress report, private communication.
The IAU is the international astronomical organisation that brings together more than 13 500 professional astronomers from more than 100 countries worldwide. Its mission is to promote and safeguard astronomy in all its aspects, including research, communication, education and development, through international cooperation. The IAU also serves as the internationally recognised authority for assigning designations to celestial bodies and the surface features on them. Founded in 1919, the IAU is the world's largest professional body for astronomers.
Advisor, IAU Executive Committee
President Commission B7
Lars Lindberg Christensen
IAU Press Officer
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