12 February 2020

IAU Issues Update on Satellite Constellations' Potential Impacts on Astronomy

This post is adapted from an IAU press release:

In June 2019 the International Astronomical Union (IAU) expressed concern about the negative impact that planned megaconstellations 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 impacts of these satellite constellations.

Satellites Crossing the Sky
Even before the launches of satellite constellations, many satellites were visible on any given night. This image is composed of 300 13-second exposures taken within 70 minutes from Waldenburg, Germany, on the night of the Perseid meteor shower in 2018. The image has a field of view of 84° x 62°. Most of the streaks are from satellites reflecting sunlight. Photo by Eckhard Slawik, courtesy the International Astronomical Union.

Following the statement of June 2019, IAU Commission B7, Protection of Existing and Potential Observatory Sites, and the IAU Executive Committee Working Group on Dark and Quiet Sky Protection were asked by the IAU Executive Committee to assess the situation and to start discussions with the companies responsible for launching and operating the megaconstellations in order to study measures to mitigate their interference.

Commission B7 has requested the input of astronomers from a variety of organizations skilled in modeling the frequency, location, and brightness of satellite megaconstellations. These include the Vera C. Rubin Observatory , the University of Michigan, the German-Spanish Astronomical Center at Calar Alto (CAHA), the European Southern Observatory (ESO), and the European Space Agency (ESA). The results of the simulations, some of which are presented below, are to be considered preliminary given the large number of parameters involved and the associated assumptions and uncertainties.

  • While there is large uncertainty about the future number of satellites, some simulations were conducted on the basis of a large sample of more than 25,000 satellites from representative satellite constellations from different companies (SpaceX, OneWeb, and Amazon). With this sample, the number of satellites above the horizon at any given time would be between ~1,500 and a few thousand, depending on the observer's latitude. Most 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° (where the sky is likely clear from obstructions and where most astronomical observations are performed). The vast majority of these will be too faint to be visible to the naked eye1, 2, 4.
  • When the Sun is 18° below the horizon (astronomical twilight, when the night becomes truly dark), the number of illuminated satellites above the horizon would be around 1,000 (with around 160 at elevations higher than 30°). The numbers decrease further toward the middle of the night, when more satellites are in Earth's shadow and don't reflect sunlight1, 2, 4.
  • At the moment it is difficult to predict how many illuminated satellites will be visible to the naked eye, partly because of uncertainties in their actual reflectivity and partly because we don't yet know the results from an experiment being carried out by SpaceX to reduce the reflectivity of one of its Starlink satellites. 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 artificial satellites. The interference with the uncontaminated view of the night sky will be particularly important in regions of the sky close to the horizon and less evident at high elevation1, 2.
  • The prominent trains of satellites (“strings of pearls”) 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 launches2.
  • 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 observations1, 3, 4.

A summary of the findings and of the actions that have so far been undertaken is presented in a specific IAU theme, Satellite Constellations.

The focus so far has been on optical (visible) wavelengths. This is not to underplay the effect on the radio and submillimeter 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 website 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, 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 artificial satellites. While earlier this was not considered a priority topic, it is now becoming increasingly relevant. Therefore the IAU will regularly present its findings at meetings of the United Nations Committee on the Peaceful Uses of Outer Space, bringing the attention of world government representatives to the threats posed by any new space initiative on astronomy and on science in general. In addition, the specific theme of megasatellites will be included in the program of the IAU/UNOOSA/IAC Conference on Dark and Quiet Skies for Science and Society, which will be held in Santa Cruz de La Palma, Canary Islands, Spain, from 5 to 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.


1Hainaut, Olivier (ESO), 2020, "On the Impact of Satellite Megaconstellations on Astronomical Observations," submitted for publication in Astronomy & Astrophysics.

2Seitzer, Pat (University of Michigan), 2020, "Presentation to the US National Science Foundation Astronomy and Astrophysics Advisory Committee" (PDF).

3Tyson, Tony (Vera C. Rubin Observatory), 2020, private communication.

4Galadí-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.

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