Understanding the Effects of Satellite Constellations on Astronomy

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In June 2019, the International Astronomical Union expressed its concerns about the astronomical observations of the planned mega constellations of communication satellites and their negative effects on the pristine appearance of the night sky when observed from a dark area. Here we provide a summary of the current understanding of the influence of these satellite constellations.

Following the announcement of June 2019, the IAU Commission B7 Protection of Existing and Potential Observation Areas and the Executive Committee Working Group asked to evaluate the situation by the Dark and Silent Sky Protection IAU Executive Committee and initiate discussions with responsible companies. launching and operating mega constellations to study measures to reduce parasites.

Commission B7 asked for the introduction of astronomers from different organizations (Vera C. Rubin Observatory, U. Michigan, CAHA, ESO and ESA) capable of modeling the frequency, location and brightness of satellite mega constellations. Some of these results are presented below. Considering the large number of parameters and related assumptions and uncertainties, the results of the simulations should be evaluated in advance.

While there is a great uncertainty about the number of satellites in the future, some simulations have been made based on a large example of more than 25,000 satellites from representative satellite constellations from different companies. With this example, the number of satellites above the horizon at any given time will be between ~ 1500 and several thousand, depending on its latitude. Most of them will look very close to the horizon, only a few will go directly from above; For example, about 250 to 300 will have a height of more than 30 degrees on the horizon (i.e. where the sky is free from obstacles and most astronomical observations are made). The vast majority of them will be too weak to be seen with the naked eye [1] [2] [3].

When the sun is 18 degrees below the horizon (that is, when the night is dark), the number of illuminated satellites above the horizon is around 1000 (about 160 degrees higher than 30 degrees). The numbers decrease even further in the middle of the night when there are more satellites in the shadow of the world (eg without reflected sunlight) [1] [2] [3].

Currently, it is difficult to estimate how many of the illuminated satellites can be seen with the naked eye, because the uncertainties in their true reflectivity (also, since experiments were carried out by SpaceX to reduce the reflection rate of a Starlink satellite by adopting a different adaptation). The appearance of the pristine night sky will still be changed, especially when observed from dark areas, because new satellites can be significantly brighter than the man-made objects in the current orbit. Intervention in the unpolluted vision of the night sky will be particularly important in areas of the sky close to the horizon and will be less pronounced at high altitudes [1] [2].

The prominent satellite (“pearl strings”) frequently seen in images and videos is important immediately after launch and at the orbital upgrade stage, when they are brighter than their operational heights and directions. The global impact depends on how long the satellites lasted at this stage and the frequency of launch [2].

In addition to being visible to the naked eye, traces of constellation satellites are predicted to be bright enough to saturate modern detectors in large telescopes. Therefore, large-field scientific astronomical observations will be severely affected. For example, in modern rapid wide-field research, such as those to be done by the Rubin Observatory (previously known as LSST), it is estimated that up to 30% of 30-second images will appear during twilight hours. To be affected. Devices with a smaller field of view are less affected. In theory, the effects of new satellites can be mitigated by accurately estimating their trajectory and interrupting observations during their passage when necessary. Data processing can then be used to further “clean” the resulting images. However, [1] [3].

A summary of the findings and actions that have been taken so far is presented in a specific IAU Theme.

The focus of this Statement is optical wavelengths. This is not to emphasize the importance of the impact on radio and submillimeter wavelength ranges that are still under investigation. The IAU finds the results of satellite constellations worrying. They will have a negative impact on the progress of ground-based astronomy, radio, optics and infrared, and will require directing human and financial resources from basic research to the study and implementation of mitigation measures.

Great importance is also given to protecting the unpolluted vision of the night sky from dark places, which should be regarded as an indispensable world human heritage. This is one of the main messages conveyed on the IAU-UNESCO dedicated website on astronomical heritage.

Understanding the Effects of Satellite Constellations on AstronomyIn order to reduce the effects of satellite constellations that can interfere with professional and amateur astronomical observations, the IAU cooperates with the American Astronomical Society (AAS).

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