Abstract
While providing important benefits to society, swarms of small satellites (also called satellite constellations) in Low Earth Orbit create sunlight streaks impacting optical/infrared observations and radio transmissions against which the highly sensitive systems at radio observatories can be unprotected. The International Astronomical Union’s Centre for the Protection of the Dark & Quiet Sky against Satellite Constellation Interference (IAU CPS),[1] co-hosted by NSF’s NOIRLab and SKA Observatory, is taking multiple approaches to mitigate negative impacts. At least twelve operators meet under their auspices to share best practices and understand astronomical concerns. Technical recommendations[2] from four expert workshops have been consolidated to provide targets for reducing reflected sunlight, defining the accuracy needed for position and optical brightness predictions, and technological approaches to limiting radio interference with astronomical observations. CPS-affiliated policy and space law experts have developed and are refining recommendations for ultimately turning best practices into a regulatory framework to be endorsed internationally and adopted by national agencies in licensing and oversight. Because the IAU raises awareness, the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) is now adopting a five-year agenda item to consider astronomy and satellite constellations. As a critical mode of exploration and use of outer space, astronomical observations merit protection as an integral part of long-term space sustainability. The high visibility that astronomy has gained, thanks to the impact of the constellations, may be used to extend its role beyond the science realm, recovering its relevance in philosophy, poetry, and theology.
Since the dawn of civilization, the vision of the starry night sky has inspired poets, philosophers, theologians, artists, and scientists. With the introduction of optical instruments, pioneered by Galileo’s cannocchiale in 1609-10, the observation of the sky acquired a new dimension: while keeping its aesthetic fascination, it opened the path to a progressive understanding of the physical reality. The latter role has become even more evident today thanks to space technology’s possibility to access the entire electromagnetic spectrum and observe physical phenomena that would be impossible to reproduce in a terrestrial laboratory.
While technological evolution was instrumental in expanding our knowledge of the Cosmos, it also negatively affected the night sky’s naked-eye appearance. The spreading of artificial illumination in densely populated regions makes it increasingly difficult to see the stars in the bright background. Entire generations of young people may grow up without knowing the magnificence of an undisturbed starry sky. Professional astronomy is also affected by the increasing artificial brightness of the sky, but, fortunately, the best astronomical sites that host the major telescope facilities succeeded in protecting them by adequate local regulations that limit the use of ALAN, the Artificial Light At Night. Similarly, an increasing number of Countries are creating “Dark Sky Oases”, i.e. touristic protected areas that offer their visitors the possibility of enjoying an almost pristine vision of the night sky.
The most recent advances in understanding the universe and its 13,8 billion years of evolutionary history are very much indebted to space technology. The possibility of observing the cosmos from outside the terrestrial atmosphere allowed astronomers to detect signals over the entire electromagnetic spectrum, unveiling phenomena that were previously unthinkable.
It is, therefore, somehow ironic that the same space technology is today threatening the very same astronomy and cosmology. We are witnessing an epochal change in space activities: the so-called New Space Economy is driven by private companies that have reached the same, if not superior, technological capabilities of the historical Space Agencies but whose main goal, differently from the latter’s, is pure profit.
One of the most visible consequences of this epochal transition is undoubtedly the massive increase in the number of communication satellites operating from the so-called LEO (Low Earth Orbit). The purpose of these large constellations, which may be composed of tens of thousands of satellites, is to offer high-speed internet connectivity from any location in the globe, including deserts and open oceans. Considering the additional high resilience of this space connectivity to external disturbances or attacks, it is clear that they represent an important progress in worldwide communication. Indeed, they will contribute to reaching the UN Sustainable Development Goals.
The other side of the coin is a significant impact on sky visibility and astronomy. Satellites reflect sunlight and, depending on the season and observer’s location, can be visible for a large part of the night. Many are visible to the naked eye, and professional astronomers’ highly sensitive telescopes can detect them all. The high density of satellites in the sky may make it impossible for telescopes with a wide field of view to avoid capturing satellite traces, potentially affecting up to 30% of collected images.
The situation is also challenging for radio astronomical observations. Satellites must transmit towards Earth in microwaves to fulfill their task. Although their frequencies are outside the radio astronomical protected range, the power of their beams, which is orders of magnitude stronger than the feeble cosmic signals, will inevitably disturb the astronomical observations.
The International Astronomical Union, founded in 1919, promotes and safeguards the science of astronomy in all its aspects. The IAU could not have remained inert in the face of threats, so it decided to take two initiatives. The first is to use its position as Permanent Observer in the UN Committee for the Peaceful Use of Outer Space (COPUOS) to raise international attention on the impact of satellite constellations on astronomy and propose voluntary mitigating measures. The second initiative was the creation of a coordinating Centre (CPS)[3] with the specific tasks of interacting directly with the constellations companies and with space industries to study and implement hardware and software solutions that can effectively lower the negative impact.
Both lines of action have produced positive results, albeit more is needed to solve the problem. At the UN COPUOS, we succeeded in including a specific Item called “Dark and Quiet Sky, astronomy and large constellations: discussion on emerging issues and challenges” in the agenda of the Scientific and Technical Subcommittee (STSC) for the next five years. That would ensure the possibility of discussing the progress and effectiveness of the mitigating measures and adopting internationally agreed-upon best practices. The interaction with the space industry and the operating companies has also been constructive. The second generation of Starlink satellites is built and operated in a way that makes them appear less bright when reflecting sunlight. Progress has also been achieved in determining the satellites’ real-time position with the necessary accuracy to avoid the crossing of their trace on the astronomical images. As for the satellites’ interference on the radio astronomical observations, the most promising mitigation can be achieved by deviating their microwave emission beam when hovering over major radio astronomical facilities.
The issue created by the advent of the large constellations is having some positive outcomes that partially compensate for their impact on astronomy. Never, in recent times, has astronomy enjoyed such high international visibility. At the last session of the COPUOS Scientific and Technical Subcommittee, in February 2024, a significant fraction of the time was dominated by discussing the impact of the constellations on astronomy and the Delegations of about 50 Countries (on a total of 102) declared that astronomy is a fundamental science and its progress must be protected.
Astronomers should take stock of this fortunate favorable situation and use it to protect their research and foster it internationally, extending and improving the presence of astronomical teaching and training in schools and universities. To effectively achieve these goals, as the recent experience at the UN level has taught us, we need to convince the governmental policymakers that astronomy is (as it has always been) a unique science. As mentioned in the introduction, it explores phenomena that cannot be reproduced in terrestrial laboratories and that are fundamental to progress in our understanding of physical reality.
However, it is much more than that. The natural, mythical relationship with the cosmos that inspired so many philosophers and poets in the past is now supported by the unexpected scientific discovery that the main characteristic of the universe is its holistic evolution, out of which our consciousness finally emerged after a 13.8-billion-year journey. We are not just living under the same sky: we are the sky! Our very existence is linked with a golden braid to every phase of cosmic history, and now that we have reconstructed it, we are its witnesses. We bear the responsibility of being its conscience.
The power and reach of this cosmic awareness are hard to grasp. It may produce a universal sentiment of fraternity and peace that philosophers and theologians, following the seminal thoughts of George Lemaître and Pierre Teilhard de Chardin, should use to offer a glimmer of hope for humanity’s future.
[1] https://cps.iau.org/
[2] https://noirlab.edu/public/media/archives/techdocs/pdf/techdoc021.pdf
[3] https://cps.iau.org/