<p class="p1">Satellite constellations like SpaceX's Starlink network could disproportionately affect science programs that require twilight observations, such as searches for Earth-threatening asteroids and comets, astronomers have warned in a new report.</p>
<p class="p1">Large constellations of bright satellites in low-Earth orbit will fundamentally change ground-based optical and infrared astronomy and could impact the appearance of the night sky for stargazers worldwide, said the Satellite Constellations 1 (Satcon1) report.</p>
<p class="p1">The report is the outcome of the recent Satcon1 virtual workshop, which brought together more than 250 scientists, engineers, satellite operators, and other stakeholders.</p>
<p class="p1">Their goals were to better quantify the scientific impacts of huge ensembles of low-Earth-orbiting satellites (LEOsats) contaminating astronomical observations and to explore possible ways to minimize those impacts.</p>
<p class="p1">In May 2019, Elon Musk-owned SpaceX launched its first batch of 60 Starlink communication satellites, which surprised astronomers and laypeople with their appearance in the night sky.</p>
<p class="p1">Astronomers have only now, a little over a year later, accumulated enough observations of constellation satellites like those being launched by SpaceX and OneWeb and run computer simulations of their likely impact to thoroughly understand the magnitude and complexity of the problem.</p>
<p class="p1">Constellations of LEOsats are designed in part to provide communication services to underserved and remote areas, a goal everyone can support.</p>
<p class="p1">Recognizing this, astronomers have engaged satellite operators in cooperative discussions about how to achieve that goal without unduly harming ground-based astronomical observations.</p>
<p class="p1">The report offers two main findings.</p>
<p class="p1">The first is that LEOsats disproportionately affect science programs that require twilight observations, such as searches for Earth-threatening asteroids and comets, outer solar system objects, and visible-light counterparts of fleeting gravitational-wave sources.</p>
<p class="p1">During twilight the Sun is below the horizon for observers on the ground, but not for satellites hundreds of kilometers overhead, which are still illuminated.</p>
<p class="p1">As long as satellites remain below 600 kilometers, their interference with astronomical observations is somewhat limited during the night's darkest hours.</p>
<p class="p1">But satellites at higher altitudes, such as the constellation planned by OneWeb that will orbit at 1,200 kilometers, maybe visible all night long during summer and for much of the night in other seasons.</p>
<p class="p1">These constellations could have serious negative consequences for many research programmes at the world's premier optical observatories, said the report.</p>
<p class="p1">Depending on their altitude and brightness, constellation satellites could also spoil starry nights for amateur astronomers, astrophotographers, and other nature enthusiasts.</p>
<p class="p1">The report's second finding is that there are at least six ways to mitigate harm to astronomy from large satellite constellations.</p>
<p class="p1">However impractical or unlikely it may sound, the astronomers found that launching fewer or no LEOsats is the only option that can achieve zero astronomical impact.</p>
<p class="p1">Among other measures, the astronomers recommended deploying satellites at orbital altitudes no higher than approximately 600 km.</p>
<p class="p1">Darkening satellites or using sunshades to shadow their reflective surfaces can also help.</p>
<p class="p1">Other recommendations suggest that controlling each satellite's orientation in space to reflect less sunlight to Earth and minimizing or eventually eliminating the effect of satellite trails during the processing of astronomical images could be useful.</p>
<p class="p1">"Make more accurate orbital information available for satellites so that observers can avoid pointing telescopes at them," said the report.</p>.