Free Astronomy Magazine May-June 2020

48 MAY-JUNE 2020 SPACE CHRONICLES where have ranked wide-field survey telescopes as a top priority for future developments in astronomy. Professional and amateur as- tronomers alike have also raised concerns about how satellite mega- constellations could impact the pris- tine views of the night sky. The study shows that about 1600 satellites from the constellations will be above the horizon of an observatory at mid-latitude, most of which will be low in the sky — within 30 de- grees of the horizon. Above this — the part of the sky where most as- tronomical observations take place — there will be about 250 constella- tion satellites at any given time. While they are all illuminated by the Sun at sunset and sunrise, more and more get into the shadow of the Earth toward the middle of the night. The ESO study assumes a brightness for all of these satellites. With this assumption, up to about 100 satellites could be bright enough to be visible with the naked eye during twilight hours, about 10 T his 2D diagram illustrates that an observer at mid-latitude would see only a fraction of the constellation satellites orbiting the Earth. To be visible, satel- lites need to be above the observer’s horizon and be illuminated by the Sun. Most satellites would be below the horizon and/or hidden by the Earth’s shadow which, for a given observer, covers increasingly more of the sky as night ad- vances. [ESO/L. Calçada] T his image shows the night sky above the construction site of ESO’s Extremely Large Telescope, which is planned to start operating in late 2025. A laser guide from the nearby Very Large Telescope is seen in the background. [ESO/M. Zamani] lites visible in the night sky to the naked eye above 30 degrees. These numbers do not include the trains of satellites visible immedi- ately after launch. Whilst spectacu- lar and bright, they are short lived and visible only briefly after sunset or before sunrise, and — at any given time — only from a very lim- ited area on Earth. The ESO study uses simplifications and assumptions to obtain conser- vative estimates of the effects, which may be smaller in reality than calculated in the paper. More so- phisticated modelling will be neces- sary to more precisely quantify the actual impacts. While the focus is on ESO telescopes, the results apply to similar non-ESO telescopes that also operate in the visible and infrared, with similar instrumentation and science cases. Satellite constellations will also have an impact on radio, millimetre and submillimetre observatories, includ- ing the Atacama Large Millimeter/ submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX). This impact will be consid- ered in further studies. of which would be higher than 30 degrees of elevation. All these num- bers plummet as the night gets darker and the satellites fall into the shadow of the Earth. Overall, these new satellite constellations would about double the number of satel- !