SPOILER! Kattints ide a szöveg elolvasásához!This study considers only the visible and IR regimes. A separate paper will deal with the millimetre and submillimetre domains. The radio domain is also to be considered separately. Keeping in mind the limitations of this study, one can already draw the following conclusions for when the 26 000 satellites from 18 representative constellations are launched and are in operation:
About 1600 satellites will be in range (over the horizon) of an observatory at mid-latitude. Among those about 250 will be above an elevation of 30° above the horizon (i.e. in the part of the sky where observations take place). At the end of the evening, that is, in astronomical twilight, or at the beginning of the morning, astronomical twilight (i.e. when the sky is dark for deep astronomical observations), the number of illuminated satellites will be around 1100 above the horizon, and 150 above 30° of elevation. Of these, about 260 satellites will be bright enough to be visible with the naked eye in exceptional conditions (6 mag or brighter); about 110 in good conditions (5 mag or brighter). Most of them will be near the horizon, with up to about 10 above 30° of elevation –contrary to claims published online that “satellites will outnumber the visible stars”. These numbers plummet as the Sun drops further below the horizon.
The trains of satellites, forming a bright “string of pearls”, brightly visible right after launch, are not an issue for telescopic observations: while they are spectacular, they are very short-lived and visible only briefly after sunset or before sunrise.
Specular flares, while potentially spectacular (iridium’s ones could reach −8 mag), are rare and short enough so that their effect on telescopic observations will be negligible even accounting very pessimistically for one reflecting surface per satellite. The occultation of an astronomical source by a passing satellite has a very low probability of occurrence, and the effect is below the precision of the measurement.
Short telescopic observations (with an exposure time of ∼1 s) with any technique will essentially be unaffected by the satellite trails. Similarly, observations in the thermal IR regimes will be unaffected by the thermal emission of the satellites.
Medium-duration exposures (100 s) with traditional fields of view are affected at a very low level during the astronomical night. Up to 0.5% of imaging observations would be ruined during the twilights.
Long exposures (1000 s) with long-slit spectrographs: 0.3−0.4% of the exposures would be ruined during the beginning and end of night, and up to 3% of the exposures taken during twilight would be rendered useless. Short-slit and fibre-fed instruments are less affected.
Wide-field imaging and spectroscopic surveys: 1−5% of the exposures would be ruined during the beginning and end of night, and at a higher level during twilight.
Very wide-field imaging observations on large telescopes (such as those of the Vera C. Rubin Observatory), for which saturation and ghosting caused by a satellite will ruin the full exposure, would be severely affected: about 30% of the exposures could be ruined at the beginning and end of the night. The situation is even worse during twilight (about 50% of ruined images during astronomical twilight). Rubin observatory published a dedicated report based on an independent study (with different assumptions) indicating “a 40% impact on twilight observing time” (Rubin Observatory Project Science Team 2020). Only nights in the middle of winter would be completely unaffected.