The effect of satellite constellations on observations in the visible and IR domains is estimated, considering 18 constellations in development by SpaceX, Amazon, OneWeb, and others, with over 26,000 satellites, constituting a representative distribution. This study uses a series of simplifications and assumptions to obtain conservative, order-of-magnitude estimates of the effects. The number of illuminated satellites from the constellations above the horizon ranges from ~1600 right after sunset, decreasing to 1100 at the end of astronomical twilight, most of them (~85%) close to the horizon (< 30deg). The large majority of these satellites will be too faint to be seen with the naked eye: at astronomical twilight, 110 brighter than mag 5. Most of them (~95%) will be close to the horizon. The number of naked-eye satellites plummets as the Sun reaches 30-40 deg below the horizon, depending on the latitude and season. The light trails caused by satellites would ruin a small fraction (below the 1% level) of exposures using narrow to normal field imaging or spectroscopic techniques in the visible and near IR during the first and last hours of the night. Similarly, the thermal emission of the satellite would affect only a negligible fraction of thermal IR observations. However, wide-field exposures, as well as long medium-field exposures,would be affected at the 3% level during the first and last hours of the night. Furthermore, ultra-wide imaging exposures on a very large telescope (eg NSFs Rubin Observatory, LSST), would be significantly affected, with 30 to 40% of such exposures being compromised during the first and last hours of the night. Coordination between the astronomical community, satellites companies, and government agencies is therefore critical to minimise and mitigate the effect on astronomical observations, in particular on survey telescopes.
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