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The population of artificial satellites and space debris orbiting the Earthimposes non-negligible constraints on both space operations and ground-basedoptical and radio astronomy. The ongoing deployment of several satellite`mega-constellations' in the 2020s represents an additional threat that raisessignificant concerns. The expected severity of its unwanted consequences isstill under study, including radio interference and information loss bysatellite streaks appearing in science images. In this Letter, we report a newskyglow effect produced by space objects: increased night sky brightness causedby sunlight reflected and scattered by that large set of orbiting bodies whosedirect radiance is a diffuse component when observed with the naked eye or withlow angular resolution photometric instruments. According to our preliminaryestimates, the zenith luminance of this additional light pollution source mayhave already reached $\sim$20 $\mu$cd m$^{-2}$, which amounts to anapproximately 10 percent increase over the brightness of the night skydetermined by natural sources of light. This is the critical limit adopted in1979 by the International Astronomical Union for the light pollution level notto be exceeded at the sites of astronomical observatories.

The proliferation of space objects is a rapidly increasing source of artificial night sky brightness