Many/most early artificial satellites were spheres with mirror-like polished outer surfaces.
I am guessing that for lower orbits the spherical shape minimized drag as well as simplified the analysis and interpretation of rate of decay, since this was critical to understand in the early days.
But the metal could have been roughened, anodized, or even painted, rather than having a mirror-like finish. Was this done for reasons of visibility and optical tracking? Or was it done for reasons of thermal management, where the emissivity in the visible determined heat absorption and in the IR determined radiation? Or another reason, or for no particular reason at all beyond aesthetics?
I'm looking for an answer supported with a source that addresses the design intention at the time.