Why were early satellites produced with polished "mirror-like" surfaces?

Question

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.

Below Sputnik-1, Luna-1, and Vanguard-1.

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Exploring the "aesthetics" angle, rendering of Vanguard-1 versus disco mirror ball:

Answer 1

Per the official NASA history of Vanguard it was for visibility and thermal control.

(Bolding mine)

Tousey had made some of the first calculations in the fall of 1955, contributing his knowledge of optics to ensure that protective coatings on the exterior of the booster and on the satellite shell would have sufficient reflec­tivity to permit telescopic observation of the course of the rocket as it rose and then optical acquisition and tracking of the satellite in space. Schach undertook the "thermal design," that is, the calculations of what tempera­tures to expect at various points in the satellite's orbit, in darkness and in daylight, the selection of the optimum thickness of coating materials to emphasize their emissiveness of solar heat radiation, and methods of keeping the satellite's surface free of contaminating substances such as soot which would ultimately raise the satellite's temperature. Hass worked out the techniques of applying the successive surface coatings-the gold plating, the chromium evaporated to vapor and deposited to serve as a primer, the silicon oxide to serve as a barrier, the thin layer of evaporated aluminum to give a mirror-like finish, and finally a film of silicon oxide to control emitted radiation. Drummeter and Schach were chiefly responsible for developing the sunlight simulator with carbon arcs as the source of high­ intensity light. Through windows in the large cylindrical vacuum tank in which the coated sphere sat for two or three days of testing, the simulated sunlight beat upon the satellite's surface and indirectly heated the inner pot of instruments. Measurements of the effects furnished means of determining the most desirable material and thickness of the layering required. LaGow acted as advisor and monitor on all these operations. Every man concerned with temperature control worked closely with every other.