Optical surface reflector, for a spacecraft such as a geostationary satellite.The exterior surface (10) of the reflector comprises a plurality of facets (14) inclined to each other and to the interior surface (12) of the reflector. The facets (14) advantageously form a pyramid with a square base in which the angle at the apex is equal to 90.degree.. However, other arrangements are possible without departing from the scope of the invention. The invention increases radiative capacity without significantly increasing mass and overall size.