Unlike gold, platinum is quite stiff. I suspect they're using it because they don't want to have to compensate for the effect of gravity on the shape when it's

Unlike gold, platinum is quite stiff. I suspect they're using it because they don't want to have to compensate for the effect of gravity on the shape when it's being machined. At a guess the properties they're going for in the alloy are: - Easy to shape - Very shiny - Won't oxidise in atmosphere prior to launch - Very susceptible to gravity (because dense) - Not susceptible to magnetism (because not ferromagnetic) - Strong enough not to sag under gravity (because Pt) At this precision I have no gut feel as to whether tidal losses might be important, but that might be a factor too. Platinum would help attenuate them, I suspect. Pure gold would be squidgy where AuPt will be (comparatively) springy.