Aluminum is a highly conductive material. Instead of insulating the system from heat, it conducts. Multi-Layer Insulation aims to protect spacecraft from extreme heat by acting as a thermal barrier. Gold plating is used to reflect solar radiation but what about aluminum? Isn't using aluminum coated on MLI decreases the thermal insulating property?
Gold is used to reflect solar radiation but what about aluminum?
Some multilayer insulation (MLI) definitely has a gold-like color, but no gold is used in MLI. The gold-like color color is a result of the combination of the plastic-like material used to separate rather thin sheets of aluminum foil. Gold would be an extremely lousy choice due to its high density and high cost. Despite the gold-like color of many satellites covered with MLI, there is no gold in that MLE>
Aluminum is a highly conductive material. Instead of insulating the system from heat, it conducts.
Conducts heat where? Not to space; there is essentially no conduction to space. The only means of transferring heat from a spacecraft to space is radiation, and the only means of transferring heat from sunlight to a spacecraft is also radiation.
The mathematics that underlie multilayer insulation say that it's best if the layers have low emissivity. Reflectivity, emissivity, and transmissivity are positive numbers between zero and one, and sum to one. This means that a good reflector is a lousy emitter, and vice versa. This in turn means that multilayer insulation that uses a highly reflective material will do an even better job of thermally insulating a spacecraft that would a multilayer insulation that uses a pitch black material.
Just to join the last comments together on David Hammen's already good answer, this was gong to be a comment but became too long.
It is worth clarifying for the uninitiated that MLI comprises layers of Kapton (thermal insulator, yellow colour) each with a vacuum deposited aluminium (VDA) layer on one or both sides of each layer, and then a spacer (something resmbling like old fashioned net curtains or a sparse tissue) to separate the Kapton layers.
There is no through path comprising alumimium and so the VDA layer can only conduct laterally through the blanket and even that will be insignificant because the VDA is microscopically thin.
The final outer layer kapton is usually either left natural, without VDA, which gives the "yellow mirror ~ looks like gold" appearance or it is "carbon loaded" meaning that it is black in colour. The choices between these seem to be down to the practices of different manufacturers, the natural finish has a lower sunlight heat load than black but is (or used to be at least) not so good at aging under UV and was less stable over 10 - 15 years.