Explanation: Closeup of Multi-layer insulation (MLI) from a spacecraft. The metal coated plastic layers and the scrim separator are visible. The layers of MLI can be arbitrarily close to each other, as long as they are not in thermal contact. To reduce weight and blanket thickness, the internal layers are made very thin, but they must be opaque to thermal radiation. Since they don't need much structural strength, these internal layers are usually made of very thin plastic, about 6 micrometres (1/4 mil) thick, such as Mylar or Kapton, coated on one side with a thin layer of metal on both sides, typically silver or aluminium.


Image credit: Wikipedia.