One of the most eye-catching things about all manner of spacecraft (such as communication satellites and scientific instrumentation packages) that get launched into orbit by the Shuttle or a rocket is the bright, shiny foil wrapping around everything. This material is actually a custom-made insulation product made of very lightweight but highly reflective foil (such as MylarŽ, KaptonŽ, TeflonŽ etc.) layered with a polyester web-type spacer (often called bridal veil). This type of thermal blanket insulation is known as Multi-Layer Insulation or MLI.

This wrapping acts as a passive thermal control system. In the extreme environment of space in earth's orbit, surface temperature for these thermal blankets can easily fluctuate between minus150F and plus250F, depending on direct, indirect and shadowed exposure to the Sun. The purpose of the thermal control materials on a spacecraft is to help keep the electronics and mechanical moving parts within the design limits of the temperature range that will allow them to function normally.

The visible layer of the MLI faces outward to space. This outer layer of the blanket is a surface mirror material with high reflectivity. It is generally a 0.001-inch Kapton film backed with a few Angstroms thick layer of silver, which results in the gold color. The Kapton can be structurally reinforced by coarse fiberglass netting, called scrim, to prevent tearing. The multiple layers in the blanket are usually 0.00025-inch Kapton with a few Angstrom thick layer of silver on each side.

The layers are separated by a dacron netting ("bridal veil") which prevents contact of the adjacent silver layers. The inside blanket layer makes contact with the spacecraft body. The blanket layers conduct heat only laterally in their respective silver layers as long as there is no contact between layers, but they do exchange heat between adjacent layers by radiation. They will radiate some spacecraft heat to space but their insulating properties increase with the number of layers used in construction of that particular MLI blanket. The more layers in the blanket, the higher the insulation factor for keeping the heat in and the cold out. It is not unusual for MLI to have up to 10 to 14 layers. The MLI blanket shapes are first made using paper patterns that snuggly fit all the various contours of the specific spacecraft. The pattern is transferred to the foil and netting configuration with the specified number of layers, which can differ with each spacecraft (and often depends on budget considerations as much as the type of instrumentation it is protecting). The pattern is then cut and sewn on a sewing machine near a very large table work area. The edges are often finished with a tape prior to sewing. There can be a number of individual blankets in many irregular shapes on any one spacecraft. The blankets are held together and attached to the spacecraft using nothing more than dacron Velcro, which is a product that was actually first designed for use in space.

This piece of MLI was constructed to fit a scientific instrumentation spacecraft. The two outside sheets of Kapton are heavy guage, gold on one side and silver on the other. Sandwiched between these two outside layers are 12 layers of bridal veil and 11 sheets of thin mylar that is silver on both sides. The MLI was cut to fit the various shapes of contours and protrusions of the specific instrumentation. The edges were sewn or taped, and sometimes both. In certain places, there are velcro strips to hold the MLI snuggly together. There is no scrim (reinforcement by fiberglass netting) or velcro on the Kapton of this specific sample.