Space blanket

Their design reduces the heat loss in a person's body, which would otherwise occur quickly due to thermal radiation, water evaporation, or convection.

Lost campers and hikers have an additional possible benefit: the shiny surface flashes in the sun, allowing its use as an improvised distress beacon for searchers and as a method of signalling over long distances to other people.

[1] First developed by NASA's Marshall Space Flight Center in 1964 for the US space program,[2][3][4] the material comprises a thin sheet of plastic (often PET film) that is coated with a metallic, reflecting agent, making it metallized polyethylene terephthalate (MPET) that is usually gold or silver in color, which reflects up to 97% of radiated heat.

Space blankets are made by vacuum-depositing a very precise amount of pure aluminum vapor onto a very thin, durable film substrate.

That, along with their low weight and ability to pack into a small space, has made them popular among outdoor enthusiasts and emergency workers.