Emergency foil space blanket
Space or Emergency blanket is an especially low-weight, low-bulk blanket made of heat-reflective thin plastic sheeting.
Their design reduces the heat loss in your body if you get into trouble and get lost, trapped or injured which would otherwise occur due to thermal radiation, water evaporation, or convection. Their compact size before unfurling and light weight makes them ideal when space is at a premium. They are a mandatory item and must be included in your first aid kits.
An added bonus if you get lost the metallic surface appearance flashes in the sun, allowing use as an improvised distress beacon for searchers.