The balloons that are the subject of this book are balloons that transport payloads ranging from several hundred kilograms up to several tons into the earth’s stratosphere. Specifically, they are stratospheric balloons that are used for scientific observations and for the development of space technology and balloons that are used for aerological observations. These balloons attain flight altitudes that are more than three times those of passenger planes. The density of the atmosphere at these altitudes is less than 1% that at the earth’s surface. In addition, as part of planetary exploration, this book includes planetary balloons sent to float over other planets that have atmospheres, such as Mars and Venus. The general term used to describe these sorts of activities is scientific ballooning.