This book, one in the series on Aerospace Education I, provides a description of some of the discoveries that spacecraft have made possible and of the experience that American astronauts have had in piloting spacecraft. The basic principles behind the operation of spacecraft and their boosters are explained. Descriptions are also included on unmanned and manned spacecraft. Brief mention is made of space stations, reusable space vehicles, and spacecraft fitted with specialized equipment for planetary exploration. The book is designed for use in the Air Force Junior ROTC program. (PS)