Gravity
(Book)

Includes bibliographical references, glossary, places to visit, web sites and index.

You have probably seen movies or television shows about space travel. When space travelers go outside their ships, they use jet packs to tie themselves to their ships. They do this because outside their spaceships there is no force that keeps them attached to the craft. Without something to keep them attached, they would just drift off into space because there is no gravity. Gravity is also the reason why our bodies don't just fly upward into space without a space vehicle. Gravity keeps us grounded.