Space travel is a form of travel in which passengers enter space, the vast region outside the protective envelope of the Earth's atmosphere. Space starts 62 miles (100 kilometers) above the Earth's crust, at a point called the Karman Line, and it continues for a distance that appears to be infinite, interrupted periodically by celestial bodies such as stars, planets, and so forth. Humans have long had romantic associations with traveling in space, with people considering space “the final frontier,” as they famously say on Star Trek.