The earth is made up of three layers, the lithosphere (land), hydrosphere (water) and the atmosphere (air). The atmosphere, a mixture of gases surrounding the earth and held by the earth's gravity, is where flight takes place.

When earth was formed, it had no atmosphere. As volcanoes grew and erupted, gases began to form an atmosphere. This early atmosphere did not contain oxygen. Plants appeared and developed a way to use the sun's energy to produce oxygen in the air. This process is called photosynthesis. The first oxygen-breathing animals appeared about 500 million years ago.

Explore Space ... Not Drugs!
Hear what astronauts have to say about staying drug-free.
Last modified: Thu May 7 13:44:35 PDT 1998

Copyright © 1997 by Cislunar Aerospace, Inc. All Rights Reserved.