astronaut:  A person who is trained to travel in space.  The word comes from the ancient Greek words for "star sailor."

atmosphere:  Above the surface of some planets and moons, a number of gases are held by their gravitational pull.  The gases form a layer called the atmosphere.  The atmosphere extends from the surface into outer space.  On Earth, the atmosphere is called air and is about 300 miles thick.  Mercury and Pluto have almost no atmosphere at all.  The gas giants have atmospheres thousands of miles thick.