The layer of air around the earth protects us from many of the dangers which lurk in space. We depend upon the sun's rays for the light and warmth which keep us alive, but there are some kinds of rays from the sun which would burn us to death if we were not protected from them.

The sun gives out a great deal of ultra-violet light, for example. These rays travel in very short waves and our eyes cannot see them. Fortunately the air stops most of them from reaching us and a very thin layer of material, such as metal, or even some kinds of glass, could give complete protection.

From far out in space come other kinds of radiation known as cosmic rays. Some of these do get through the air to reach us, and out in space there must be far more of them. As yet we do not know very much about these rays or how great a danger they really are.

Every day our earth is bombarded by a rain of dust and pieces of stone and metal known as meteors. Most of these are burned up as they rush into the atmosphere. We see them flash across the night sky in the form of shooting stars although, of course, this name has nothing to do with the real stars. In space, however, where there is no atmosphere to shield the traveller, there must be a much greater chance of being hit by meteors and some of them may be quite large pieces of rock. At the high speeds of forty to sixty miles per second at which they move, they would certainly be able to do serious damage to a space-craft.