Hydrogen

Liquid hydrogen, combined with liquid oxygen, is the fuel that sends space shuttles into orbit. The reaction between hydrogen and oxygen to form water gives off a large amount of energy. They are useful as a rocket fuel because in their liquid forms, large quantities of them can be stored in a small space. They are very dangerous to handle, however, because unless they are kept well below their boiling points (hundreds of degrees below zero), they will boil and change into gases. Under certain conditions, hydrogen gas in the air can explode, while oxygen gas can feed the slightest spark into a fiery inferno if there is anything combustible around. Mixed together, they make a highly explosive mixture. These sobering facts turned into disaster on January 28, 1986, when the Challenger space shuttle exploded shortly after liftoff, killing all seven astronauts aboard. A rubber seal had failed, spilling the explosive gases out into the jet of flame that resulted in the explosion of the center fuel tank.

An earlier flying tragedy caused by hydrogen was the explosion on May 6, 1937 of the German zeppelin (a dirigible, or blimp), Hindenburg. At that time, hydrogen was used as the lighter-than-air filling in dirigibles. The Hindenburg caught fire while mooring at Lakehurst, New Jersey after a transatlantic flight, and 36 people were killed. Ever since then, nonflammable helium gas has been used instead of hydrogen as the filling in dirigibles. It is not as buoyant, but it is completely safe.