Although the Space Shuttle can travel faster and farther than any other aircraft yet flying, unless it is on a space voyage, it must depend on other means of transportation just to get from one side of the country to the other. Since it must use such large external boosters to place it into the air, it is not feasible to launch the shuttle for anything less than orbital flight.
To provide for "local" hops, NASA obtained a giant Boeing 747 airliner from American Airlines, and altered it to carry the shuttle in a special cradle mounted to the fuselage. In addition,a pair of vertical stabilizers was installed to increase the stability of the 747 during the captive flights, and to provide the necessary vertical surface for safe flight in the event the shuttle collided with the 747's large tail fin.
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