The last of the Mariner series, Mariner 10 launches
with a two-planet objective: Mercury and Venus.
For the first time, engineers experiment
with an ingenious way of traveling through the solar system using
the gravity of one planet to help propel the craft on to the next
destination -- somewhat like a series of bank shots in a game of
billiards. With Mercury as its ultimate target, Mariner 10 pioneers
the use of a "gravity assist" swing by the planet Venus
to bend its flight path toward Mercury.