After decades of flirting with the moon, scientists hope to give it a great big smack this week. On Saturday, they intend to slam the Lunar Prospector craft into the lunar south pole with an force equal to that of a two-ton car hurtling in at more than 1,000 miles an hour. Their purpose is to kick up enough water vapor to answer conclusively the question of whether there is frozen water on the moon. If it is present in great enough quntities, as strongly suggested by recent evidence, lunar water could be of immeasurable scientific and possibly economic value.