Add the chunks of cantaloupe to a blender and pulse until the fruit is completely pureed. Pour the cantaloupe puree through a fine mesh sieve, pressing on the solids. It should yield about 2 cups of fresh juice - enough for 4 cocktails. Discard solids.
In a cocktail shaker, muddle the rosemary until it's bruised and fragrant. Add the cantaloupe juice, triple sec, simple syrup and vodka. Add ice, secure the top of the shaker and shake vigorously. Pour into a glass (removing the bruised rosemary leaves if desired). Garnish with a fresh sprig of rosemary and cantaloupe. Serve.