In a medium saucepan, mix the sugar, and water and heat gently until the sugar dissolves. Bring to the boil and simmer for 4-5 minutes, then remove from heat and let cool.
Scrape out the seeds, then scoop out the flesh from the cantaloupe. Puree in a food processor/blender with the cooled syrup and lime juice.
Stir in the mint and pour the melon mixture into an ice-cream maker. Churn, until the sherbet is smooth and firm. Alternatively, pour the mixture into a suitable container and freeze until icy around the edges. Transfer to a food processor/blender and process til smooth.
Repeat the freezing and processing two or three times until the mixture is smooth and holding its shape. Then, freeze until firm.
Scrape out the seeds, then scoop out the flesh from the cantaloupe. Puree the melon in a food processor/blender. Pour the puree into a bowl and stir in the orange and lemon juice.
Place the soup in refrigerator for 30-40 minutes, but do not chill it for too long, as this will dull its flavor.
Ladle soup into bowls and add a large scoop of melon and mint sherbet to each.