A radiation-soaked planet belonging to the Sheliak under the Treaty of Armens, where Data must persuade over fifteen thousand human colonists to abandon their home in The Ensigns of Command.
Tau Cygni V is a desert world bathed in hyperonic radiation, thought to be deadly to humans. The Treaty of Armens assigned it to the Sheliak. Unknown to the Federation, the colony ship Artemis went off course and landed there some ninety years before 2366. The survivors adapted, and their descendants number 15,253.
In The Ensigns of Command the Sheliak demand the humans be removed within days. Because the radiation rules out transporters, Data, who is immune, goes down alone by shuttle. The colony's leader Gosheven refuses to give up what his ancestors built.
Helped by Ard'rian McKenzie, Data finally destroys the prized aqueduct with a modified phaser to show how helpless the colonists would be. Picard meanwhile wins three weeks for the evacuation.