The Thor-Ablestar, or Thor Able-Star, also known as Thor-Epsilon was an early American expendable launch system consisting of a PGM-17 Thor missile, with an Ablestar upper stage. It was a member of the Thor family of rockets, and was derived from the Thor-Able. Nineteen Thor-Ablestars were launched between 1960-65, of which four failed, and a fifth resulted in a partial failure, as only one of two payloads separated from the upper stage. The launch of a Transit satellite on 22 February 1961 was successful, but its companion Lofti satellite failed to separate from the second stage.