Thor DM-21 Agena-B or just Thor-Agena was a series of orbital launch vehicles.
The rockets used the Douglas-built Thor first stage and the Lockheed-built Agena second stages.
They are thus cousins of the more-famous Thor-Deltas, which founded the Delta rocket family.
The first attempted launch of a Thor DM-21 Agena-B was in January 1959.
The first successful launch was on 28 February 1959, launching Discoverer 17.