Thor DM-18 Agena or just Thor-Agena was a series of orbital launch vehicles.
The launch vehicles used the Douglas-built Thor first stage and the Lockheed-built Agena second stages.
They are thus cousins of the more-famous Thor-Delta, which founded the Delta launch vehicle family.
The first attempted launch of a Thor-Agena was on 28 February 1959.
It was the first two-stage rocket to place a satellite into orbit.