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. The series includes a variant with Solid Rocket Boosters (TAT Agena-B or Thor-SLV2A Agena-B)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.
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