A Thor DM-21 Agena-B rocket was used, flying from launch pad 75-3-5 at the Vandenberg Air Force Base.

[2] Upon successfully reaching orbit, it was assigned the Harvard designation 1961 Zeta 1.

Discoverer 21 was operated in a low Earth orbit, with a perigee of 243 kilometres (151 mi), an apogee of 1,026 kilometres (638 mi), 80.7 degrees of inclination, and a period of 97.4 minutes.

[3] The satellite had a mass of 1,110 kilograms (2,450 lb),[4] and was used to demonstrate an engine restart,[5] and to test infrared sensors for the Midas programme.

[4] It remained in orbit until 20 April 1962,[3] when it decayed and reentered the atmosphere.