Kosmos 21 was launched at 06:23:34 GMT on 11 November 1963, atop a Molniya 8K78 s/n G103-18 carrier rocket flying from Site 1/5 at the Baikonur Cosmodrome.

The designation of this mission as an intended planetary probe is based on evidence from Soviet and non-Soviet sources and historical documents.

Typically Soviet planetary missions were initially put into an Earth parking orbit as a launch platform with a rocket engine and attached probe.

[1] These "Object-Probes" (ob'yekt-zond) were designed to verify key technological systems during simpler missions into deep space and back to Earth.

With this mission, the Soviets began the practice of giving Kosmos designations to obscure the failure of lunar and planetary probes that remained stranded in Earth orbit.