Kosmos 21 ( meaning Cosmos 21) was a Soviet spacecraft. This mission has been tentatively identified by NASA as a technology test of the Venera series space probes. It may have been an attempted Venus impact, presumably similar to the later Kosmos 27 mission, or it may have been intended from the beginning to remain in geocentric orbit. In any case, the spacecraft never left Earth orbit after insertion by the Molniya launcher. The orbit decayed on 14 November 1963, three days after launch.
Launch
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.
Spacecraft designation
Beginning in 1963, the name Kosmos was given to Soviet spacecraft which remained in Earth orbit, regardless of whether that was their intended final destination. 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designation of this mission as an intended planetary probe is based on evidence from Soviet and non-Soviet sources and historical documents.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. The probes were then launched toward their targets with an engine burn with a duration of roughly 4 minutes. If the engine misfired or the burn was not completed, the probes would be left in Earth orbit and given a Kosmos designation.
The spacecraft's original development name before being given the Kosmos 21 denomination once it reached orbit was 3MV-1 No.1.
This was the first "third-generation" deep space planetary probes of the 3MV series of the Soviet Union. Like the second generation, Soviet engineers planned four types of the 3MV, the 3MV-1 (for Venus impact), 3MV-2 (for Venus flyby), 3MV-3 (for Mars impact), and 3MV-4 (for Mars flyby). The primary difference over the second-generation was vastly improved (and in many cases doubled) orientation system elements as well as improved onboard propulsion systems. While these four versions were meant to study Mars and Venus, the Soviets conceived of two additional variants of the series, similar but not
identical to the 3MV-1 and 3MV-4 versions, with the designations 3MV-1A and 3MV-4A.