Kosmos 21 (Russian: Космос 21 meaning Cosmos 21) was a Soviet spacecraft with an unknown mission. 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 flyby, 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 SL-6/A-2-e launcher. The orbit decayed on November 14, three days after launch.
Cosmos 21 was launched at 06:23:35 UTC on 11 November 1963, atop a Molniya 8K78 carrier rocket flying from Site 1/5 at the Baikonur Cosmodrome. Its original development name before being given the Cosmos 21 denomination once it reached orbit was 3MV-1 No. 1.
Beginning in 1962, 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designation of this mission as an intended planetary probe is based on evidence from Soviet and non-Soviet sources and historical documents.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.