Editing Kosmos 21 (section)

== Spacecraft designation ==
Beginning in 1963, the name [[Kosmos (satellite)|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designation of this mission as an intended planetary probe is based on evidence from Soviet and non-Soviet sources and historical documents.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.<ref name="nssdc.gsfc.nasa.gov">https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1963-044A - 27 February 2020</ref>

The spacecraft's original development name before being given the Kosmos 21 denomination once it reached orbit was '''3MV-1 No.1'''.<ref name="LL">{{cite web|url=http://planet4589.org/space/log/launchlog.txt |title=Launch Log|last=McDowell|first=Jonathan|website=Jonathan's Space Page|access-date=10 January 2011}}</ref>

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.<ref name=siddiqi/>

These "Object-Probes" (ob'yekt-zond) were designed to verify key technological systems during simpler missions into deep space and back to Earth. A government decree on 21 March 1963 had approved two to three such "object-probe" missions, one of which (a 3MV-1A) was designed to depart from Earth's ecliptic (the orbital plane of Earth around the Sun) out to 12–16 million kilometers from Earth and then return to Earth after about six months when its orbit intersected with that of Earth again, aided by two mid-course corrections using its S5.45 main engine. The latter, capable of two firings, was a lighter version of that used on the 2MV model with higher specific impulse and longer burn time.<ref name=siddiqi/>