▁carried ▁out ▁six ▁cre wed ▁space fl ights ▁between ▁ 1 9 6 1 ▁and ▁ 1 9 6 3 . ▁The ▁longest ▁flight ▁last ed ▁nearly ▁five ▁days , ▁and ▁the ▁last ▁four ▁were ▁launched ▁in ▁pairs , ▁one ▁day ▁apart . ▁This ▁exceed ed ▁Project ▁Mercur y ' s ▁demonstrated ▁capabilities ▁of ▁a ▁longest ▁flight ▁of ▁just ▁over ▁ 3 4 ▁hours , ▁and ▁of ▁single ▁miss ions . ▁ ▁V ost ok ▁was ▁succeeded ▁by ▁two ▁V os kh od ▁programme ▁fl ights ▁in ▁ 1 9 6 4 ▁and ▁ 1 9 6 5 , ▁which ▁used ▁three - ▁and ▁two - man ▁modifications ▁of ▁the ▁V ost ok ▁caps ule ▁and ▁a ▁larger ▁launch ▁ro cket . ▁ ▁Background ▁ ▁The ▁world ' s ▁first ▁artificial ▁satellite , ▁S put nik ▁ 1 , ▁had ▁been ▁put ▁into ▁orbit ▁by ▁the ▁S ovi ets ▁in ▁ 1 9 5 7 . ▁The ▁next ▁mil estone ▁in ▁the ▁history ▁of ▁space ▁expl oration ▁would ▁be ▁to ▁put ▁a ▁human ▁in ▁space , ▁and ▁both ▁the ▁S ovi ets ▁and ▁the ▁Americans ▁wanted ▁to ▁be ▁the ▁first . ▁ ▁Cos mon aut ▁selection ▁and ▁training ▁ ▁By ▁January ▁ 1 9 5 9 , ▁the ▁S ovi ets ▁had ▁begun ▁prepar ations ▁for ▁human ▁space fl ight . ▁Phys icians ▁from ▁the ▁Soviet ▁Air ▁Force ▁ins isted ▁that ▁the ▁potential ▁cos mon aut ▁candidates ▁be ▁qualified ▁Air ▁Force ▁pil ots , ▁arg uing ▁that ▁they ▁would ▁have ▁relevant ▁skills ▁such