▁ ▁Dis adv antage ▁L un ar - or bit ▁rende z vous ▁was ▁considered ▁ris ky ▁as ▁of ▁ 1 9 6 2 , ▁because ▁space ▁rende z vous ▁had ▁not ▁been ▁achieved , ▁even ▁in ▁Earth ▁orbit . ▁If ▁the ▁LE M ▁could ▁not ▁reach ▁the ▁C SM , ▁two ▁astr onaut s ▁would ▁be ▁str and ed ▁with ▁no ▁way ▁to ▁get ▁back ▁to ▁Earth ▁or ▁surv ive ▁re - entry ▁into ▁the ▁atmosphere . ▁The ▁fear ▁proved ▁to ▁be ▁unf ounded , ▁as ▁rende z vous ▁was ▁successfully ▁demonstrated ▁in ▁ 1 9 6 5 ▁and ▁ 1 9 6 6 ▁on ▁six ▁Project ▁Gem ini ▁miss ions ▁with ▁the ▁aid ▁of ▁rad ar ▁and ▁on - board ▁computers . ▁It ▁was ▁also ▁successfully ▁done ▁each ▁of ▁the ▁eight ▁times ▁it ▁was ▁tried ▁on ▁Apol lo ▁miss ions . ▁ ▁Apol lo ▁Mission ▁mode ▁selection ▁▁ ▁When ▁the ▁Apol lo ▁Moon ▁landing ▁program ▁was ▁started ▁in ▁ 1 9 6 1 , ▁it ▁was ▁assumed ▁that ▁the ▁three - man ▁command ▁and ▁service ▁module ▁combination ▁( CS M ) ▁would ▁be ▁used ▁for ▁take off ▁from ▁the ▁lun ar ▁surface , ▁and ▁return ▁to ▁Earth . ▁It ▁would ▁therefore ▁have ▁to ▁be ▁land ed ▁on ▁the ▁Moon ▁by ▁a ▁larger ▁ro cket ▁stage ▁with ▁landing ▁g ear ▁legs , ▁resulting ▁in ▁a ▁very ▁large ▁space craft ▁( in ▁excess ▁of ▁) ▁to ▁be ▁sent ▁to ▁the ▁Moon . ▁ ▁If ▁this ▁were ▁done ▁by ▁direct ▁as cent ▁( on