▁ 1 6 9   km ) ▁where ▁there ▁was ▁significant ▁atmos pher ic ▁drag . ▁But ▁it ▁was ▁partially ▁overcome ▁by ▁continuous ▁vent ing ▁of ▁hydro gen ▁from ▁the ▁third ▁stage ▁of ▁the ▁Sat urn ▁V , ▁and ▁was ▁in ▁any ▁event ▁toler able ▁for ▁the ▁short ▁stay . ▁ ▁Rob otic ▁miss ions ▁do ▁not ▁require ▁an ▁abort ▁cap ability ▁or ▁radiation ▁minim ization , ▁and ▁because ▁modern ▁launch ers ▁rout in ely ▁meet ▁" inst ant aneous " ▁launch ▁windows , ▁space ▁prob es ▁to ▁the ▁Moon ▁and ▁other ▁plan ets ▁generally ▁use ▁direct ▁injection ▁to ▁maxim ize ▁performance . ▁Although ▁some ▁might ▁coast ▁briefly ▁during ▁the ▁launch ▁sequence , ▁they ▁do ▁not ▁complete ▁one ▁or ▁more ▁full ▁par king ▁or bits ▁before ▁the ▁burn ▁that ▁inject s ▁them ▁onto ▁an ▁Earth ▁escape ▁traject ory . ▁ ▁Note ▁that ▁the ▁escape ▁velocity ▁from ▁a ▁cel est ial ▁body ▁decre ases ▁with ▁alt itude ▁above ▁that ▁body . ▁However , ▁it ▁is ▁more ▁fuel - efficient ▁for ▁a ▁craft ▁to ▁burn ▁its ▁fuel ▁as ▁close ▁to ▁the ▁ground ▁as ▁possible ; ▁see ▁Ober th ▁effect ▁and ▁reference . ▁This ▁is ▁another ▁way ▁to ▁explain ▁the ▁performance ▁penalty ▁associated ▁with ▁establish ing ▁the ▁safe ▁per ige e ▁of ▁a ▁par king ▁orbit . ▁ ▁Pl ans ▁for ▁future ▁cre wed ▁inter plan et ary ▁space fl ight ▁miss ions ▁often ▁include ▁final ▁vehicle ▁assembly ▁in ▁Earth ▁orbit , ▁such ▁as ▁NASA ' s ▁Or ion ▁space craft ▁and ▁Russia ' s ▁K