▁traject ory . ▁This ▁costs ▁additional ▁prop ell ant ▁because ▁the ▁par king ▁orbit ▁per ige e ▁must ▁be ▁high ▁enough ▁to ▁prevent ▁re entry ▁while ▁direct ▁injection ▁can ▁have ▁an ▁arbitr arily ▁low ▁per ige e ▁because ▁it ▁will ▁never ▁be ▁reached . ▁ ▁However , ▁the ▁par king ▁orbit ▁approach ▁greatly ▁simplified ▁Apol lo ▁mission ▁planning ▁in ▁several ▁important ▁ways . ▁It ▁substantial ly ▁w iden ed ▁the ▁allow able ▁launch ▁windows , ▁increasing ▁the ▁chance ▁of ▁a ▁successful ▁launch ▁despite ▁minor ▁technical ▁problems ▁during ▁the ▁count down . ▁The ▁par king ▁orbit ▁was ▁a ▁stable ▁" mission ▁plate au " ▁that ▁gave ▁the ▁crew ▁and ▁controllers ▁several ▁hours ▁to ▁thoroughly ▁check ▁out ▁the ▁space craft ▁after ▁the ▁str esses ▁of ▁launch ▁before ▁comm itting ▁it ▁to ▁a ▁long ▁lun ar ▁flight ; ▁the ▁crew ▁could ▁quickly ▁return ▁to ▁Earth , ▁if ▁necessary , ▁or ▁an ▁alternate ▁Earth - orb ital ▁mission ▁could ▁be ▁conducted . ▁The ▁par king ▁orbit ▁also ▁enabled ▁transl un ar ▁traject ories ▁that ▁avoided ▁the ▁dens est ▁parts ▁of ▁the ▁Van ▁Allen ▁radiation ▁bel ts . ▁ ▁Apol lo ▁miss ions ▁minim ized ▁the ▁performance ▁penalty ▁of ▁the ▁par king ▁orbit ▁by ▁keeping ▁its ▁alt itude ▁as ▁low ▁as ▁possible . ▁For ▁example , ▁Apol lo ▁ 1 5 ▁used ▁an ▁unus ually ▁low ▁par king ▁orbit ▁( even ▁for ▁Apol lo ) ▁of ▁ 9 2 . 5 ▁n mi ▁by ▁ 9 1 . 5 ▁n mi ▁( 1 7 1   km ▁by