▁exp el ▁prop ell ant ▁to ▁provide ▁forward ▁thrust ▁that ▁generates ▁enough ▁delta - v ▁( change ▁in ▁velocity ) ▁to ▁reach ▁orbit . ▁ ▁For ▁cre wed ▁launch ▁systems ▁launch ▁escape ▁systems ▁are ▁frequently ▁fitted ▁to ▁allow ▁astr onaut s ▁to ▁escape ▁in ▁the ▁case ▁of ▁emer gency . ▁ ▁Altern atives ▁▁ ▁Many ▁ways ▁to ▁reach ▁space ▁other ▁than ▁rock ets ▁have ▁been ▁proposed . ▁Ide as ▁such ▁as ▁the ▁space ▁elev ator , ▁and ▁momentum ▁exchange ▁t eth ers ▁like ▁ro to v ators ▁or ▁sky ho oks ▁require ▁new ▁materials ▁much ▁stronger ▁than ▁any ▁currently ▁known . ▁Elect romagnet ic ▁launch ers ▁such ▁as ▁launch ▁loops ▁might ▁be ▁feas ible ▁with ▁current ▁technology . ▁Other ▁ideas ▁include ▁ro cket ▁assist ed ▁aircraft / space plan es ▁such ▁as ▁Re action ▁Eng ines ▁Sk yl on ▁( current ly ▁in ▁early ▁stage ▁development ), ▁sc ram jet ▁power ed ▁space plan es , ▁and ▁R B CC ▁power ed ▁space plan es . ▁Gun ▁launch ▁has ▁been ▁proposed ▁for ▁cargo . ▁ ▁Le aving ▁orbit ▁▁ ▁Ach ieving ▁a ▁closed ▁orbit ▁is ▁not ▁essential ▁to ▁lun ar ▁and ▁inter plan et ary ▁voy ages . ▁Early ▁Russian ▁space ▁vehicles ▁successfully ▁achieved ▁very ▁high ▁alt itudes ▁without ▁going ▁into ▁orbit . ▁NASA ▁considered ▁launch ing ▁Apol lo ▁miss ions ▁directly ▁into ▁lun ar ▁traject ories ▁but ▁adopted ▁the ▁strategy ▁of ▁first ▁entering ▁a ▁temporary ▁par king ▁orbit ▁and ▁then ▁performing ▁a ▁separate ▁burn ▁several ▁or bits ▁later ▁onto ▁a ▁lun ar