li per / Par om ▁tan dem . ▁ ▁Ast rod ynam ics ▁▁ ▁Ast rod ynam ics ▁is ▁the ▁study ▁of ▁space craft ▁traject ories , ▁particularly ▁as ▁they ▁relate ▁to ▁gravit ational ▁and ▁prop ulsion ▁effects . ▁Ast rod ynam ics ▁allows ▁for ▁a ▁space craft ▁to ▁arrive ▁at ▁its ▁destination ▁at ▁the ▁correct ▁time ▁without ▁excess ive ▁prop ell ant ▁use . ▁An ▁orb ital ▁man eu ver ing ▁system ▁may ▁be ▁needed ▁to ▁maintain ▁or ▁change ▁or bits . ▁ ▁Non - ro cket ▁orb ital ▁prop ulsion ▁methods ▁include ▁solar ▁s ails , ▁magnetic ▁s ails , ▁pl asma - b ubble ▁magnetic ▁systems , ▁and ▁using ▁gravit ational ▁s ling shot ▁effects . ▁ ▁Transfer ▁energy ▁ ▁The ▁term ▁" trans fer ▁energy " ▁means ▁the ▁total ▁amount ▁of ▁energy ▁im part ed ▁by ▁a ▁ro cket ▁stage ▁to ▁its ▁payload . ▁ ▁This ▁can ▁be ▁the ▁energy ▁im part ed ▁by ▁a ▁first ▁stage ▁of ▁a ▁launch ▁vehicle ▁to ▁an ▁upper ▁stage ▁plus ▁payload , ▁or ▁by ▁an ▁upper ▁stage ▁or ▁space craft ▁kick ▁motor ▁to ▁a ▁space craft . ▁ ▁Re entry ▁▁ ▁Ve h icles ▁in ▁orbit ▁have ▁large ▁amounts ▁of ▁kin etic ▁energy . ▁This ▁energy ▁must ▁be ▁disc arded ▁if ▁the ▁vehicle ▁is ▁to ▁land ▁safely ▁without ▁v apor izing ▁in ▁the ▁atmosphere . ▁Typ ically ▁this ▁process ▁requires ▁special ▁methods ▁to ▁protect ▁against ▁aer od ynamic ▁he ating . ▁The ▁theory ▁behind ▁re entry ▁was ▁developed ▁by ▁Harry ▁Julian ▁Allen .