. ▁Therefore , ▁the ▁oper ational ▁life ▁of ▁the ▁space craft ▁is ▁determined ▁by ▁the ▁amount ▁of ▁prop ell ant ▁carried , ▁and ▁the ▁rate ▁at ▁which ▁the ▁prop ell ant ▁is ▁used ▁up . ▁Prop ell ant ▁is ▁used ▁for ▁two ▁main ▁purposes : ▁to ▁maintain ▁the ▁space craft ▁in ▁orbit , ▁and ▁to ▁control ▁rotation . ▁Therefore , ▁the ▁less ▁prop ell ant ▁that ▁has ▁to ▁be ▁used ▁for ▁cont rolling ▁rotation , ▁the ▁more ▁that ▁is ▁available ▁for ▁maintain ing ▁orbit , ▁and ▁the ▁longer ▁the ▁lifetime ▁of ▁the ▁space craft . ▁ ▁However , ▁momentum ▁storage ▁devices ▁have ▁a ▁limited ▁capacity , ▁and ▁that ▁capacity ▁soon ▁becomes ▁s atur ated ▁when ▁they ▁are ▁required ▁to ▁abs orb ▁space craft ▁disturb ance ▁tor ques ▁caused ▁by ▁( gravity ▁gradient , ▁solar ▁wind , ▁and ▁aer od ynamic ▁drag ); ▁when ▁in ▁other ▁words ▁they ▁reach ▁their ▁momentum ▁storage ▁limit . ▁Once ▁s atur ation ▁is ▁reached , ▁momentum ▁storage ▁devices ▁cannot ▁apply ▁tor que ▁to ▁control ▁the ▁space craft ' s ▁orientation . ▁The ▁space craft ▁then ▁typically ▁requires ▁thrust ers ▁using ▁prop ell ant ▁to ▁' des atur ate ' ▁the ▁storage ▁devices , ▁in ▁other ▁words ▁to ▁un load ▁the ▁accum ulated ▁momentum , ▁and ▁so ▁to ▁restore ▁the ▁space craft ' s ▁full ▁ability ▁to ▁carry ▁out ▁rot ational ▁operations . ▁ ▁Space craft ▁experience ▁orb ital ▁decay ▁due ▁to ▁drag . ▁To ▁maintain ▁their ▁orbit , ▁thrust ers ▁are ▁used ▁to ▁re boost ▁the ▁space craft