I'm not really sure but I thought that Earth research for space will define a big destroying alien ship and a mother ship, some 2 CS volunteers getting transported to space to the mother ship so they can hack the force field on the destroying ship so the aircraft can use explosive plasma missiles. so after they hack the mothership for that, the CS aircraft start shooting missiles on the destroying space ship... until they reach to the ship's gun that destroys earth.. so none got enough missiles so somebody sacrifices and fly into the gun while it's activated and when his ship crashes into their gun his aircraft blows up and also the gun exploded and destroy the entire destroying alien ship.. after that the 2 volunteers shot a missile to the alien who controls the mother ship and they go out... then the rocket missile explodes and destroys the entire ship and then the human mankind has been saved by the Civilian security forces.. not by the marine or noir lime or proxy...
but basically, that would be possible in the game series cuz CS would have been too late and the marine made things worse towards CS.. and also no clue knew information about usurpations space ships so whatever.. (bad story for PB3) so perhaps the marine cannot do shit and cannot find the c9 but while he's in the past he can contact with the CS army and C9 to get prepared to plan about all of that if there will be an alien ship encountered and needs to be destroyed..then they get prepared and ready doing all of this plans then after doing all of this they destroy the mother ship and the big alien destroying ship... that doesn't make sense anyway.