Select the "Risk/Reward" mission on Earth to search the Cosmodrome and complete your new weapon.
"All we need now are some Arc conductors. Take this Ether key. It's Hunter tech. Was a gift. Should get you to a stash 'near a gap' in the old Cosmodrome.
"Track down and defeat Fallen Captains. Their Ether profiles will activate the key. And, hey… If you see my friend—or his chicken—tell him not to be a stranger." —A note from Banshee-44