The 3rd part of this quest line is The Taken War: Earth. This particular quest requires you to kill the Taken Descendant at the Divide, Skywatch and the Rocketyard on Earth. I managed to kill the first two with the assistance of public players. However, no one ever ventures as far as the Rocketyard. When I try to complete this quest, there usually aren't any public players around. Unfortunately I don't have any IRL friends who play this game. After this quest I will have to do the same on Venus and on Mars. If you just started the quest, I don't mind helping you out.