In the Citadel area on Venus (two areas left from spawn), if you go up on that platform where the 2 level 20 Axis Minotaurs are... notice that passage in the wall behind them? Go on back there and find the grav lift. Now, there is a forcefield around it that prevents simply walking into it, but if you go off to the left wall and jump on the rods and antennae sticking out, it is possible to jump OVER the forcefield and get sent up in that Vex tower in the sky. There are 2 ghosts up there.