Lost Brother
Locations: Nexus(Hyperion), Eos
This mission begins with Nigel McCoy, who's sitting on the first hospital bed on the left when you enter the Cryogenics area.
He'll give you a pendant to lay in the sands on a certain point on Eos. Surprisingly, all you have to do is go to that point (an overlook above the Promise settlement) and set down the pendant.
You do need the NOMAD to get there, but there are no other complications. Just take the hill-that-requires-six-wheel-drive out of town and turn right, rather than left toward the monoliths.