This is the first real main mission of the game, so it's unmissable.
After either completing
or just skipping the prologue, which I personally wouldn't recommend the first time at least. You'll beam down to your first planet. Your AI will tell you to explore a bit, very soon after it'll detect something, and give you a waypoint on your top right where your quest objective is. Following it you'll find an odd looking gate like structure. Where you'll be told by your AI you'll need 20 Core Fragments to power it up. You find Core Fragments either on the Mine on the planet, or by going to the core of planet. I'd recommend going to the Mine. To find the Mine just keep going left or right on the planet's surface, as all planets in the game loop. In the Mine you'll find the Fragments in the boxes and chests scattered around.
You'll also have the opportunity to run into and maybe even kill the Mother Poptop for the
achievement.
After obtaining the 20 Core Fragments and going back to the gate, you'll power it up and enter the Outpost, which is the central hub of the game. Just keep heading to the right and you'll find and meet Esther.