Transport Center
From M2K2: Metroid Prime 2 Wiki
Submitted By: Shinin
When you first enter Dark Aether from the Portal Chamber and open the door to the Transport Center/Crossroads, you can get back to the Light Portal without getting the Space Jump Boots. Because of this, you can get the Missile Expansion (behind the door) before facing the Jump Guardian.
Added by: MMcLean54
The easiest way I've found to do this is to jump on the Light Crystal and then jump on the platform from there. Make sure your jump on the platform is very precise. Also worth noting, you should be on the very edge of the Light Crystal. Most of the time, you will not make the second jump because you're not on the Light Crystal's edge completely.