Early Missile Expansion: Crossway

Discovered by

Varsis Erion, Radix, kip

Notes:

Look for a set of brown bars by the Chozo Lores in the room after crossing to the other side as detailed in Skipping the Boost Ball: Crossway. These bars run all the way to the ceiling, and you can land at enough places that it's possible to climb up by space jumping.
Stand near the bars so that they're easily within the distance of one jump, but not right up to them (she'll hit her head whenever you try to climb them if you are). Jump straight up, and at the peak jump toward the bars; you'll land somewhere on them. If you repeat this you'll land higher on the bars, then land at the top, but move back slightly during the first jump, to avoid hitting her head on the bars.
Once you're at the top, you should see three small ledges in the distance. Space jump to either the highest ledge or the one closest to it.
Now space jump to the ledge furthest away, which happens to be right next to the small tunnel with the missile expansion.
The hard part is here. From this ledge it's possible to space jump and get right into to the tunnel, then morph and collect the expansion, but it's unclear if there's anything you can do that makes it more consistent. It seems to help if you reach the tunnel early in your first jump, then jump again when you are below the tunnel's ceiling level.
To speed up the third step, try the Dash Jump shown below (US version only).