Interplanetary Portals adds a late-game fast-travel network that fits naturally into Space Age progression: no rockets required after you've already blazed the trail.

How it works:
Build one Interplanetary Portal on each planet you want to link. Open the portal and a Travel panel docks alongside its native inventory window. Each destination starts locked; to unlock it, craft and slot the matching Warp Module for that planet (one module per planet: Nauvis, Vulcanus, Fulgora, Gleba, Aquilo). Once a module is in the portal, the Travel button lights up and you can warp instantly.

Warp cost and modules:
Every trip consumes resources from the portal's own inventory - by default 50 processing units, 50 low-density structures, and 50 rocket fuel. You stock the portal; it pays the fare. The cost scales with the Travel Cost Multiplier server setting (set it to 0 for free travel without any module).

Two optional upgrade modules change the rules:
Cargo Warp Module - by default you must travel empty-handed (your inventory stays behind, you arrive with nothing). Install this module in the portal to bring everything with you. Without it, portals are commuter lanes, not cargo haulers.
Free Travel Module - install this to waive the per-trip resource cost entirely. A late-game convenience upgrade, not a starting freebie.