Jump node
From FreeSpace Wiki
A jump node is a place where the second type of subspace jump, the intersystem subspace jump is possible. Jump nodes are shown in-game as a wire frame HUD diagram. Without assistance, it is impossible to locate a jump node visually as they are invisible to the unaided eye.
It is theorized by fans that jumping near to a jump node is impossible because it would make escort missions pointless. However, this is completely non-canon, and there is also evidence against this theory.
Most jump nodes are unstable and cannot be traveled by GTVA ships. These nodes can be stabilized permanently (or for a very long time) by Knossos portals. It is also thought that Shivan ships can traverse unstable nodes more easily than Terran and Vasudan ships.