From Halopedia, the Halo wiki
Slipspace portals are a method of slipstream space transit developed by the Forerunners. Essentially a large-scale application of slipspace translocation, slipspace portals are designed to transport enormous amounts of mass between two fixed points on a continuing basis.
Large constructs, such as the Halo installations, could be transported across interstellar distances through the use of portals. The transition of such a large object would slow down other slipspace traffic in the entire galaxy due to the buildup of an effect known as reconciliation. While designed for large-scale transit, portals have a limited transit capacity and can be strained or even destroyed if this capacity is exceeded. Portals also have a system that filters unnecessary objects such as debris, discarding them into the volume of slipspace. While passage through a portal is remarkably faster than an ordinary slipspace jump, portal transit is not instantaneous; for example, the passage from the Earth portal to the Ark took over three weeks. The portal constructs have unmediated slipspace cores bound to them that generate stable, pre-computed slipspace routes between synchronised endpoints. Due to the fact that these devices have fixed endpoints, destination flexibility is sacrificed for safety, speed, and security.
Portals may be generated by groundside facilities which vary in overall architecture but are typically enormous, or generator systems situated in space. Portals generated within an atmosphere are often accompanied by severe weather disturbances. Dedicated ancillas were often tasked with maintaining portals. Certain portals are designed to activate only when a specific code sequence is transmitted or a device, such as the Conduit, is inserted into a machine capable of generating such portals. Others, such as the one on Earth, require a keyship to function. Halo Installations are capable of generating slipspace portals if necessary to reach a destination.
Portals are similar in appearance to a slipspace rupture generated by a slipspace drive, appearing as holes into the pitch-black void of slipspace and surrounded by blue-violet energy and supported by a web of hard light. Some portals, however, allow visibility to the destination on the other side.
The Forerunners had a network of portals that allowed fast passage throughout their ecumene. Some systems, including that of the capital, had portal installations that connected to multiple worlds. The Librarian used an array of portals to travel to the worlds where she collected the various species of the galaxy for indexing at the Ark. The same network was used to transport the Halo rings. The portals in this network could only be activated by a special type of vessel, known as a keyship, most of which were eventually destroyed by the Librarian in order to prevent the Flood from reaching the Ark. The generator device for one of the Librarian's portals was buried near the town of Voi on Earth, and was later activated by the Covenant, who were in possession of the last known remaining keyship, known as the Forerunner Dreadnought. Faber, the Master Builder, had a secret private portal constructed allowing transit from the vicinity of the greater Ark to Installation 00. This portal could only be activated by inserting a specific code known only to the Master Builder. Transit through the portal was also distinctly more comfortable than in most normal slipspace portals due to the Master Builder's wealth and power. On Reach was a portal complex that could be used to connect to the Ark. Unlike the portal on Earth, this one only remained open for as long as the local charge lasted and could close in minutes to hours.
An unknown variant of a portal-generating machine was present in the jungles of Installation 03, and was only able to be activated via the Conduit's interaction with another machine found at the installation's Altar complex. Furthermore, the activation of the Altar triggered the opening of slipspace portals around the galaxy by way of these generator machines; one such machine in the Halo's jungle directly linked to another machine residing in an ONI facility in Arizona on Earth, enabling slipspace travel between the two locations. It is unknown if these portals provide two-way travel between Installation 03 and Earth exclusively, or are part of the Forerunner's vast network of portals at large.
Though the Librarian's portal network was utilized to transport the Halo rings, the rings themselves are capable of generating portals for transport if necessary. After Installation 07's near-collision with the wolf-faced planet, the IsoDidact had the installation, boosted by power from Forerunner dreadnoughts, generate a limited portal to travel to the greater Ark for repairs, but the limited power and severe damage meant that the installation had to shed 20,000 kilometers of its mass to make it through. Similarly, Installation 09 was able to open a portal for its transit to the Soell system when it was launched in 2559.
Several of the structures on Sarcophagus contained slipspace portals not just to other parts of the shield world, but to other planets entirely including Hesduros in an entirely different star system and Mackintosh which was not only within the Zeta Doradus system, but fell into orbit of Sarcophagus when the shield world was removed from its slipspace bubble. These two portals were used by Jul 'Mdama to escape and the Servants of the Abiding Truth to travel to Mackintosh respectively.
The Halo: Reach multiplayer map High Noon is set in an observation platform of a portal generator. This portal facility is noticeably different from the Earth portal; in addition to a different structural design, the area of space beyond the portal is visible, as opposed to complete blackness. This particular portal device also generates a storm of swirling debris in the immediate area around the portal, just beyond the map's borders.
List of appearances