E-Space

The Doctor's TARDIS travels through the green-tinted E-Space. (TV: State of Decay)

E-Space — alternately known as Exo-Space, Ecto-Space or Exo-Space-time continuum — was a smaller universe existing alongside its prime universe, N-Space. It possessed an unusual greenish tint. (TV: Full Circle, State of Decay)

Locations in E-Space had negative co-ordinates, where N-Space was positive. When the Doctor's TARDIS materialised on the planet Alzarius after it had travelled into E-space, the TARDIS scanner continued to show images of Gallifrey, which had the positive equivalent of Alzarius' spatial co-ordinates. Rather than being a simple optical relay, the system apparently depicted whatever existed at the ship's current co-ordinates. The Doctor suggested that the system operated off the absolute value of the co-ordinates and thus could not process negative co-ordinates. The scanner system was made functional again simply by replacing its original N-Space image translator with one native to E-Space. (TV: Full Circle)

Transit between the universes

The Fifth Doctor stated that the negative co-ordinates would make it very difficult for his TARDIS to travel there deliberately and not accidentally. (TV: Earthshock)

By his seventh incarnation the Doctor was able to pilot his TARDIS between E- and N-space, and was able to retrieve Romana II from E-space. (PROSE: Blood Harvest) K9 was also retrieved. (PROSE: Lungbarrow)

CVEs

Travel between N-Space and E-Space could occur via a Charged Vacuum Emboitment, or "CVE". This was a hazardous and apparently random process, even using a TARDIS. (TV: Full Circle)

The Logopolitans created the CVEs which linked together the two universes as part of their plan to forestall the overdue entropic collapse of their universe, N-Space, by opening up what had previously been a closed system and thus dissipating the accumulated entropy.

Following the disastrous consequences of the Master's visit to Logopolis, only one CVE remained open, in the constellation of Cassiopeia (from Earth's perspective). (TV: Logopolis) Unless further CVEs were later created, it was highly likely this was the route through which the Hydrax would later enter E-Space (TV: State of Decay) in the 22nd century. (PROSE: Lucifer Rising)

The Gateway

On his return journey to N-Space the Fourth Doctor and his companions passed through a strange white void known to its most frequent inhabitants, the Tharils, as the Gateway. (TV: Warriors' Gate)

Planets of E-Space

Planets in E-Space included Alzarius, Ballustra, the planet which was the hiding place of the last Great Vampire, and Terradon, the origin world of the starliner which crashed on Alzarius. (TV: Full Circle, State of Decay, Earthshock, AUDIO: The Invasion of E-Space) The Tharils presumably had a homeworld of some kind as well, as did the humanoid races which they variously enslaved and were enslaved by; Shapeer was a place named for hunting them. (TV: Warriors' Gate)

History

The Fourth Doctor, Romana II and K9 Mark II accidentally entered E-Space through a CVE. He landed on Alzarius where he met Adric, an inhabitant of the universe who secreted himself aboard the Doctor's TARDIS to join the Time Lord on his travels. (TV: Full Circle, State of Decay) They spent months trying to find another CVE. During this time, they encountered the Farrian, who had entered E-Space through a CVE. They had opened it themselves using Block Transfer Computation, a skill they learnt from a prisoner. The Farrian attempted to conquer the planet Ballustra. The Doctor destroyed the device that generated the CVE, forcing them to retreat. (AUDIO: The Invasion of E-Space)

The Doctor and Adric left E-Space through the Gateway. Romana and K9 chose to the stay behind. Romana did so to avoid a return to Gallifrey and K9 accompanied her. (TV: Warriors' Gate)