I have submitted a report to Starfleet, bearing the signatures of both myself¹ and
The first known crossover event involved Captain James Tiberius Kirk of the (Constitution class) USS Enterprise while in orbit around the Halkan homeworld. A freak ion storm, occurring simultaneously in the same corresponding space/time locus in two universes, managed to somehow lower the field density between the two universes sufficiently that a transporter beam from each universe was able to "punch" through to the other side, transposing the analogous crewmembers from each universe into the other. The lowered field density was only temporary, however; and Kirk's landing party was able to duplicate the conditions to transport back home to their own universe (this one) before conditions made such a transposal impossible. It is important to note that such a cross-universal transporter accident is normally impossible, and was only able to occur due to the lowered intra-universal field densities, which "heal" over time.
The next recorded² crossover event occurred many years later near space station Deep Space Nine in the Bajor system. Bajoran Major Kira Nerys and Starfleet Medical Officer Julian Bashir, both assigned to Deep Space Nine, were accidentally transposed into the Mirror Universe while their runabout's warp field collapsed improperly as they entered the Bajoran wormhole. Once again, this incident "punched" a hole between the two universes; a much larger one than the hole caused by the Halkan ion storm. Because of this, intra-universal transport was once again possible; and it took approximately four point seven years before the field densities between the universes healed sufficiently to render intra-universal transport once again impossible. The last recorded crossover event before that occurred involved the Ferengi Grand Nagus Zek and two other Ferengi from Deep Space Nine.
The most recent incident once again involved a runabout "punching" through to the Mirror Universe, this one involving Lieutenant
The danger is not, however, immediate. In the original incident, Kirk's landing party were able to easily duplicate the conditions of the crossover because the ion storm had been observed and recorded by the ISS Enterprise. Every intra-universal fistula has a different harmonic; and since the personnel on Terok Nor in the Mirror Universe did not directly observe the crossover of Nerys and Bashir's runabout, it took a great deal more effort (the actual amount of time and personnel involved in the effort is unknown) before they were able to duplicate it (approximately a year later, "Smiley" O'Brien was able to transport himself from Terok Nor to Deep Space Nine in order to abduct Captain Benjamin Sisko). Given that, it should be a matter of some months, at minimum, before the residents of that universe are able to once again duplicate the conditions allowing them to transport into this one (as the report filed by officers Diziara,
¹As much of this speculation and analysis is more within my field than my analog's, I decided that the report would carry more credibility were my name attached along with his.
²The data suggests that another crossover incident occurred within a year of the Halkan crossover incident, but I can find no record of such an event occurring. It is possible that it did not occur in Federation space.