Voyager engages the quantum slipstream drive (see data on it below) in order to travel swiftly straight back to the Alpha Quadrant. The ship is preceded by the Delta Flyer. On board the Delta Flyer it is Kim's task to transmit phase corrections to Voyager to keep the ship within a properly stable slipstream. However, Kim makes an error and while the Delta Flyer travels on to the Alpha Quadrant, Voyager falls out of the slipstream. The ship crashes on an ice-planet near the Alpha Quadrant, killing all aboard. In 2390, fifteen years after the tragic event, Kim and Chakotay put into action a secret plan to change history. Having stolen the Delta Flyer from a Federation shipyard, they go to the ice-planet and salvage the Doctor. He agrees to obtain data from Seven's Borg interplexing beacon so that Kim, using a salvaged Borg temporal transmitter, can transmit the correct phase corrections back in time and, not only avert the disaster, but also bring ship and crew home safely. Unable to correct his original error, Kim settles for transmiting a phase correction back in time which disperses the quantum slipstream entirely. Back in 2375, both Voyager and the Delta Flyer are thrown out of the slipstream, effectively erasing the future - they are safe, though still in the Delta Quadrant. However, the flight in the quantum slipstream was not wholly a failure as they are ten years closer to home.

Voyager in the quantum slipstream

The construction of the quantum slipstream drive, which has been adapted from Arturis' original model featured in [#94 Hope And Fear], includes a quantum matrix, benamite crystals and Borg technology. The benamite crystals at the heart of the engine have already started to decay even before the drive is first used and Kim notes that it could take years to synthesise more. From flight simulations, Paris finds a .42 phase variance in the slipstream threshold, a problem which arises about 17 seconds into the flight and renders the slipstream unstable.

modified warp core

At the ceremony christening the quantum slipstream drive, Janeway observes that the crew has been in the Delta Quadrant precisely four years, two months and eleven days.

In the eventually unrealised timeline, Voyager crashes on a class-L planet in the Takara sector, just outside the Alpha Quadrant (i.e. in the Beta Quadrant). In the revised timeline, Voyager is still in the Delta Quadrant, but on leaving the quantum slipstream they are ten years closer to home.

in the unrealised timeline, Voyager crashes onto the ice-planet and skids to a halt

Kim and Chakotay, having successfully reached the Alpha Quadrant in the Delta Flyer, in 2390 steal it from a Federation shipyard. As a result of the thefts and their activity they are wanted by Starfleet on two counts of high treason and conspiracy to violate the Temporal Prime Directive. In the eventually unrealised future, the Delta Flyer suffers a warp core breach and is destroyed. In the revised timeline the Delta Flyer does not sustain any damage.

In the eventually unrealised timeline, in 2375, Voyager crashes on the class-L planet mentioned above, and all aboard are killed.

In the Starfleet of 2390 in the eventually unrealised future, high treason and conspiracy to violate the Temporal Prime Directive are offences for which suspects can expect to be arrested and charged. (The Temporal Prime Directive is mentioned in [#50 and #51 Future's End], [#118 Relativity] and [#171 and #172 Endgame] and, I think, [DS9: Trials And Tribble-lations].) In addition, the Galaxy-class starships are still in use, originally introduced in the 2360s.