Poul Anderson's "To Outlive Eternity" (novella, space opera, free): When a journey in space turned to one beyond the end of time!
Note many reviewers describe this story as "hard sf". I don't know enough physics to conclude that; some of its physics is obviously cooked up. To me it sounded like space opera, but without guns & aliens.
Story is not always an easy read. Particularly early parts include infodumps that were just meaningless babble for me. But I'm no physicist.
Story summary.Leonora Christine, a human colonizing ship with 50 men & women aboard, encounters a "small" nebula on its way & cannot avoid unplanned collision with the gaseous body. All is fine after the collision, except the ship has lost ability to decelerate - hence to arrive at its destination!
Damaged portions can be repaired, but they're outside the hull, & for complicated reasons, cannot be repaired except in intergalactic empty space. Thus begins a grand adventure with an unusual happy ending.
Fact sheet.First published: Galaxy, June & August 1967 as a 2-part serial.
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Related: Stories of Poul Anderson.