Fusion-powered space travel has long held the promise of rapid trips across the solar system: Mars in weeks, Saturn in months, Pluto in years. For decades, such possibilities have remained theoretical, like something plucked out of a science fiction novel. But several companies are now working to build practical nuclear fusion propulsion engines, with significant milestones being hit. Pulsar Fusion, a U.K.-based startup, hopes to launch a demonstration mission to space in 2027, while Princeton University and Helicity Space in the U.S. are continuing their own work on fusion drives.Richard Dinan, the CEO of Pulsar Fusion, displays the Sunbird spacecraft at the company's facility in Bletchley,...