: The term, propulsion breakthrough, refers to concepts like propellantless space drives and faster‐than‐light travel, the kind of breakthroughs that would make interstellar exploration practical. Although no such breakthroughs appear imminent, a variety of investigations have begun. During 1996–2002 NASA supported the breakthrough propulsion physics project to examine physics in the context of breakthrough spaceflight. Three facets of these assessments are now reported: (1) predicting benefits, (2) selecting research, and (3) recent technical progress. Predicting benefits is challenging, since the breakthroughs are still only notional concepts, but energy can serve as a basis for comparison. A hypothetical space drive would require many orders of magnitude less energy than a rocket for journeys to our nearest neighboring star. Assessing research options is challenging when the goals are beyond known physics and when the implications of success are profound. To mitigate the challenges, a selection process is described where: (1) research tasks are constrained to only address the immediate unknowns, curious effects, or critical issues; (2) reliability of assertions is more important than their implications; and (3) reviewers judge credibility rather than feasibility. The recent findings of a number of tasks, some selected using this process, are discussed. Of the 14 tasks included, six reached null conclusions, four remain unresolved, and four have opportunities for sequels. A dominant theme with the sequels is research about the properties of space, inertial frames, and the quantum vacuum.

To travel to our neighboring stars as practically as envisioned by science fiction, breakthroughs in science are required. One of these breakthroughs is to discover a self-contained means of propulsion that requires no propellant. To chart a path toward such a discovery, seven hypothetical space drives are presented to illustrate the specific unsolved challenges and associated research objectives toward this ambition. One research objective is to discover a means to asymmetrically interact with the electromagnetic fluctuations of the vacuum. Another is to develop a physics that describes inertia, gravity, or the properties of space-time as a function of electromagnetics that leads to using electromagnetic technology for inducing propulsive forces. Another is to determine if negative mass exists or if its properties can be synthesized. An alternative approach that covers the possibility that negative mass might not exist is to develop a formalism of Mach's principle or reformulate ether concepts to lay a foundation for addressing reaction forces and conservation of momentum with space drives.

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