Trying to fathom the concept of a light-year is not by any means easy - it’s literally how far light travels in a year, roughly 9,460 billion kilometers or 5,880 billion miles. That’s a lot of zeros.

Spacecrafts of the future could be propelled by in-space engines that require zero fuel, if one engineer’s proposal comes to fruition. David Burns , an engineer based at NASA’s Marshall Space Flight ...

David Burns’s idea for a “helical engine” with no propellant is ingenious (19 October, p 15). He imagines a movable ring, whose mass is much greater when it slides in one direction than the other. But ...