This is all very anglocentric - what expression might be used in koine Greek, Church Slavonic, and Romanian? For Greek how about διαστημοπλεόντων? Or does make the Greeks here fall about laughing??
Diastemopleonton would mean "those sailing (in) space". Nautical terms have, since the beginning, carried over into the language of aviation, such as (air)ship. (air)craft, fleet, and (flight) deck. This has also been the case in space terminology, particularly astronaut and, the more accurate, cosmonaut. So, while it sounds somewhat awkward at first, diastemopleonton may well be a suitable new word for those who travel in space.
I suspect the first local church to introduce a new petition along these lines would be the Russian rather than the Greek, given that they have been at the forefront of space exploration since its inception more than 55 years ago. If they do, their choice of terminology should be instructive.