The species in question that can survive in the vacuum of space for extended periods of time (say months). They can feed on stellar radiation and have an innate sense of orbital dynamics. They come from a planet with several moons and they frequently travel from the planet to the moons and back, and occasionally venture out to other planets. The planet has similar gravity to Earth.
Moving in space seems somewhat plausible. They could use thin membranes as solar sails and bladders of gas for maneuvering.
How do they get into orbit?
I'm looking for a mechanism that is primarily biological rather than technological. I recognize that the line is fuzzy, so the main criteria I'm looking for is that one of these animals could make it happen purely of their own means, without assistance from any others. Beavers can make a lodge, but each one can do it by their own power without reliance on the work of other beavers, so I'd count this as biological. Bees making a hive takes a lot of bees to do and no single bee can reasonably do it by itself; this, I'd classify as the lowest threshold of technological. A human making fire with a bow drill that they can make themselves out of materials in nature is grey area; they need knowledge passed on by other people, but otherwise can do it of their own means.
If the planet needs to be substantially less massive that Earth to make this possible, that's an option, but the closer to Earth's gravity, the better.
- The organism should be big enough to be visible unaided, but beyond that is whatever size makes this plausible.
- The entire organism does not need to make it into space (and it seems unlikely that this would happen). All that needs to happen is an egg, seed, or larva capable of growing a new organism makes it into space.
- A given member of this species does not need to make this trip multiple times in a lifetime, and the trip to space could definitely be part of a very specific lifecycle phase (like Salmon spawning, only much more extreme).
- The organism does not need to be an animal. If there's going to be a lot of photosynthesizing or if it would need to have a large stationary component, it could be a plant.