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Many science fiction stories describe soldiers who perform spaceship boarding/defense or planetary invasions as "space marines." This is by analogy to modern marines who historically specialize in defending/assaulting naval vessels and amphibious combat.
Is there any term (neologism?) for this kind of soldier beyond adding "space" in front of an existing modern word?
Use Portmanteaus
War Dogs by Greg Bear uses the term Skyrines, a portmanteau of "sky" and "marine". If you don't like the use of the term "sky", you could substitute it for another term like say, "cosmo", to get "Cosmorine."
Other examples may include: astroldier - combining Astro and Soldier. A bit hard on your tongues though, decreasing efficiency in a military setting.
Plasfo - PLanetary ASsault FOrce. This one is a bit more heavy on stereotypical military terminology, but gets the job done.
Look Into Linguistic Roots
The term "marine" is derived from French (French "marine", from Latin "marinus", derived from "mare"). We could do the same but with the term "space". In French, the term for space is "espace" according to Google Translate, so space marines could potentially be called "espatiers" (pronounced "espaciers" as in "spatial").
(I got the above example from http://www.projectrho.com/public_html/rocket/astromilitary.php#terminology)
But this isn't the 1800s and presumably not everyone speaks a Latin-derived language. So you could potentially play around with linguistics and lore to create your terminology. For instance, I could have a world where there is a second space race and China comes out on top, and have terminologies be based on Chinese.
But regardless of language, we need to consider what would be practical. In a military setting, communication needs to be short and precise, and easy to say, so we should decrease the number of syllables while keeping the meaning clear. If you want to stick to English and keep it linguistically reasonable, I say you should get common phrases like in the other answers and play around with them (see above section).
In the context of your world, if the vast majority of engagements are fought ship-to-ship, or ship-to-station, they could simply be "soldiers" or "troopers" without any particular need for a prefix.
Essentially these types of actions have become so commonplace that doesn't make sense to draw attention to it - it's just a part of an ordinary soldiers job.
Other ideas:
@Clay Deitas mentioned referring to them like modern special forces, e.g. "Green Beret", which I think is a good idea
Tough call.
I tend to refer to people who specialize in fighting within ships as "Boarding soldier", but that doesn't seem like what your talking about.
I guess "Exo-trooper" could work, referring to combatants who operate outside atmospheres.
Personally, I'd be tempted to refer to these guys as "Rocketeers", but that's technically already in use. Plus it might be a bit too retro-sounding.
If I was allowed to get fancy, I'd claim that Russians established the first military bases and space-specialized soldiers. The russian weapons program to arm these soilders was designated operatsiya: kosmicheskiy rytsar' (operation: space knight), so the term for any null-gravity soilder became "Raystar."
The most generic-sounding one would probably be "Spacer", which I kind of like because it sounds equal parts insulting, pessimistic, and threatening.
I think that's all I got in my arsenal at the moment, but I might edit more in later.
Wait, you could call these guys "Kents" after the secret identity of superman. Might be a fun way of poking fun at their egos, you know?
Edit: 30-secs would be a fun nickname at least. That's about how long humans can survive in a hard vacuum.
I always liked the idea of space being referred to as the Abyss. You could call the more hardened soldiers "Abyssers" or something along those lines.
A more technical term would have to use "Astro" somehow, but Astrosoldiers sounds lame.
Cosmonauts.
Astrotroopers.
Star Soldiers.
Or any feature given to the standard space soldier, like the name Green Barrett.
