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For any sort of long-range communication, though, Astropaths are required. This is true throughout the Imperium. Local, in-system comms can be handled by comm-lasers or vox-casters, but anything to talk to another star-system requires an Astropath.

The First Company is typically veterans and Terminators. Companies 2 through 5 are the "Battle Companies" and have a mix of Tactical, Assault and Devastator Marines. Companies 6 through 9 are "Reserve Companies", Companies 6 & 7 being all Tactical Marines, Company 8 being Assault Squads and Company 9 being Devastators. Company 10 is the Scout Company, and is almost entirely Scouts (save whatever Marines are responsible for their training).

So, in most cases, deploying a single Battle Company should be enough to execute most SM missions, and the Chapter can thusly deal with numerous missions at once.

The above assumes Codex-adherent Chapters. If the Chapter is not by-the-book, then none of the above may apply.]]> http://www.dakkadakka.com/dakkaforum/posts/preList/587896/6688784.page http://www.dakkadakka.com/dakkaforum/posts/preList/587896/6688784.page GMT
Astropaths are necessary for long range communication, Librarians are also periodically noted as being capable of functioning as short-range Astropaths.

The Marines Errant are an extreme example of operations, their crusade fleets can operate out of contact for decades at a time. ]]> http://www.dakkadakka.com/dakkaforum/posts/preList/587896/6688846.page http://www.dakkadakka.com/dakkaforum/posts/preList/587896/6688846.page GMT