Cosmonauts are astronauts sent to Moon Base Solaris in order to help the Blisk.
Cosmonauts are KGB agents tasked with going to the moon to help the Blisk. They do not yet know the Blisk in fact plan to take over the Earth with spores until Crypto (disguised as Leonid) convinces them with a speech and destroys their friendship. The Blisk then barge in and a massive fight breaks out, destroying the main dome. Any Cosmonaut will attack a Blisk if they see each other. After the Blisk are destroyed by Crypto, an unnamed Cosmonaut tasks Crypto with destroying the rest of their kind on the moon. After that they contact Russia for help back to Earth. It is unknown if they get it or not.
There are two kinds of different Cosmonauts.
Cosmonauts dressed in either green or white suits are scientists, there to study the Moon. They will run away from Crypto or Blisk.
Cosmonauts dressed in orange are tasked with protecting Solaris from either the Blisk or Crypto.
Cosmonauts in orange are armed with a weapon similar to a Majestic Assault Rifle. It shoots an orange bullet rapidly, which does more damage to Crypto then most other weapons.
Like locations on Earth, they also have Tanks and Army Transports to defend themselves.