There are five factions of enemy Astronauts, and a single variety of allied (friendly) Astronauts. An Astronaut's affiliation can be determined by the color of his space suit. Each type of Astronaut varies in speed, deadliness and difficulty to kill.
Astronauts are about 3.33 blocks tall, much taller than the player. This means that they require a 4 block tall space in order to move around. Astronauts of different colors will occasionaly attack each other. If you encounter a random Disk lying around, or craters made by Brown Mobs, enemy Astronauts may be in the vicinity. Astronauts will also attack other Mobs; finding Biogel may also mean that Astronauts are in the area. They also may spawn from out of nowhere. Another way to find Astronauts is if you encouter an Astronaut Lair which is surrounded by Polymer on all four sides and is always deeper than 15 blocks in depth. However, not all Astronaut Lairs have Astronauts.
White/Mint Green AstronautsWhite Astronauts are the slowest and easiest to kill. They are also the most common
Pink/Powder Blue AstronautsPink Astronauts move faster and are harder to kill. They are trouble in numbers.
Yellow Astronauts are very fast and deadly. They often pair up and remain hostile towards oposing Astronauts including the player.
Friendly (Bright Green) Astronauts
If the player places throws a Refined Gold bar near an Astronaut, the Astronaut might pick it up; and if it does, it becomes friendly. (In order to throw a gold bar, go into the inventory and move the gold bar to the edge of the screen). After the player throws refined gold at the Astronaut or placed by a beacon,the friendly Astronaut seems to run away from the player backwards.
A bright green Astronaut will fire at other Mobs, including other friendly Astronauts, but not at the player. All Astronauts become this color when made friendly, regardless of their original color.
If you fire at a friendly Astronaut and hit it, it turns back into its original color and begins to fire back at you.
Updates 1.7.0 and above have a bug that causes an unending repeated pistol noise, that is heard even when there is no Astronaut in sight. This seems to happen as a direct result of making an Astronaut friendly, and it also happens in Update 1.8 when you place a Beacon. After a little while, the gameplay slows down so much as to be unplayable.