CH252 helmet
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The CH252 Helmet is the standard, typical issue helmet that the UNSC use for the Marines. The helmet isn't seen in Halo: Combat Evolved however, though it could be the fact there were none onboard the Pillar of Autumn, or the Marines wearing them were not seen in-game.
In Halo 2, both regular Marines and ODST Helljumpers wear versions of the helmet - Marine helmets are painted green, while ODST helmets are painted black, like their combat suits, and feature a faceplate complete with HUD display. The standard helmet weighs about 3.3 pounds according to crates found in the game. In Halo 2 and Halo 3, you can shoot off their helmets. They will not pick them back up.
The Marine pilot helmet is a version of this type of helmet. This was most likely manufactured around the end of 2552. It is not seen until Halo 3. This helmet is almost a replica of the ODST helmet. It does have a HUD and weighs about 4 pounds. It is not black like the ODST helmet, instead having a clear visor and is green. Each also bears a wing symbol, likely denoting their particular specialty.
According to the description for the ODST helmet permutation, UNSC combat armor, including that used by the UNSC Marine Corps, utilizes CTC's for protection enhancement.
Similar helmets
The helmet used by marines in the five long years cinematic heavily resembles the CH252. The "prongs" on either side of the CH252 are not present on this version of helmet, but apart from that and other minor ascetic differences both share the same physical characteristics, such as goggles for example.
Halo: Combat evolved
The helmet in halo combat evolved typically serves the same purpose with little difference other than physical traits. The frontal area of the helmet sticks out more than on the CH252. It also lacks the two prongs. It is interesting to note that the helmet used by marines in the first trailer for Halo Wars is very similar if not the same to this type. It could be speculated then that this is an older type helmet that may have still be in use aboard the Pillar of Autumn.
