Where would the space marine be without his trusty suit? Not space, that's for damn sure.
But how exactly does one design space armor? Thus far, it's been about all we can do to make space suits to allow something resembling space survival. With some help from Atomic Rockets and a degree in engineering, I've been working on that.
There are several major considerations with any space suit. Most habitats maintained by humanity will be pressurized with air, rather than pure oxygen, due to the sheer danger of a large, pure oxygen environment. Current protocol is to pressurize to a full 1 Atmosphere, 101kPa. Most of our space suits, due to being internally pressurized, however, are only pressurized to 32 kPa. This leads to a problem: The Bends. Going rapidly between pressures will lead to terrible problems, which is itself a problem for anyone in space, and even moreso for the space combatant, who may need to suit up and launch with very little warning, and no time to prebreathe to vent the inert gasses in their system that leads to the bends.
So how do you get around this? There's a few ways. Space combat ships are likely to be kept at 50-60 kPa, which permits the use of air rather than pure oxygen, but is substantially closer to the low pressure that works for the human system. That will substantially lessen the severity and chance of the bends, even without other systems to resist the bends, while not converting the ship into a missile with an oxygen warhead.
The next step is the suit itself. There are 4 major types of spacesuit: Hard, Soft, and Compression. Compression suits are popular in fiction, and modern tech is trying it's hand at them. Basically a fancy spandex jumpsuit, it pushes on the body with the same pressure as atmosphere would, applying that same 32 kPa through mechanical tension instead of air pressure. A hole in the suit just leads to a bruise, and the suit adds very little difficulty to tasks due to its form-fitting nature. Still, it can only apply low pressure or the wearer will be unable to breathe. Some fiction has these compression suits being used as space underwear, allowing spacers to don a helmet in case of emergency and survive even exposure to vacuum. Soft suits are mostly what has been used for human space exploration: semi rigid, with constant volume joints. Unfortunately, they can ONLY be used with low pressure, or the suit would inflate so hard the astronaut could not move. They also make just about all tasks FAR harder as they fight your every move. Hard suits are a fully rigid suit permitting high pressure, but they're exceptionally bulky, complex, and resist all motion even harder than a soft suit. A fourth style, the hybrid suit, incorporates elements of both hard and soft suits.
Unfortunately, the already complex task of surviving in space is complicated by the fact that an interstellar warrior may need to fight in other environments, including those that swing to the other end of the spectrum and place far greater pressure upon their environment suit than Earth atmosphere.
For the purposes of a space combat force, and indeed any space travelers, a compression skinsuit makes sense, worn as an undergarment. The suits can be customized as desired and worn as a base layer, or even an outer layer for the daring and immodest in accepting company. For a space combatant, this will be worn even inside of a full spacesuit as an emergency layer; in case of the suit taking damage sufficient to decompress, the wearer remains vacuum resistant. Unfortunately, this suit for all its advantages, offers no further resistance to high pressure than that of a normal human, which is good to just shy of 8 Atmospheres, or 804 kPa. Still, combined with another suit, it serves as a vital base layer for combat sustainability, and may be used for suits intended exclusively for use no higher than slightly above 1 Atmosphere.
To form the main suit of the space marine, a hybrid suit makes the most sense. A heavy duty, armored torso minimizes the bulk of the hardsuit, allowing the same higher pressure to be used as the space station or ship, eliminating the need for prebreathing. Limbs are enclosed in a semi-rigid suit, kept at minimal pressure as an assistance to the compression suit, to prevent undue difficulty due to fighting the pressure. The entire armor system is rigid enough to withstand pressures far above those of an unequipped human, permitting the combatant to engage in battle under any circumstances where human interests are likely to be found and fought for.
This suit will be armored for combat, using materials such as steel armor plate, titanium, or other materials that provide multiple hit survivability; the weight of the armor, if high tech low-weight armors are unavailable, is offset by obviating the need to carry spare plates and increased combatant survivability.
Gloves. For a great deal of time, it was uncertain if space-capable gloves would be possible. We managed, but they are immensely difficult to operate, often leading to a great deal of pain and suffering for those using them. Rather than the multi-purpose hooks and pincers originally theorized, space combat forces will wear a simple glove inside of the arms of their suit, within the pressure unit. Via mechanical or electronic control, this glove will operate a separate hand built into the suit, possibly providing as much or greater dexterity and feedback to the suit's wearer than pressure gloves might.
Biological Considerations: On assignment, a space combatant may be required to eat, sleep, and perform vital human necessities for days or weeks at a time in their suit. Current systems manage this by high tech underwear for the short durations in suit. For combat spacers, a double catheter system will both permit the suit's wearer to deal with physiological needs as required and prevent issues caused by human waste disposal systems not working properly in freefall (the bladder doesn't register as full until it's absolutely at capacity without the effects of gravity).
Food: Extended duration operations that require multiple meals to be had in suit will make use of a failsafe micro-airlock, that can be loaded with meal bars or similar to be loaded into place. Combat drugs, water canteens, and other such systems may also be similarly reloadable while in vacuum.
Heat: A combat suit will need a more robust than standard heat ventilation system, and a combination of available and proposed mechanisms will be used.
Injury prevention/maintenance: All portions of the suit will be separated, and decompression issues are resisted by the compression undersuit, but exposure to hostile environments is to be avoided. Suits may include materials that will foam when the suit is punctured, sealing the hole and applying pressure to a wound inside. An application of liquid wound sealant may be included in one of these layers. Alternatively, such a Foam Bandage may be stored as a syringe, and may even be usable on internal injuries to the wearer's torso with or without a suit.
0G afflictions: It has been found recently that a suit with resistance bands built in to apply an imitation of gravity between the heels and shoulders of an astronaut largely mitigates bone and muscle loss due to freefall. Such a suit will be standard when not in armor, and may be an adjustable part of the compression space underwear. A similar system may resist arm motion as well to retain muscle.