Humans are physiologically well-adapted to life on Earth. We developed with gravity and we need it.
The most significant adverse effects of long-term weightlessness are muscle atrophy and deterioration of the skeleton. Without the effects of gravity, skeletal muscle is no longer required to maintain posture and the muscle groups used in moving around in a weightless environment differ from those required in terrestrial locomotion. In a weightless environment, astronauts put almost no weight on the back muscles or leg muscles used for standing up. Astronauts who have been in space more or longer than they can take run the risk of snapping bones when re-introduced to a weighted environment. The bone has lost enough mineral material to well - dry twigs come to mind. After a 3-4 month trip into space, it takes about 2–3 years to regain lost bone density.
Without regular exercise astronauts can lose up to 20% of their muscle mass in just 5 to 11 days.
Normally, bone is laid down in the direction of mechanical stress, however in a Microgravity environment there is very little mechanical stress. This results in a loss of bone tissue approximately 1.5% per month especially from the lower vertebrae, hip and femur.
For game purposes - we're hand-waving this one a little bit, cause: ow. Basically all medical attention in game has been focused on figuring out how to fix this before it destroys the population. So expect vitamin supplements, mineralization in drinking water, required daily exercise or deliberate exposure to gravity fields by going on away-missions.
Other significant effects include a slowing of cardiovascular system functions, decreased production of red blood cells, balance disorders, and a weakening of the immune system. In space, astronauts lose fluid volume—including up to 22% of their blood volume. Because it has less blood to pump, the heart will atrophy.
Lesser symptoms include fluid redistribution (causing the "moon-face" appearance typical in pictures of astronauts experiencing weightlessness), loss of body mass, nasal congestion, sleep disturbance, and excess flatulence.
"One effect of weightlessness on humans is that some astronauts report a change in their sense of taste when in space."
The majority component of taste is smell. In a weightless / windless environment you're relying on osmosis principles only to convey the volatile oils of a food stuff. Side note: the way smell works is small molecules of whatever it is have floated off and entered your nose and are directly interacting with your follicles. Which is to say: YES an extremely sensitive peanut allergy *can* be lethal if you open a bag of nuts in a contained air environment. If you can smell the peanut - you're in *contact* with it. (Think about that next time you hit a morgue and catch a whiff...)
"Tears cannot be shed while crying, as they stick together into a ball."
Water, anything with surface tension forms a ball and likes to float around under minor inertial forces. No gravity to create a 'drip'. This makes water one of the most dangerous things to have free floating in a ship.
Short-term exposure to hard vaccum
So to sum up, if exposed to the near vacuum of space, as long as you don’t try to hold your breath or impeded its decompression, you’d:
Remain conscious for about 10-15 seconds, during which time you’d feel the water evaporating off your tongue and the moisture on your skin doing the same, such as if you were sweating. (This would make the vacuum feel cold.)
You may or may not projectile vomit and defecate, as the gasses in your stomach and bowels are ejected rapidly (Mental note: might want to avoid chili and Coke before going into space.)
If your Eustachian tubes in your ears are blocked by ear wax or the like, you may have some inner-ear problems that result, but otherwise should be fine there.
Your heart rate will spike up, then steadily fall thereafter, as will your arterial blood pressure. Your venous pressure will steadily rise as gasses form.
Your body will swell up to as much as twice its normal size as your skin stretches, assuming you weren’t wearing a suit that constricted things. According to the Bioastronautics Data Book, with a properly designed and fitted elastic suit, experiments have shown that the formation of gas bubbles in your body fluids can be completely prevented as low as 15 torr (for reference 760 torr is normal atmospheric pressure and atmospheric pressure on the moon is 10-11 torr. Further, 47 torr is the point at which your blood would normally boil.) The swelling of your body is due to the moisture in your soft tissue turning to a gaseous state. However, your skin is strong enough to hold it in. So you won’t explode, you’ll simply expand.
During this process, your body will continually eject gas and water vapor through your mouth and nose, resulting in these getting colder and colder as the moisture evaporates, possibly even freezing your mouth or tongue.
If you happen to be in direct sunlight, you can expect extreme sunburns without the Earth’s atmosphere or other medium to protect you from the intense UV rays of the Sun.
Your skin will start to turn blue-ish purple from lack of oxygen, a condition known as cyanosis.
Your brain and heart will remain relatively undamaged for a time and your heart will continue to beat until around the 90-180 second mark. As your blood pressure drops, your blood itself will begin to boil once the pressure drops below 47 torr, resulting in your heart stopping beating, among other problems. This doesn’t happen instantly, though, as is depicted in the movies. No animal or human has ever been successfully resuscitated in these instances once the heart stops.
If pressure is restored in time, you’ll find yourself temporarily blind and unable to move, but both of these symptoms will pass. You also apparently will lose your sense of taste for a few days.
On the flip-side, if you hold your breath or otherwise try to impede the rate at which air is exhaled during explosive decompression, the “lungs and thorax will become over-expanded by the excessively high intrapulmonic pressure, causing actual tearing and rupture of the lung tissues and capillaries. The trapped air is forced through the lungs into the thoracic cage, and air can be injected directly into the general circulation by way of the ruptured blood vessels, with massive air bubbles moving throughout the body and lodging in vital organs such as the heart and brain.” This goes for decompression in a commercial airplane at high altitude too, so make sure you don’t try to hold your breath if that ever happens to you on a plane.
Rules for Handling Exposure to Hard Vacuum
How to handle someone being vented into space or exposed to hard vacuum vary round by round.
Rules for Round...
- C rolls DEX and END vs. difficulty 25 to react to the situation (breathe out, curl up, etc). They take 25 damage minus the PP generated (to a minimum of zero). Wearing a vacuum suit or other pressure suit with no helmet reduces the base damage to 10. Wearing a vacuum suit with a helmet and available oxygen means no roll is required at all until the oxygen runs out.
- C rolls STR and END vs. difficulty 40 to maintain consciousness and resist the cold and pressure. Again, base damage 25 minus PP generated. Pressure suit with no helmet is base damage 10 instead.
- C rolls STR and END vs. +10 difficulty to resist further damage as above (base damage numbers don't change). All other actions also suffer +10 difficulty to reflect the situation, regardless of the outcome of the resistance roll. By this point, many characters will be unconscious or otherwise need the aid of others to survive, which is reflexive of being vented into space.
All remaining rounds follow the same rules as round 3 - difficulties for resisting the Void and for performing other actions go up by 10 each round they remain in the vacuum.
Do these rules apply to Metal-born as well? Yes. If you're metal-born and expect to be in space a lot - make sure to take a skill to reflect any vacuum hardening your body has.
Many items won't work well in vacuum: Various compounds outgas from many plastics, lubricants evaporate, etc. Vacuum is also a very good isolator, making overheating a problem. In addition, sharp thermal changes can cause metal fatigue, ceramics to become brittle, and prevent many chemical reactions. In a pure vacum environment where micro-rusting is not occurring metals can fuse together as well (aka "cold welding") - all of these things can result in physical damage and data corruption.