Most planets are far from habitable, or at least extremely uncomfortable, for the average colonist. Many are lifeless, atmosphere-lacking rocks, or so extreme in their conditions that life as we know it might not even have a chance, no matter how hardy the organism. This is for you, the not-me-as-DM-for-once.
Primary Factors are generally those of a hazardous environment, where exposure is almost certainly death. Characters might get some leeway in expeditious corrective measures, though most often their chances are as slim as their environment is deadly. Being exposed to vacuum will not instantly kill someone, but getting a face-full of pressurized Venus is definitely the last thing they'll ever do.
Secondary Factors are those more likely to be survived, should exposure occur. These factors are more likely to be evident on more moderate planets that aren't quite right. With the exception of antiquated space suits, most feature auto-sealing countermeasures, or are easily patched in the event of integrity rupture.
- no atmosphere
- very thin atmosphere
- extreme atmospheric pressure
- extreme temperature
- heavily radiated
- extreme gravity
- dangerously saturated air
- unbreathable air
- dangerous micro-organisms/bacteria in air
- poisonous elements in air
- unsafe radiation levels
- regular acid/sulfur rain
Planets without atmospheres are often smaller and possess lower gravity than most are accustomed to, and often are subject to massive amounts of unimpeded solar radiation and extremely cold temperatures during the night cycle. Asphyxiation is not fun.
A planetoid with a notably thin atmosphere might consist of breathable air, however there is never enough air density to breath, leading to hypoxia and death. These planets are often colder and subject to almost unimpeded solar radiation. Often the cause of receding atmosphere relates to the planet being remarkably close to a larger source of gravity or exposure to extreme solar radiation.
Extreme atmospheric pressure is caused by an overdeveloped atmosphere often laden with extremely high levels of greenhouse gasses, or the result of a very high gravity, such as the conditions of a gas giant. This environment is exceedingly dangerous, requiring the most advanced of protection systems. Failure of these systems can mean swift death.
Deep cold and exceeding heat both prove dangerous to any would-be adventurer. Depending on atmospheric conditions, either of these extremes can also accompany extreme pressure ranges. Exposure means quick painful death.
Most common among worlds that survive their parent star going nova, heavily radiated worlds often lack atmospheres and are devoid of life. Even in heavily shielded suits, people quickly die from radiation bombardment.
Extremely high gravity can crush and flatten man and machine alike, and is often accompanied by heightened atmospheric pressure. Low gravity is an impediment and often a cause of very thin atmospheres. While movement and physical exertion is often taxed in high-gravity worlds, higher speeds can prove especially dangerous in low-gravity conditions.
Dangerously saturated air is one that is technically breathable, however the overabundance of a not-normally-dangerous element in the atmosphere can effectively drown or suffocate people without proper breathing apparatus, within any number of minutes.
Unbreathable air is simply the result of an atmosphere that our bodies can not draw oxygen from. Asphyxiation is not fun.
On some primitive or overly-aggressive life-bearing worlds, there can be any manner or fungi, spores, virus, or airborne bacteria that cause anaphylactic shock when accidentally inhaled or exposed to through suit ruptures. Other effects might include sickness and infections only later apparent. This is the hazardous garden that looks welcoming...
Poisoned air contains elements naturally dangerous to our bodies with appropriate dosage. An overabundance of dangerous gasses and chemicals pollute air that might have almost been otherwise near breathable conditions. Gasmasks, or similar air filters, may suffice in allowing safe respiration to explorers.
While not as extreme as heavily-radiated worlds, others may possess thin ozone layers, or be exposed to radiation from a nearby star or other presence that can be harmful to unprotected individuals, though not necessarily to native life, assuming that the radiation levels aren't too high.
Atmospheric composition and conditions can lead to noxious concentrations of chemical acids and similar destructive compounds that pollute the environment, leading to acid rain and rapid corrosion. This environment can quickly wear out equipment, destroy materials, and kill living beings.